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rfc:rfc5321

Network Working Group J. Klensin Request for Comments: 5321 October 2008 Obsoletes: 2821 Updates: 1123 Category: Standards Track

                   Simple Mail Transfer Protocol

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document is a specification of the basic protocol for Internet
 electronic mail transport.  It consolidates, updates, and clarifies
 several previous documents, making all or parts of most of them
 obsolete.  It covers the SMTP extension mechanisms and best practices
 for the contemporary Internet, but does not provide details about
 particular extensions.  Although SMTP was designed as a mail
 transport and delivery protocol, this specification also contains
 information that is important to its use as a "mail submission"
 protocol for "split-UA" (User Agent) mail reading systems and mobile
 environments.

Klensin Standards Track [Page 1] RFC 5321 SMTP October 2008

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.1.  Transport of Electronic Mail . . . . . . . . . . . . . . .  5
   1.2.  History and Context for This Document  . . . . . . . . . .  5
   1.3.  Document Conventions . . . . . . . . . . . . . . . . . . .  6
 2.  The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.1.  Basic Structure  . . . . . . . . . . . . . . . . . . . . .  7
   2.2.  The Extension Model  . . . . . . . . . . . . . . . . . . .  9
     2.2.1.  Background . . . . . . . . . . . . . . . . . . . . . .  9
     2.2.2.  Definition and Registration of Extensions  . . . . . . 10
     2.2.3.  Special Issues with Extensions . . . . . . . . . . . . 11
   2.3.  SMTP Terminology . . . . . . . . . . . . . . . . . . . . . 11
     2.3.1.  Mail Objects . . . . . . . . . . . . . . . . . . . . . 11
     2.3.2.  Senders and Receivers  . . . . . . . . . . . . . . . . 12
     2.3.3.  Mail Agents and Message Stores . . . . . . . . . . . . 12
     2.3.4.  Host . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.3.5.  Domain Names . . . . . . . . . . . . . . . . . . . . . 13
     2.3.6.  Buffer and State Table . . . . . . . . . . . . . . . . 14
     2.3.7.  Commands and Replies . . . . . . . . . . . . . . . . . 14
     2.3.8.  Lines  . . . . . . . . . . . . . . . . . . . . . . . . 14
     2.3.9.  Message Content and Mail Data  . . . . . . . . . . . . 15
     2.3.10. Originator, Delivery, Relay, and Gateway Systems . . . 15
     2.3.11. Mailbox and Address  . . . . . . . . . . . . . . . . . 15
   2.4.  General Syntax Principles and Transaction Model  . . . . . 16
 3.  The SMTP Procedures: An Overview . . . . . . . . . . . . . . . 17
   3.1.  Session Initiation . . . . . . . . . . . . . . . . . . . . 18
   3.2.  Client Initiation  . . . . . . . . . . . . . . . . . . . . 18
   3.3.  Mail Transactions  . . . . . . . . . . . . . . . . . . . . 19
   3.4.  Forwarding for Address Correction or Updating  . . . . . . 21
   3.5.  Commands for Debugging Addresses . . . . . . . . . . . . . 22
     3.5.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . 22
     3.5.2.  VRFY Normal Response . . . . . . . . . . . . . . . . . 24
     3.5.3.  Meaning of VRFY or EXPN Success Response . . . . . . . 25
     3.5.4.  Semantics and Applications of EXPN . . . . . . . . . . 26
   3.6.  Relaying and Mail Routing  . . . . . . . . . . . . . . . . 26
     3.6.1.  Source Routes and Relaying . . . . . . . . . . . . . . 26
     3.6.2.  Mail eXchange Records and Relaying . . . . . . . . . . 26
     3.6.3.  Message Submission Servers as Relays . . . . . . . . . 27
   3.7.  Mail Gatewaying  . . . . . . . . . . . . . . . . . . . . . 28
     3.7.1.  Header Fields in Gatewaying  . . . . . . . . . . . . . 28
     3.7.2.  Received Lines in Gatewaying . . . . . . . . . . . . . 29
     3.7.3.  Addresses in Gatewaying  . . . . . . . . . . . . . . . 29
     3.7.4.  Other Header Fields in Gatewaying  . . . . . . . . . . 29
     3.7.5.  Envelopes in Gatewaying  . . . . . . . . . . . . . . . 30
   3.8.  Terminating Sessions and Connections . . . . . . . . . . . 30
   3.9.  Mailing Lists and Aliases  . . . . . . . . . . . . . . . . 31
     3.9.1.  Alias  . . . . . . . . . . . . . . . . . . . . . . . . 31

Klensin Standards Track [Page 2] RFC 5321 SMTP October 2008

     3.9.2.  List . . . . . . . . . . . . . . . . . . . . . . . . . 31
 4.  The SMTP Specifications  . . . . . . . . . . . . . . . . . . . 32
   4.1.  SMTP Commands  . . . . . . . . . . . . . . . . . . . . . . 32
     4.1.1.  Command Semantics and Syntax . . . . . . . . . . . . . 32
     4.1.2.  Command Argument Syntax  . . . . . . . . . . . . . . . 41
     4.1.3.  Address Literals . . . . . . . . . . . . . . . . . . . 43
     4.1.4.  Order of Commands  . . . . . . . . . . . . . . . . . . 44
     4.1.5.  Private-Use Commands . . . . . . . . . . . . . . . . . 46
   4.2.  SMTP Replies . . . . . . . . . . . . . . . . . . . . . . . 46
     4.2.1.  Reply Code Severities and Theory . . . . . . . . . . . 48
     4.2.2.  Reply Codes by Function Groups . . . . . . . . . . . . 50
     4.2.3.  Reply Codes in Numeric Order . . . . . . . . . . . . . 52
     4.2.4.  Reply Code 502 . . . . . . . . . . . . . . . . . . . . 53
     4.2.5.  Reply Codes after DATA and the Subsequent
             <CRLF>.<CRLF>  . . . . . . . . . . . . . . . . . . . . 53
   4.3.  Sequencing of Commands and Replies . . . . . . . . . . . . 54
     4.3.1.  Sequencing Overview  . . . . . . . . . . . . . . . . . 54
     4.3.2.  Command-Reply Sequences  . . . . . . . . . . . . . . . 55
   4.4.  Trace Information  . . . . . . . . . . . . . . . . . . . . 57
   4.5.  Additional Implementation Issues . . . . . . . . . . . . . 61
     4.5.1.  Minimum Implementation . . . . . . . . . . . . . . . . 61
     4.5.2.  Transparency . . . . . . . . . . . . . . . . . . . . . 62
     4.5.3.  Sizes and Timeouts . . . . . . . . . . . . . . . . . . 62
       4.5.3.1.  Size Limits and Minimums . . . . . . . . . . . . . 62
         4.5.3.1.1.  Local-part . . . . . . . . . . . . . . . . . . 63
         4.5.3.1.2.  Domain . . . . . . . . . . . . . . . . . . . . 63
         4.5.3.1.3.  Path . . . . . . . . . . . . . . . . . . . . . 63
         4.5.3.1.4.  Command Line . . . . . . . . . . . . . . . . . 63
         4.5.3.1.5.  Reply Line . . . . . . . . . . . . . . . . . . 63
         4.5.3.1.6.  Text Line  . . . . . . . . . . . . . . . . . . 63
         4.5.3.1.7.  Message Content  . . . . . . . . . . . . . . . 63
         4.5.3.1.8.  Recipients Buffer  . . . . . . . . . . . . . . 64
         4.5.3.1.9.  Treatment When Limits Exceeded . . . . . . . . 64
         4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . . 64
       4.5.3.2.  Timeouts . . . . . . . . . . . . . . . . . . . . . 65
         4.5.3.2.1.  Initial 220 Message: 5 Minutes . . . . . . . . 65
         4.5.3.2.2.  MAIL Command: 5 Minutes  . . . . . . . . . . . 65
         4.5.3.2.3.  RCPT Command: 5 Minutes  . . . . . . . . . . . 65
         4.5.3.2.4.  DATA Initiation: 2 Minutes . . . . . . . . . . 66
         4.5.3.2.5.  Data Block: 3 Minutes  . . . . . . . . . . . . 66
         4.5.3.2.6.  DATA Termination: 10 Minutes.  . . . . . . . . 66
         4.5.3.2.7.  Server Timeout: 5 Minutes. . . . . . . . . . . 66
     4.5.4.  Retry Strategies . . . . . . . . . . . . . . . . . . . 66
     4.5.5.  Messages with a Null Reverse-Path  . . . . . . . . . . 68
 5.  Address Resolution and Mail Handling . . . . . . . . . . . . . 69
   5.1.  Locating the Target Host . . . . . . . . . . . . . . . . . 69
   5.2.  IPv6 and MX Records  . . . . . . . . . . . . . . . . . . . 71
 6.  Problem Detection and Handling . . . . . . . . . . . . . . . . 71

Klensin Standards Track [Page 3] RFC 5321 SMTP October 2008

   6.1.  Reliable Delivery and Replies by Email . . . . . . . . . . 71
   6.2.  Unwanted, Unsolicited, and "Attack" Messages . . . . . . . 72
   6.3.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 73
   6.4.  Compensating for Irregularities  . . . . . . . . . . . . . 73
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 75
   7.1.  Mail Security and Spoofing . . . . . . . . . . . . . . . . 75
   7.2.  "Blind" Copies . . . . . . . . . . . . . . . . . . . . . . 76
   7.3.  VRFY, EXPN, and Security . . . . . . . . . . . . . . . . . 76
   7.4.  Mail Rerouting Based on the 251 and 551 Response Codes . . 77
   7.5.  Information Disclosure in Announcements  . . . . . . . . . 77
   7.6.  Information Disclosure in Trace Fields . . . . . . . . . . 78
   7.7.  Information Disclosure in Message Forwarding . . . . . . . 78
   7.8.  Resistance to Attacks  . . . . . . . . . . . . . . . . . . 78
   7.9.  Scope of Operation of SMTP Servers . . . . . . . . . . . . 78
 8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 79
 9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 80
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 81
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 81
   10.2. Informative References . . . . . . . . . . . . . . . . . . 82
 Appendix A.  TCP Transport Service . . . . . . . . . . . . . . . . 85
 Appendix B.  Generating SMTP Commands from RFC 822 Header
              Fields  . . . . . . . . . . . . . . . . . . . . . . . 85
 Appendix C.  Source Routes . . . . . . . . . . . . . . . . . . . . 86
 Appendix D.  Scenarios . . . . . . . . . . . . . . . . . . . . . . 87
   D.1.  A Typical SMTP Transaction Scenario  . . . . . . . . . . . 88
   D.2.  Aborted SMTP Transaction Scenario  . . . . . . . . . . . . 89
   D.3.  Relayed Mail Scenario  . . . . . . . . . . . . . . . . . . 90
   D.4.  Verifying and Sending Scenario . . . . . . . . . . . . . . 92
 Appendix E.  Other Gateway Issues  . . . . . . . . . . . . . . . . 92
 Appendix F.  Deprecated Features of RFC 821  . . . . . . . . . . . 93
   F.1.  TURN . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
   F.2.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 93
   F.3.  HELO . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
   F.4.  #-literals . . . . . . . . . . . . . . . . . . . . . . . . 94
   F.5.  Dates and Years  . . . . . . . . . . . . . . . . . . . . . 94
   F.6.  Sending versus Mailing . . . . . . . . . . . . . . . . . . 94

Klensin Standards Track [Page 4] RFC 5321 SMTP October 2008

1. Introduction

1.1. Transport of Electronic Mail

 The objective of the Simple Mail Transfer Protocol (SMTP) is to
 transfer mail reliably and efficiently.
 SMTP is independent of the particular transmission subsystem and
 requires only a reliable ordered data stream channel.  While this
 document specifically discusses transport over TCP, other transports
 are possible.  Appendices to RFC 821 [1] describe some of them.
 An important feature of SMTP is its capability to transport mail
 across multiple networks, usually referred to as "SMTP mail relaying"
 (see Section 3.6).  A network consists of the mutually-TCP-accessible
 hosts on the public Internet, the mutually-TCP-accessible hosts on a
 firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN
 environment utilizing a non-TCP transport-level protocol.  Using
 SMTP, a process can transfer mail to another process on the same
 network or to some other network via a relay or gateway process
 accessible to both networks.
 In this way, a mail message may pass through a number of intermediate
 relay or gateway hosts on its path from sender to ultimate recipient.
 The Mail eXchanger mechanisms of the domain name system (RFC 1035
 [2], RFC 974 [12], and Section 5 of this document) are used to
 identify the appropriate next-hop destination for a message being
 transported.

1.2. History and Context for This Document

 This document is a specification of the basic protocol for the
 Internet electronic mail transport.  It consolidates, updates and
 clarifies, but does not add new or change existing functionality of
 the following:
 o  the original SMTP (Simple Mail Transfer Protocol) specification of
    RFC 821 [1],
 o  domain name system requirements and implications for mail
    transport from RFC 1035 [2] and RFC 974 [12],
 o  the clarifications and applicability statements in RFC 1123 [3],
    and
 o  material drawn from the SMTP Extension mechanisms in RFC 1869
    [13].

Klensin Standards Track [Page 5] RFC 5321 SMTP October 2008

 o  Editorial and clarification changes to RFC 2821 [14] to bring that
    specification to Draft Standard.
 It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC
 1123 (replacing the mail transport materials of RFC 1123).  However,
 RFC 821 specifies some features that were not in significant use in
 the Internet by the mid-1990s and (in appendices) some additional
 transport models.  Those sections are omitted here in the interest of
 clarity and brevity; readers needing them should refer to RFC 821.
 It also includes some additional material from RFC 1123 that required
 amplification.  This material has been identified in multiple ways,
 mostly by tracking flaming on various lists and newsgroups and
 problems of unusual readings or interpretations that have appeared as
 the SMTP extensions have been deployed.  Where this specification
 moves beyond consolidation and actually differs from earlier
 documents, it supersedes them technically as well as textually.
 Although SMTP was designed as a mail transport and delivery protocol,
 this specification also contains information that is important to its
 use as a "mail submission" protocol, as recommended for Post Office
 Protocol (POP) (RFC 937 [15], RFC 1939 [16]) and IMAP (RFC 3501
 [17]).  In general, the separate mail submission protocol specified
 in RFC 4409 [18] is now preferred to direct use of SMTP; more
 discussion of that subject appears in that document.
 Section 2.3 provides definitions of terms specific to this document.
 Except when the historical terminology is necessary for clarity, this
 document uses the current 'client' and 'server' terminology to
 identify the sending and receiving SMTP processes, respectively.
 A companion document, RFC 5322 [4], discusses message header sections
 and bodies and specifies formats and structures for them.

1.3. Document Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [5].  As each
 of these terms was intentionally and carefully chosen to improve the
 interoperability of email, each use of these terms is to be treated
 as a conformance requirement.
 Because this document has a long history and to avoid the risk of
 various errors and of confusing readers and documents that point to
 this one, most examples and the domain names they contain are
 preserved from RFC 2821.  Readers are cautioned that these are

Klensin Standards Track [Page 6] RFC 5321 SMTP October 2008

 illustrative examples that should not actually be used in either code
 or configuration files.

2. The SMTP Model

2.1. Basic Structure

 The SMTP design can be pictured as:
                +----------+                +----------+
    +------+    |          |                |          |
    | User |<-->|          |      SMTP      |          |
    +------+    |  Client- |Commands/Replies| Server-  |
    +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
    | File |<-->|          |    and Mail    |          |<-->| File |
    |System|    |          |                |          |    |System|
    +------+    +----------+                +----------+    +------+
                 SMTP client                SMTP server
 When an SMTP client has a message to transmit, it establishes a two-
 way transmission channel to an SMTP server.  The responsibility of an
 SMTP client is to transfer mail messages to one or more SMTP servers,
 or report its failure to do so.
 The means by which a mail message is presented to an SMTP client, and
 how that client determines the identifier(s) ("names") of the
 domain(s) to which mail messages are to be transferred, is a local
 matter, and is not addressed by this document.  In some cases, the
 designated domain(s), or those determined by an SMTP client, will
 identify the final destination(s) of the mail message.  In other
 cases, common with SMTP clients associated with implementations of
 the POP (RFC 937 [15], RFC 1939 [16]) or IMAP (RFC 3501 [17])
 protocols, or when the SMTP client is inside an isolated transport
 service environment, the domain determined will identify an
 intermediate destination through which all mail messages are to be
 relayed.  SMTP clients that transfer all traffic regardless of the
 target domains associated with the individual messages, or that do
 not maintain queues for retrying message transmissions that initially
 cannot be completed, may otherwise conform to this specification but
 are not considered fully-capable.  Fully-capable SMTP
 implementations, including the relays used by these less capable
 ones, and their destinations, are expected to support all of the
 queuing, retrying, and alternate address functions discussed in this
 specification.  In many situations and configurations, the less-
 capable clients discussed above SHOULD be using the message
 submission protocol (RFC 4409 [18]) rather than SMTP.

Klensin Standards Track [Page 7] RFC 5321 SMTP October 2008

 The means by which an SMTP client, once it has determined a target
 domain, determines the identity of an SMTP server to which a copy of
 a message is to be transferred, and then performs that transfer, is
 covered by this document.  To effect a mail transfer to an SMTP
 server, an SMTP client establishes a two-way transmission channel to
 that SMTP server.  An SMTP client determines the address of an
 appropriate host running an SMTP server by resolving a destination
 domain name to either an intermediate Mail eXchanger host or a final
 target host.
 An SMTP server may be either the ultimate destination or an
 intermediate "relay" (that is, it may assume the role of an SMTP
 client after receiving the message) or "gateway" (that is, it may
 transport the message further using some protocol other than SMTP).
 SMTP commands are generated by the SMTP client and sent to the SMTP
 server.  SMTP replies are sent from the SMTP server to the SMTP
 client in response to the commands.
 In other words, message transfer can occur in a single connection
 between the original SMTP-sender and the final SMTP-recipient, or can
 occur in a series of hops through intermediary systems.  In either
 case, once the server has issued a success response at the end of the
 mail data, a formal handoff of responsibility for the message occurs:
 the protocol requires that a server MUST accept responsibility for
 either delivering the message or properly reporting the failure to do
 so (see Sections 6.1, 6.2, and 7.8, below).
 Once the transmission channel is established and initial handshaking
 is completed, the SMTP client normally initiates a mail transaction.
 Such a transaction consists of a series of commands to specify the
 originator and destination of the mail and transmission of the
 message content (including any lines in the header section or other
 structure) itself.  When the same message is sent to multiple
 recipients, this protocol encourages the transmission of only one
 copy of the data for all recipients at the same destination (or
 intermediate relay) host.
 The server responds to each command with a reply; replies may
 indicate that the command was accepted, that additional commands are
 expected, or that a temporary or permanent error condition exists.
 Commands specifying the sender or recipients may include server-
 permitted SMTP service extension requests, as discussed in
 Section 2.2.  The dialog is purposely lock-step, one-at-a-time,
 although this can be modified by mutually agreed upon extension
 requests such as command pipelining (RFC 2920 [19]).
 Once a given mail message has been transmitted, the client may either
 request that the connection be shut down or may initiate other mail

Klensin Standards Track [Page 8] RFC 5321 SMTP October 2008

 transactions.  In addition, an SMTP client may use a connection to an
 SMTP server for ancillary services such as verification of email
 addresses or retrieval of mailing list subscriber addresses.
 As suggested above, this protocol provides mechanisms for the
 transmission of mail.  Historically, this transmission normally
 occurred directly from the sending user's host to the receiving
 user's host when the two hosts are connected to the same transport
 service.  When they are not connected to the same transport service,
 transmission occurs via one or more relay SMTP servers.  A very
 common case in the Internet today involves submission of the original
 message to an intermediate, "message submission" server, which is
 similar to a relay but has some additional properties; such servers
 are discussed in Section 2.3.10 and at some length in RFC 4409 [18].
 An intermediate host that acts as either an SMTP relay or as a
 gateway into some other transmission environment is usually selected
 through the use of the domain name service (DNS) Mail eXchanger
 mechanism.
 Usually, intermediate hosts are determined via the DNS MX record, not
 by explicit "source" routing (see Section 5 and Appendix C and
 Appendix F.2).

2.2. The Extension Model

2.2.1. Background

 In an effort that started in 1990, approximately a decade after RFC
 821 was completed, the protocol was modified with a "service
 extensions" model that permits the client and server to agree to
 utilize shared functionality beyond the original SMTP requirements.
 The SMTP extension mechanism defines a means whereby an extended SMTP
 client and server may recognize each other, and the server can inform
 the client as to the service extensions that it supports.
 Contemporary SMTP implementations MUST support the basic extension
 mechanisms.  For instance, servers MUST support the EHLO command even
 if they do not implement any specific extensions and clients SHOULD
 preferentially utilize EHLO rather than HELO.  (However, for
 compatibility with older conforming implementations, SMTP clients and
 servers MUST support the original HELO mechanisms as a fallback.)
 Unless the different characteristics of HELO must be identified for
 interoperability purposes, this document discusses only EHLO.
 SMTP is widely deployed and high-quality implementations have proven
 to be very robust.  However, the Internet community now considers
 some services to be important that were not anticipated when the
 protocol was first designed.  If support for those services is to be

Klensin Standards Track [Page 9] RFC 5321 SMTP October 2008

 added, it must be done in a way that permits older implementations to
 continue working acceptably.  The extension framework consists of:
 o  The SMTP command EHLO, superseding the earlier HELO,
 o  a registry of SMTP service extensions,
 o  additional parameters to the SMTP MAIL and RCPT commands, and
 o  optional replacements for commands defined in this protocol, such
    as for DATA in non-ASCII transmissions (RFC 3030 [20]).
 SMTP's strength comes primarily from its simplicity.  Experience with
 many protocols has shown that protocols with few options tend towards
 ubiquity, whereas protocols with many options tend towards obscurity.
 Each and every extension, regardless of its benefits, must be
 carefully scrutinized with respect to its implementation, deployment,
 and interoperability costs.  In many cases, the cost of extending the
 SMTP service will likely outweigh the benefit.

2.2.2. Definition and Registration of Extensions

 The IANA maintains a registry of SMTP service extensions.  A
 corresponding EHLO keyword value is associated with each extension.
 Each service extension registered with the IANA must be defined in a
 formal Standards-Track or IESG-approved Experimental protocol
 document.  The definition must include:
 o  the textual name of the SMTP service extension;
 o  the EHLO keyword value associated with the extension;
 o  the syntax and possible values of parameters associated with the
    EHLO keyword value;
 o  any additional SMTP verbs associated with the extension
    (additional verbs will usually be, but are not required to be, the
    same as the EHLO keyword value);
 o  any new parameters the extension associates with the MAIL or RCPT
    verbs;
 o  a description of how support for the extension affects the
    behavior of a server and client SMTP; and

Klensin Standards Track [Page 10] RFC 5321 SMTP October 2008

 o  the increment by which the extension is increasing the maximum
    length of the commands MAIL and/or RCPT, over that specified in
    this Standard.
 In addition, any EHLO keyword value starting with an upper or lower
 case "X" refers to a local SMTP service extension used exclusively
 through bilateral agreement.  Keywords beginning with "X" MUST NOT be
 used in a registered service extension.  Conversely, keyword values
 presented in the EHLO response that do not begin with "X" MUST
 correspond to a Standard, Standards-Track, or IESG-approved
 Experimental SMTP service extension registered with IANA.  A
 conforming server MUST NOT offer non-"X"-prefixed keyword values that
 are not described in a registered extension.
 Additional verbs and parameter names are bound by the same rules as
 EHLO keywords; specifically, verbs beginning with "X" are local
 extensions that may not be registered or standardized.  Conversely,
 verbs not beginning with "X" must always be registered.

2.2.3. Special Issues with Extensions

 Extensions that change fairly basic properties of SMTP operation are
 permitted.  The text in other sections of this document must be
 understood in that context.  In particular, extensions can change the
 minimum limits specified in Section 4.5.3, can change the ASCII
 character set requirement as mentioned above, or can introduce some
 optional modes of message handling.
 In particular, if an extension implies that the delivery path
 normally supports special features of that extension, and an
 intermediate SMTP system finds a next hop that does not support the
 required extension, it MAY choose, based on the specific extension
 and circumstances, to requeue the message and try later and/or try an
 alternate MX host.  If this strategy is employed, the timeout to fall
 back to an unextended format (if one is available) SHOULD be less
 than the normal timeout for bouncing as undeliverable (e.g., if
 normal timeout is three days, the requeue timeout before attempting
 to transmit the mail without the extension might be one day).

2.3. SMTP Terminology

2.3.1. Mail Objects

 SMTP transports a mail object.  A mail object contains an envelope
 and content.
 The SMTP envelope is sent as a series of SMTP protocol units
 (described in Section 3).  It consists of an originator address (to

Klensin Standards Track [Page 11] RFC 5321 SMTP October 2008

 which error reports should be directed), one or more recipient
 addresses, and optional protocol extension material.  Historically,
 variations on the reverse-path (originator) address specification
 command (MAIL) could be used to specify alternate delivery modes,
 such as immediate display; those variations have now been deprecated
 (see Appendix F and Appendix F.6).
 The SMTP content is sent in the SMTP DATA protocol unit and has two
 parts: the header section and the body.  If the content conforms to
 other contemporary standards, the header section consists of a
 collection of header fields, each consisting of a header name, a
 colon, and data, structured as in the message format specification
 (RFC 5322 [4]); the body, if structured, is defined according to MIME
 (RFC 2045 [21]).  The content is textual in nature, expressed using
 the US-ASCII repertoire [6].  Although SMTP extensions (such as
 "8BITMIME", RFC 1652 [22]) may relax this restriction for the content
 body, the content header fields are always encoded using the US-ASCII
 repertoire.  Two MIME extensions (RFC 2047 [23] and RFC 2231 [24])
 define an algorithm for representing header values outside the US-
 ASCII repertoire, while still encoding them using the US-ASCII
 repertoire.

2.3.2. Senders and Receivers

 In RFC 821, the two hosts participating in an SMTP transaction were
 described as the "SMTP-sender" and "SMTP-receiver".  This document
 has been changed to reflect current industry terminology and hence
 refers to them as the "SMTP client" (or sometimes just "the client")
 and "SMTP server" (or just "the server"), respectively.  Since a
 given host may act both as server and client in a relay situation,
 "receiver" and "sender" terminology is still used where needed for
 clarity.

2.3.3. Mail Agents and Message Stores

 Additional mail system terminology became common after RFC 821 was
 published and, where convenient, is used in this specification.  In
 particular, SMTP servers and clients provide a mail transport service
 and therefore act as "Mail Transfer Agents" (MTAs).  "Mail User
 Agents" (MUAs or UAs) are normally thought of as the sources and
 targets of mail.  At the source, an MUA might collect mail to be
 transmitted from a user and hand it off to an MTA; the final
 ("delivery") MTA would be thought of as handing the mail off to an
 MUA (or at least transferring responsibility to it, e.g., by
 depositing the message in a "message store").  However, while these
 terms are used with at least the appearance of great precision in
 other environments, the implied boundaries between MUAs and MTAs
 often do not accurately match common, and conforming, practices with

Klensin Standards Track [Page 12] RFC 5321 SMTP October 2008

 Internet mail.  Hence, the reader should be cautious about inferring
 the strong relationships and responsibilities that might be implied
 if these terms were used elsewhere.

2.3.4. Host

 For the purposes of this specification, a host is a computer system
 attached to the Internet (or, in some cases, to a private TCP/IP
 network) and supporting the SMTP protocol.  Hosts are known by names
 (see the next section); they SHOULD NOT be identified by numerical
 addresses, i.e., by address literals as described in Section 4.1.2.

2.3.5. Domain Names

 A domain name (or often just a "domain") consists of one or more
 components, separated by dots if more than one appears.  In the case
 of a top-level domain used by itself in an email address, a single
 string is used without any dots.  This makes the requirement,
 described in more detail below, that only fully-qualified domain
 names appear in SMTP transactions on the public Internet,
 particularly important where top-level domains are involved.  These
 components ("labels" in DNS terminology, RFC 1035 [2]) are restricted
 for SMTP purposes to consist of a sequence of letters, digits, and
 hyphens drawn from the ASCII character set [6].  Domain names are
 used as names of hosts and of other entities in the domain name
 hierarchy.  For example, a domain may refer to an alias (label of a
 CNAME RR) or the label of Mail eXchanger records to be used to
 deliver mail instead of representing a host name.  See RFC 1035 [2]
 and Section 5 of this specification.
 The domain name, as described in this document and in RFC 1035 [2],
 is the entire, fully-qualified name (often referred to as an "FQDN").
 A domain name that is not in FQDN form is no more than a local alias.
 Local aliases MUST NOT appear in any SMTP transaction.
 Only resolvable, fully-qualified domain names (FQDNs) are permitted
 when domain names are used in SMTP.  In other words, names that can
 be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
 in Section 5) are permitted, as are CNAME RRs whose targets can be
 resolved, in turn, to MX or address RRs.  Local nicknames or
 unqualified names MUST NOT be used.  There are two exceptions to the
 rule requiring FQDNs:
 o  The domain name given in the EHLO command MUST be either a primary
    host name (a domain name that resolves to an address RR) or, if
    the host has no name, an address literal, as described in
    Section 4.1.3 and discussed further in the EHLO discussion of
    Section 4.1.4.

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 o  The reserved mailbox name "postmaster" may be used in a RCPT
    command without domain qualification (see Section 4.1.1.3) and
    MUST be accepted if so used.

2.3.6. Buffer and State Table

 SMTP sessions are stateful, with both parties carefully maintaining a
 common view of the current state.  In this document, we model this
 state by a virtual "buffer" and a "state table" on the server that
 may be used by the client to, for example, "clear the buffer" or
 "reset the state table", causing the information in the buffer to be
 discarded and the state to be returned to some previous state.

2.3.7. Commands and Replies

 SMTP commands and, unless altered by a service extension, message
 data, are transmitted from the sender to the receiver via the
 transmission channel in "lines".
 An SMTP reply is an acknowledgment (positive or negative) sent in
 "lines" from receiver to sender via the transmission channel in
 response to a command.  The general form of a reply is a numeric
 completion code (indicating failure or success) usually followed by a
 text string.  The codes are for use by programs and the text is
 usually intended for human users.  RFC 3463 [25], specifies further
 structuring of the reply strings, including the use of supplemental
 and more specific completion codes (see also RFC 5248 [26]).

2.3.8. Lines

 Lines consist of zero or more data characters terminated by the
 sequence ASCII character "CR" (hex value 0D) followed immediately by
 ASCII character "LF" (hex value 0A).  This termination sequence is
 denoted as <CRLF> in this document.  Conforming implementations MUST
 NOT recognize or generate any other character or character sequence
 as a line terminator.  Limits MAY be imposed on line lengths by
 servers (see Section 4).
 In addition, the appearance of "bare" "CR" or "LF" characters in text
 (i.e., either without the other) has a long history of causing
 problems in mail implementations and applications that use the mail
 system as a tool.  SMTP client implementations MUST NOT transmit
 these characters except when they are intended as line terminators
 and then MUST, as indicated above, transmit them only as a <CRLF>
 sequence.

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2.3.9. Message Content and Mail Data

 The terms "message content" and "mail data" are used interchangeably
 in this document to describe the material transmitted after the DATA
 command is accepted and before the end of data indication is
 transmitted.  Message content includes the message header section and
 the possibly structured message body.  The MIME specification (RFC
 2045 [21]) provides the standard mechanisms for structured message
 bodies.

2.3.10. Originator, Delivery, Relay, and Gateway Systems

 This specification makes a distinction among four types of SMTP
 systems, based on the role those systems play in transmitting
 electronic mail.  An "originating" system (sometimes called an SMTP
 originator) introduces mail into the Internet or, more generally,
 into a transport service environment.  A "delivery" SMTP system is
 one that receives mail from a transport service environment and
 passes it to a mail user agent or deposits it in a message store that
 a mail user agent is expected to subsequently access.  A "relay" SMTP
 system (usually referred to just as a "relay") receives mail from an
 SMTP client and transmits it, without modification to the message
 data other than adding trace information, to another SMTP server for
 further relaying or for delivery.
 A "gateway" SMTP system (usually referred to just as a "gateway")
 receives mail from a client system in one transport environment and
 transmits it to a server system in another transport environment.
 Differences in protocols or message semantics between the transport
 environments on either side of a gateway may require that the gateway
 system perform transformations to the message that are not permitted
 to SMTP relay systems.  For the purposes of this specification,
 firewalls that rewrite addresses should be considered as gateways,
 even if SMTP is used on both sides of them (see RFC 2979 [27]).

2.3.11. Mailbox and Address

 As used in this specification, an "address" is a character string
 that identifies a user to whom mail will be sent or a location into
 which mail will be deposited.  The term "mailbox" refers to that
 depository.  The two terms are typically used interchangeably unless
 the distinction between the location in which mail is placed (the
 mailbox) and a reference to it (the address) is important.  An
 address normally consists of user and domain specifications.  The
 standard mailbox naming convention is defined to be
 "local-part@domain"; contemporary usage permits a much broader set of
 applications than simple "user names".  Consequently, and due to a
 long history of problems when intermediate hosts have attempted to

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 optimize transport by modifying them, the local-part MUST be
 interpreted and assigned semantics only by the host specified in the
 domain part of the address.

2.4. General Syntax Principles and Transaction Model

 SMTP commands and replies have a rigid syntax.  All commands begin
 with a command verb.  All replies begin with a three digit numeric
 code.  In some commands and replies, arguments are required following
 the verb or reply code.  Some commands do not accept arguments (after
 the verb), and some reply codes are followed, sometimes optionally,
 by free form text.  In both cases, where text appears, it is
 separated from the verb or reply code by a space character.  Complete
 definitions of commands and replies appear in Section 4.
 Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command
 and extension name keywords) are not case sensitive, with the sole
 exception in this specification of a mailbox local-part (SMTP
 Extensions may explicitly specify case-sensitive elements).  That is,
 a command verb, an argument value other than a mailbox local-part,
 and free form text MAY be encoded in upper case, lower case, or any
 mixture of upper and lower case with no impact on its meaning.  The
 local-part of a mailbox MUST BE treated as case sensitive.
 Therefore, SMTP implementations MUST take care to preserve the case
 of mailbox local-parts.  In particular, for some hosts, the user
 "smith" is different from the user "Smith".  However, exploiting the
 case sensitivity of mailbox local-parts impedes interoperability and
 is discouraged.  Mailbox domains follow normal DNS rules and are
 hence not case sensitive.
 A few SMTP servers, in violation of this specification (and RFC 821)
 require that command verbs be encoded by clients in upper case.
 Implementations MAY wish to employ this encoding to accommodate those
 servers.
 The argument clause consists of a variable-length character string
 ending with the end of the line, i.e., with the character sequence
 <CRLF>.  The receiver will take no action until this sequence is
 received.
 The syntax for each command is shown with the discussion of that
 command.  Common elements and parameters are shown in Section 4.1.2.
 Commands and replies are composed of characters from the ASCII
 character set [6].  When the transport service provides an 8-bit byte
 (octet) transmission channel, each 7-bit character is transmitted,
 right justified, in an octet with the high-order bit cleared to zero.
 More specifically, the unextended SMTP service provides 7-bit

Klensin Standards Track [Page 16] RFC 5321 SMTP October 2008

 transport only.  An originating SMTP client that has not successfully
 negotiated an appropriate extension with a particular server (see the
 next paragraph) MUST NOT transmit messages with information in the
 high-order bit of octets.  If such messages are transmitted in
 violation of this rule, receiving SMTP servers MAY clear the high-
 order bit or reject the message as invalid.  In general, a relay SMTP
 SHOULD assume that the message content it has received is valid and,
 assuming that the envelope permits doing so, relay it without
 inspecting that content.  Of course, if the content is mislabeled and
 the data path cannot accept the actual content, this may result in
 the ultimate delivery of a severely garbled message to the recipient.
 Delivery SMTP systems MAY reject such messages, or return them as
 undeliverable, rather than deliver them.  In the absence of a server-
 offered extension explicitly permitting it, a sending SMTP system is
 not permitted to send envelope commands in any character set other
 than US-ASCII.  Receiving systems SHOULD reject such commands,
 normally using "500 syntax error - invalid character" replies.
 8-bit message content transmission MAY be requested of the server by
 a client using extended SMTP facilities, notably the "8BITMIME"
 extension, RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP
 servers.  However, it MUST NOT be construed as authorization to
 transmit unrestricted 8-bit material, nor does 8BITMIME authorize
 transmission of any envelope material in other than ASCII. 8BITMIME
 MUST NOT be requested by senders for material with the high bit on
 that is not in MIME format with an appropriate content-transfer
 encoding; servers MAY reject such messages.
 The metalinguistic notation used in this document corresponds to the
 "Augmented BNF" used in other Internet mail system documents.  The
 reader who is not familiar with that syntax should consult the ABNF
 specification in RFC 5234 [7].  Metalanguage terms used in running
 text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
 The reader is cautioned that the grammar expressed in the
 metalanguage is not comprehensive.  There are many instances in which
 provisions in the text constrain or otherwise modify the syntax or
 semantics implied by the grammar.

3. The SMTP Procedures: An Overview

 This section contains descriptions of the procedures used in SMTP:
 session initiation, mail transaction, forwarding mail, verifying
 mailbox names and expanding mailing lists, and opening and closing
 exchanges.  Comments on relaying, a note on mail domains, and a
 discussion of changing roles are included at the end of this section.
 Several complete scenarios are presented in Appendix D.

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3.1. Session Initiation

 An SMTP session is initiated when a client opens a connection to a
 server and the server responds with an opening message.
 SMTP server implementations MAY include identification of their
 software and version information in the connection greeting reply
 after the 220 code, a practice that permits more efficient isolation
 and repair of any problems.  Implementations MAY make provision for
 SMTP servers to disable the software and version announcement where
 it causes security concerns.  While some systems also identify their
 contact point for mail problems, this is not a substitute for
 maintaining the required "postmaster" address (see Section 4).
 The SMTP protocol allows a server to formally reject a mail session
 while still allowing the initial connection as follows: a 554
 response MAY be given in the initial connection opening message
 instead of the 220.  A server taking this approach MUST still wait
 for the client to send a QUIT (see Section 4.1.1.10) before closing
 the connection and SHOULD respond to any intervening commands with
 "503 bad sequence of commands".  Since an attempt to make an SMTP
 connection to such a system is probably in error, a server returning
 a 554 response on connection opening SHOULD provide enough
 information in the reply text to facilitate debugging of the sending
 system.

3.2. Client Initiation

 Once the server has sent the greeting (welcoming) message and the
 client has received it, the client normally sends the EHLO command to
 the server, indicating the client's identity.  In addition to opening
 the session, use of EHLO indicates that the client is able to process
 service extensions and requests that the server provide a list of the
 extensions it supports.  Older SMTP systems that are unable to
 support service extensions, and contemporary clients that do not
 require service extensions in the mail session being initiated, MAY
 use HELO instead of EHLO.  Servers MUST NOT return the extended EHLO-
 style response to a HELO command.  For a particular connection
 attempt, if the server returns a "command not recognized" response to
 EHLO, the client SHOULD be able to fall back and send HELO.
 In the EHLO command, the host sending the command identifies itself;
 the command may be interpreted as saying "Hello, I am <domain>" (and,
 in the case of EHLO, "and I support service extension requests").

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3.3. Mail Transactions

 There are three steps to SMTP mail transactions.  The transaction
 starts with a MAIL command that gives the sender identification.  (In
 general, the MAIL command may be sent only when no mail transaction
 is in progress; see Section 4.1.4.)  A series of one or more RCPT
 commands follows, giving the receiver information.  Then, a DATA
 command initiates transfer of the mail data and is terminated by the
 "end of mail" data indicator, which also confirms the transaction.
 The first step in the procedure is the MAIL command.
    MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>
 This command tells the SMTP-receiver that a new mail transaction is
 starting and to reset all its state tables and buffers, including any
 recipients or mail data.  The <reverse-path> portion of the first or
 only argument contains the source mailbox (between "<" and ">"
 brackets), which can be used to report errors (see Section 4.2 for a
 discussion of error reporting).  If accepted, the SMTP server returns
 a "250 OK" reply.  If the mailbox specification is not acceptable for
 some reason, the server MUST return a reply indicating whether the
 failure is permanent (i.e., will occur again if the client tries to
 send the same address again) or temporary (i.e., the address might be
 accepted if the client tries again later).  Despite the apparent
 scope of this requirement, there are circumstances in which the
 acceptability of the reverse-path may not be determined until one or
 more forward-paths (in RCPT commands) can be examined.  In those
 cases, the server MAY reasonably accept the reverse-path (with a 250
 reply) and then report problems after the forward-paths are received
 and examined.  Normally, failures produce 550 or 553 replies.
 Historically, the <reverse-path> was permitted to contain more than
 just a mailbox; however, contemporary systems SHOULD NOT use source
 routing (see Appendix C).
 The optional <mail-parameters> are associated with negotiated SMTP
 service extensions (see Section 2.2).
 The second step in the procedure is the RCPT command.  This step of
 the procedure can be repeated any number of times.
    RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
 The first or only argument to this command includes a forward-path
 (normally a mailbox and domain, always surrounded by "<" and ">"
 brackets) identifying one recipient.  If accepted, the SMTP server
 returns a "250 OK" reply and stores the forward-path.  If the

Klensin Standards Track [Page 19] RFC 5321 SMTP October 2008

 recipient is known not to be a deliverable address, the SMTP server
 returns a 550 reply, typically with a string such as "no such user -
 " and the mailbox name (other circumstances and reply codes are
 possible).
 The <forward-path> can contain more than just a mailbox.
 Historically, the <forward-path> was permitted to contain a source
 routing list of hosts and the destination mailbox; however,
 contemporary SMTP clients SHOULD NOT utilize source routes (see
 Appendix C).  Servers MUST be prepared to encounter a list of source
 routes in the forward-path, but they SHOULD ignore the routes or MAY
 decline to support the relaying they imply.  Similarly, servers MAY
 decline to accept mail that is destined for other hosts or systems.
 These restrictions make a server useless as a relay for clients that
 do not support full SMTP functionality.  Consequently, restricted-
 capability clients MUST NOT assume that any SMTP server on the
 Internet can be used as their mail processing (relaying) site.  If a
 RCPT command appears without a previous MAIL command, the server MUST
 return a 503 "Bad sequence of commands" response.  The optional
 <rcpt-parameters> are associated with negotiated SMTP service
 extensions (see Section 2.2).
 Since it has been a common source of errors, it is worth noting that
 spaces are not permitted on either side of the colon following FROM
 in the MAIL command or TO in the RCPT command.  The syntax is exactly
 as given above.
 The third step in the procedure is the DATA command (or some
 alternative specified in a service extension).
    DATA <CRLF>
 If accepted, the SMTP server returns a 354 Intermediate reply and
 considers all succeeding lines up to but not including the end of
 mail data indicator to be the message text.  When the end of text is
 successfully received and stored, the SMTP-receiver sends a "250 OK"
 reply.
 Since the mail data is sent on the transmission channel, the end of
 mail data must be indicated so that the command and reply dialog can
 be resumed.  SMTP indicates the end of the mail data by sending a
 line containing only a "." (period or full stop).  A transparency
 procedure is used to prevent this from interfering with the user's
 text (see Section 4.5.2).
 The end of mail data indicator also confirms the mail transaction and
 tells the SMTP server to now process the stored recipients and mail

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 data.  If accepted, the SMTP server returns a "250 OK" reply.  The
 DATA command can fail at only two points in the protocol exchange:
 If there was no MAIL, or no RCPT, command, or all such commands were
 rejected, the server MAY return a "command out of sequence" (503) or
 "no valid recipients" (554) reply in response to the DATA command.
 If one of those replies (or any other 5yz reply) is received, the
 client MUST NOT send the message data; more generally, message data
 MUST NOT be sent unless a 354 reply is received.
 If the verb is initially accepted and the 354 reply issued, the DATA
 command should fail only if the mail transaction was incomplete (for
 example, no recipients), if resources were unavailable (including, of
 course, the server unexpectedly becoming unavailable), or if the
 server determines that the message should be rejected for policy or
 other reasons.
 However, in practice, some servers do not perform recipient
 verification until after the message text is received.  These servers
 SHOULD treat a failure for one or more recipients as a "subsequent
 failure" and return a mail message as discussed in Section 6 and, in
 particular, in Section 6.1.  Using a "550 mailbox not found" (or
 equivalent) reply code after the data are accepted makes it difficult
 or impossible for the client to determine which recipients failed.
 When the RFC 822 format ([28], [4]) is being used, the mail data
 include the header fields such as those named Date, Subject, To, Cc,
 and From.  Server SMTP systems SHOULD NOT reject messages based on
 perceived defects in the RFC 822 or MIME (RFC 2045 [21]) message
 header section or message body.  In particular, they MUST NOT reject
 messages in which the numbers of Resent-header fields do not match or
 Resent-to appears without Resent-from and/or Resent-date.
 Mail transaction commands MUST be used in the order discussed above.

3.4. Forwarding for Address Correction or Updating

 Forwarding support is most often required to consolidate and simplify
 addresses within, or relative to, some enterprise and less frequently
 to establish addresses to link a person's prior address with a
 current one.  Silent forwarding of messages (without server
 notification to the sender), for security or non-disclosure purposes,
 is common in the contemporary Internet.
 In both the enterprise and the "new address" cases, information
 hiding (and sometimes security) considerations argue against exposure
 of the "final" address through the SMTP protocol as a side effect of
 the forwarding activity.  This may be especially important when the

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 final address may not even be reachable by the sender.  Consequently,
 the "forwarding" mechanisms described in Section 3.2 of RFC 821, and
 especially the 251 (corrected destination) and 551 reply codes from
 RCPT must be evaluated carefully by implementers and, when they are
 available, by those configuring systems (see also Section 7.4).
 In particular:
 o  Servers MAY forward messages when they are aware of an address
    change.  When they do so, they MAY either provide address-updating
    information with a 251 code, or may forward "silently" and return
    a 250 code.  However, if a 251 code is used, they MUST NOT assume
    that the client will actually update address information or even
    return that information to the user.
 Alternately,
 o  Servers MAY reject messages or return them as non-deliverable when
    they cannot be delivered precisely as addressed.  When they do so,
    they MAY either provide address-updating information with a 551
    code, or may reject the message as undeliverable with a 550 code
    and no address-specific information.  However, if a 551 code is
    used, they MUST NOT assume that the client will actually update
    address information or even return that information to the user.
 SMTP server implementations that support the 251 and/or 551 reply
 codes SHOULD provide configuration mechanisms so that sites that
 conclude that they would undesirably disclose information can disable
 or restrict their use.

3.5. Commands for Debugging Addresses

3.5.1. Overview

 SMTP provides commands to verify a user name or obtain the content of
 a mailing list.  This is done with the VRFY and EXPN commands, which
 have character string arguments.  Implementations SHOULD support VRFY
 and EXPN (however, see Section 3.5.2 and Section 7.3).
 For the VRFY command, the string is a user name or a user name and
 domain (see below).  If a normal (i.e., 250) response is returned,
 the response MAY include the full name of the user and MUST include
 the mailbox of the user.  It MUST be in either of the following
 forms:
    User Name <local-part@domain>
    local-part@domain

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 When a name that is the argument to VRFY could identify more than one
 mailbox, the server MAY either note the ambiguity or identify the
 alternatives.  In other words, any of the following are legitimate
 responses to VRFY:
    553 User ambiguous
 or
    553- Ambiguous; Possibilities are
    553-Joe Smith <jsmith@foo.com>
    553-Harry Smith <hsmith@foo.com>
    553 Melvin Smith <dweep@foo.com>
 or
    553-Ambiguous; Possibilities
    553- <jsmith@foo.com>
    553- <hsmith@foo.com>
    553 <dweep@foo.com>
 Under normal circumstances, a client receiving a 553 reply would be
 expected to expose the result to the user.  Use of exactly the forms
 given, and the "user ambiguous" or "ambiguous" keywords, possibly
 supplemented by extended reply codes, such as those described in RFC
 3463 [25], will facilitate automated translation into other languages
 as needed.  Of course, a client that was highly automated or that was
 operating in another language than English might choose to try to
 translate the response to return some other indication to the user
 than the literal text of the reply, or to take some automated action
 such as consulting a directory service for additional information
 before reporting to the user.
 For the EXPN command, the string identifies a mailing list, and the
 successful (i.e., 250) multiline response MAY include the full name
 of the users and MUST give the mailboxes on the mailing list.
 In some hosts, the distinction between a mailing list and an alias
 for a single mailbox is a bit fuzzy, since a common data structure
 may hold both types of entries, and it is possible to have mailing
 lists containing only one mailbox.  If a request is made to apply
 VRFY to a mailing list, a positive response MAY be given if a message
 so addressed would be delivered to everyone on the list, otherwise an
 error SHOULD be reported (e.g., "550 That is a mailing list, not a
 user" or "252 Unable to verify members of mailing list").  If a
 request is made to expand a user name, the server MAY return a

Klensin Standards Track [Page 23] RFC 5321 SMTP October 2008

 positive response consisting of a list containing one name, or an
 error MAY be reported (e.g., "550 That is a user name, not a mailing
 list").
 In the case of a successful multiline reply (normal for EXPN),
 exactly one mailbox is to be specified on each line of the reply.
 The case of an ambiguous request is discussed above.
 "User name" is a fuzzy term and has been used deliberately.  An
 implementation of the VRFY or EXPN commands MUST include at least
 recognition of local mailboxes as "user names".  However, since
 current Internet practice often results in a single host handling
 mail for multiple domains, hosts, especially hosts that provide this
 functionality, SHOULD accept the "local-part@domain" form as a "user
 name"; hosts MAY also choose to recognize other strings as "user
 names".
 The case of expanding a mailbox list requires a multiline reply, such
 as:
    C: EXPN Example-People
    S: 250-Jon Postel <Postel@isi.edu>
    S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>
    S: 250 Sam Q. Smith <SQSmith@specific.generic.com>
 or
    C: EXPN Executive-Washroom-List
    S: 550 Access Denied to You.
 The character string arguments of the VRFY and EXPN commands cannot
 be further restricted due to the variety of implementations of the
 user name and mailbox list concepts.  On some systems, it may be
 appropriate for the argument of the EXPN command to be a file name
 for a file containing a mailing list, but again there are a variety
 of file naming conventions in the Internet.  Similarly, historical
 variations in what is returned by these commands are such that the
 response SHOULD be interpreted very carefully, if at all, and SHOULD
 generally only be used for diagnostic purposes.

3.5.2. VRFY Normal Response

 When normal (2yz or 551) responses are returned from a VRFY or EXPN
 request, the reply MUST include the <Mailbox> name using a
 "<local-part@domain>" construction, where "domain" is a fully-
 qualified domain name.  In circumstances exceptional enough to
 justify violating the intent of this specification, free-form text
 MAY be returned.  In order to facilitate parsing by both computers

Klensin Standards Track [Page 24] RFC 5321 SMTP October 2008

 and people, addresses SHOULD appear in pointed brackets.  When
 addresses, rather than free-form debugging information, are returned,
 EXPN and VRFY MUST return only valid domain addresses that are usable
 in SMTP RCPT commands.  Consequently, if an address implies delivery
 to a program or other system, the mailbox name used to reach that
 target MUST be given.  Paths (explicit source routes) MUST NOT be
 returned by VRFY or EXPN.
 Server implementations SHOULD support both VRFY and EXPN.  For
 security reasons, implementations MAY provide local installations a
 way to disable either or both of these commands through configuration
 options or the equivalent (see Section 7.3).  When these commands are
 supported, they are not required to work across relays when relaying
 is supported.  Since they were both optional in RFC 821, but VRFY was
 made mandatory in RFC 1123 [3], if EXPN is supported, it MUST be
 listed as a service extension in an EHLO response.  VRFY MAY be
 listed as a convenience but, since support for it is required, SMTP
 clients are not required to check for its presence on the extension
 list before using it.

3.5.3. Meaning of VRFY or EXPN Success Response

 A server MUST NOT return a 250 code in response to a VRFY or EXPN
 command unless it has actually verified the address.  In particular,
 a server MUST NOT return 250 if all it has done is to verify that the
 syntax given is valid.  In that case, 502 (Command not implemented)
 or 500 (Syntax error, command unrecognized) SHOULD be returned.  As
 stated elsewhere, implementation (in the sense of actually validating
 addresses and returning information) of VRFY and EXPN are strongly
 recommended.  Hence, implementations that return 500 or 502 for VRFY
 are not in full compliance with this specification.
 There may be circumstances where an address appears to be valid but
 cannot reasonably be verified in real time, particularly when a
 server is acting as a mail exchanger for another server or domain.
 "Apparent validity", in this case, would normally involve at least
 syntax checking and might involve verification that any domains
 specified were ones to which the host expected to be able to relay
 mail.  In these situations, reply code 252 SHOULD be returned.  These
 cases parallel the discussion of RCPT verification in Section 2.1.
 Similarly, the discussion in Section 3.4 applies to the use of reply
 codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are
 recognized but that would be forwarded or rejected were mail received
 for them.  Implementations generally SHOULD be more aggressive about
 address verification in the case of VRFY than in the case of RCPT,
 even if it takes a little longer to do so.

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3.5.4. Semantics and Applications of EXPN

 EXPN is often very useful in debugging and understanding problems
 with mailing lists and multiple-target-address aliases.  Some systems
 have attempted to use source expansion of mailing lists as a means of
 eliminating duplicates.  The propagation of aliasing systems with
 mail on the Internet for hosts (typically with MX and CNAME DNS
 records), for mailboxes (various types of local host aliases), and in
 various proxying arrangements has made it nearly impossible for these
 strategies to work consistently, and mail systems SHOULD NOT attempt
 them.

3.6. Relaying and Mail Routing

3.6.1. Source Routes and Relaying

 In general, the availability of Mail eXchanger records in the domain
 name system (RFC 1035 [2], RFC 974 [12]) makes the use of explicit
 source routes in the Internet mail system unnecessary.  Many
 historical problems with the interpretation of explicit source routes
 have made their use undesirable.  SMTP clients SHOULD NOT generate
 explicit source routes except under unusual circumstances.  SMTP
 servers MAY decline to act as mail relays or to accept addresses that
 specify source routes.  When route information is encountered, SMTP
 servers MAY ignore the route information and simply send to the final
 destination specified as the last element in the route and SHOULD do
 so.  There has been an invalid practice of using names that do not
 appear in the DNS as destination names, with the senders counting on
 the intermediate hosts specified in source routing to resolve any
 problems.  If source routes are stripped, this practice will cause
 failures.  This is one of several reasons why SMTP clients MUST NOT
 generate invalid source routes or depend on serial resolution of
 names.
 When source routes are not used, the process described in RFC 821 for
 constructing a reverse-path from the forward-path is not applicable
 and the reverse-path at the time of delivery will simply be the
 address that appeared in the MAIL command.

3.6.2. Mail eXchange Records and Relaying

 A relay SMTP server is usually the target of a DNS MX record that
 designates it, rather than the final delivery system.  The relay
 server may accept or reject the task of relaying the mail in the same
 way it accepts or rejects mail for a local user.  If it accepts the
 task, it then becomes an SMTP client, establishes a transmission
 channel to the next SMTP server specified in the DNS (according to
 the rules in Section 5), and sends it the mail.  If it declines to

Klensin Standards Track [Page 26] RFC 5321 SMTP October 2008

 relay mail to a particular address for policy reasons, a 550 response
 SHOULD be returned.
 This specification does not deal with the verification of return
 paths for use in delivery notifications.  Recent work, such as that
 on SPF [29] and DKIM [30] [31], has been done to provide ways to
 ascertain that an address is valid or belongs to the person who
 actually sent the message.  A server MAY attempt to verify the return
 path before using its address for delivery notifications, but methods
 of doing so are not defined here nor is any particular method
 recommended at this time.

3.6.3. Message Submission Servers as Relays

 Many mail-sending clients exist, especially in conjunction with
 facilities that receive mail via POP3 or IMAP, that have limited
 capability to support some of the requirements of this specification,
 such as the ability to queue messages for subsequent delivery
 attempts.  For these clients, it is common practice to make private
 arrangements to send all messages to a single server for processing
 and subsequent distribution.  SMTP, as specified here, is not ideally
 suited for this role.  A standardized mail submission protocol has
 been developed that is gradually superseding practices based on SMTP
 (see RFC 4409 [18]).  In any event, because these arrangements are
 private and fall outside the scope of this specification, they are
 not described here.
 It is important to note that MX records can point to SMTP servers
 that act as gateways into other environments, not just SMTP relays
 and final delivery systems; see Sections 3.7 and 5.
 If an SMTP server has accepted the task of relaying the mail and
 later finds that the destination is incorrect or that the mail cannot
 be delivered for some other reason, then it MUST construct an
 "undeliverable mail" notification message and send it to the
 originator of the undeliverable mail (as indicated by the reverse-
 path).  Formats specified for non-delivery reports by other standards
 (see, for example, RFC 3461 [32] and RFC 3464 [33]) SHOULD be used if
 possible.
 This notification message must be from the SMTP server at the relay
 host or the host that first determines that delivery cannot be
 accomplished.  Of course, SMTP servers MUST NOT send notification
 messages about problems transporting notification messages.  One way
 to prevent loops in error reporting is to specify a null reverse-path
 in the MAIL command of a notification message.  When such a message
 is transmitted, the reverse-path MUST be set to null (see

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 Section 4.5.5 for additional discussion).  A MAIL command with a null
 reverse-path appears as follows:
    MAIL FROM:<>
 As discussed in Section 6.4, a relay SMTP has no need to inspect or
 act upon the header section or body of the message data and MUST NOT
 do so except to add its own "Received:" header field (Section 4.4)
 and, optionally, to attempt to detect looping in the mail system (see
 Section 6.3).  Of course, this prohibition also applies to any
 modifications of these header fields or text (see also Section 7.9).

3.7. Mail Gatewaying

 While the relay function discussed above operates within the Internet
 SMTP transport service environment, MX records or various forms of
 explicit routing may require that an intermediate SMTP server perform
 a translation function between one transport service and another.  As
 discussed in Section 2.3.10, when such a system is at the boundary
 between two transport service environments, we refer to it as a
 "gateway" or "gateway SMTP".
 Gatewaying mail between different mail environments, such as
 different mail formats and protocols, is complex and does not easily
 yield to standardization.  However, some general requirements may be
 given for a gateway between the Internet and another mail
 environment.

3.7.1. Header Fields in Gatewaying

 Header fields MAY be rewritten when necessary as messages are
 gatewayed across mail environment boundaries.  This may involve
 inspecting the message body or interpreting the local-part of the
 destination address in spite of the prohibitions in Section 6.4.
 Other mail systems gatewayed to the Internet often use a subset of
 the RFC 822 header section or provide similar functionality with a
 different syntax, but some of these mail systems do not have an
 equivalent to the SMTP envelope.  Therefore, when a message leaves
 the Internet environment, it may be necessary to fold the SMTP
 envelope information into the message header section.  A possible
 solution would be to create new header fields to carry the envelope
 information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this
 would require changes in mail programs in foreign environments and
 might risk disclosure of private information (see Section 7.2).

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3.7.2. Received Lines in Gatewaying

 When forwarding a message into or out of the Internet environment, a
 gateway MUST prepend a Received: line, but it MUST NOT alter in any
 way a Received: line that is already in the header section.
 "Received:" header fields of messages originating from other
 environments may not conform exactly to this specification.  However,
 the most important use of Received: lines is for debugging mail
 faults, and this debugging can be severely hampered by well-meaning
 gateways that try to "fix" a Received: line.  As another consequence
 of trace header fields arising in non-SMTP environments, receiving
 systems MUST NOT reject mail based on the format of a trace header
 field and SHOULD be extremely robust in the light of unexpected
 information or formats in those header fields.
 The gateway SHOULD indicate the environment and protocol in the "via"
 clauses of Received header field(s) that it supplies.

3.7.3. Addresses in Gatewaying

 From the Internet side, the gateway SHOULD accept all valid address
 formats in SMTP commands and in the RFC 822 header section, and all
 valid RFC 822 messages.  Addresses and header fields generated by
 gateways MUST conform to applicable standards (including this one and
 RFC 5322 [4]).  Gateways are, of course, subject to the same rules
 for handling source routes as those described for other SMTP systems
 in Section 3.3.

3.7.4. Other Header Fields in Gatewaying

 The gateway MUST ensure that all header fields of a message that it
 forwards into the Internet mail environment meet the requirements for
 Internet mail.  In particular, all addresses in "From:", "To:",
 "Cc:", etc., header fields MUST be transformed (if necessary) to
 satisfy the standard header syntax of RFC 5322 [4], MUST reference
 only fully-qualified domain names, and MUST be effective and useful
 for sending replies.  The translation algorithm used to convert mail
 from the Internet protocols to another environment's protocol SHOULD
 ensure that error messages from the foreign mail environment are
 delivered to the reverse-path from the SMTP envelope, not to an
 address in the "From:", "Sender:", or similar header fields of the
 message.

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3.7.5. Envelopes in Gatewaying

 Similarly, when forwarding a message from another environment into
 the Internet, the gateway SHOULD set the envelope return path in
 accordance with an error message return address, if supplied by the
 foreign environment.  If the foreign environment has no equivalent
 concept, the gateway must select and use a best approximation, with
 the message originator's address as the default of last resort.

3.8. Terminating Sessions and Connections

 An SMTP connection is terminated when the client sends a QUIT
 command.  The server responds with a positive reply code, after which
 it closes the connection.
 An SMTP server MUST NOT intentionally close the connection under
 normal operational circumstances (see Section 7.8) except:
 o  After receiving a QUIT command and responding with a 221 reply.
 o  After detecting the need to shut down the SMTP service and
    returning a 421 response code.  This response code can be issued
    after the server receives any command or, if necessary,
    asynchronously from command receipt (on the assumption that the
    client will receive it after the next command is issued).
 o  After a timeout, as specified in Section 4.5.3.2, occurs waiting
    for the client to send a command or data.
 In particular, a server that closes connections in response to
 commands that are not understood is in violation of this
 specification.  Servers are expected to be tolerant of unknown
 commands, issuing a 500 reply and awaiting further instructions from
 the client.
 An SMTP server that is forcibly shut down via external means SHOULD
 attempt to send a line containing a 421 response code to the SMTP
 client before exiting.  The SMTP client will normally read the 421
 response code after sending its next command.
 SMTP clients that experience a connection close, reset, or other
 communications failure due to circumstances not under their control
 (in violation of the intent of this specification but sometimes
 unavoidable) SHOULD, to maintain the robustness of the mail system,
 treat the mail transaction as if a 451 response had been received and
 act accordingly.

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3.9. Mailing Lists and Aliases

 An SMTP-capable host SHOULD support both the alias and the list
 models of address expansion for multiple delivery.  When a message is
 delivered or forwarded to each address of an expanded list form, the
 return address in the envelope ("MAIL FROM:") MUST be changed to be
 the address of a person or other entity who administers the list.
 However, in this case, the message header section (RFC 5322 [4]) MUST
 be left unchanged; in particular, the "From" field of the header
 section is unaffected.
 An important mail facility is a mechanism for multi-destination
 delivery of a single message, by transforming (or "expanding" or
 "exploding") a pseudo-mailbox address into a list of destination
 mailbox addresses.  When a message is sent to such a pseudo-mailbox
 (sometimes called an "exploder"), copies are forwarded or
 redistributed to each mailbox in the expanded list.  Servers SHOULD
 simply utilize the addresses on the list; application of heuristics
 or other matching rules to eliminate some addresses, such as that of
 the originator, is strongly discouraged.  We classify such a pseudo-
 mailbox as an "alias" or a "list", depending upon the expansion
 rules.

3.9.1. Alias

 To expand an alias, the recipient mailer simply replaces the pseudo-
 mailbox address in the envelope with each of the expanded addresses
 in turn; the rest of the envelope and the message body are left
 unchanged.  The message is then delivered or forwarded to each
 expanded address.

3.9.2. List

 A mailing list may be said to operate by "redistribution" rather than
 by "forwarding".  To expand a list, the recipient mailer replaces the
 pseudo-mailbox address in the envelope with each of the expanded
 addresses in turn.  The return (backward-pointing) address in the
 envelope is changed so that all error messages generated by the final
 deliveries will be returned to a list administrator, not to the
 message originator, who generally has no control over the contents of
 the list and will typically find error messages annoying.  Note that
 the key difference between handling aliases (Section 3.9.1) and
 forwarding (this subsection) is the change to the backward-pointing
 address in this case.  When a list constrains its processing to the
 very limited set of modifications and actions described here, it is
 attempting to emulate an MTA; such lists can be treated as a
 continuation in email transit.

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 There exist mailing lists that perform additional, sometimes
 extensive, modifications to a message and its envelope.  Such mailing
 lists need to be viewed as full MUAs, which accept a delivery and
 post a new message.

4. The SMTP Specifications

4.1. SMTP Commands

4.1.1. Command Semantics and Syntax

 The SMTP commands define the mail transfer or the mail system
 function requested by the user.  SMTP commands are character strings
 terminated by <CRLF>.  The commands themselves are alphabetic
 characters terminated by <SP> if parameters follow and <CRLF>
 otherwise.  (In the interest of improved interoperability, SMTP
 receivers SHOULD tolerate trailing white space before the terminating
 <CRLF>.)  The syntax of the local part of a mailbox MUST conform to
 receiver site conventions and the syntax specified in Section 4.1.2.
 The SMTP commands are discussed below.  The SMTP replies are
 discussed in Section 4.2.
 A mail transaction involves several data objects that are
 communicated as arguments to different commands.  The reverse-path is
 the argument of the MAIL command, the forward-path is the argument of
 the RCPT command, and the mail data is the argument of the DATA
 command.  These arguments or data objects must be transmitted and
 held, pending the confirmation communicated by the end of mail data
 indication that finalizes the transaction.  The model for this is
 that distinct buffers are provided to hold the types of data objects;
 that is, there is a reverse-path buffer, a forward-path buffer, and a
 mail data buffer.  Specific commands cause information to be appended
 to a specific buffer, or cause one or more buffers to be cleared.
 Several commands (RSET, DATA, QUIT) are specified as not permitting
 parameters.  In the absence of specific extensions offered by the
 server and accepted by the client, clients MUST NOT send such
 parameters and servers SHOULD reject commands containing them as
 having invalid syntax.

4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO)

 These commands are used to identify the SMTP client to the SMTP
 server.  The argument clause contains the fully-qualified domain name
 of the SMTP client, if one is available.  In situations in which the
 SMTP client system does not have a meaningful domain name (e.g., when
 its address is dynamically allocated and no reverse mapping record is

Klensin Standards Track [Page 32] RFC 5321 SMTP October 2008

 available), the client SHOULD send an address literal (see
 Section 4.1.3).
 RFC 2821, and some earlier informal practices, encouraged following
 the literal by information that would help to identify the client
 system.  That convention was not widely supported, and many SMTP
 servers considered it an error.  In the interest of interoperability,
 it is probably wise for servers to be prepared for this string to
 occur, but SMTP clients SHOULD NOT send it.
 The SMTP server identifies itself to the SMTP client in the
 connection greeting reply and in the response to this command.
 A client SMTP SHOULD start an SMTP session by issuing the EHLO
 command.  If the SMTP server supports the SMTP service extensions, it
 will give a successful response, a failure response, or an error
 response.  If the SMTP server, in violation of this specification,
 does not support any SMTP service extensions, it will generate an
 error response.  Older client SMTP systems MAY, as discussed above,
 use HELO (as specified in RFC 821) instead of EHLO, and servers MUST
 support the HELO command and reply properly to it.  In any event, a
 client MUST issue HELO or EHLO before starting a mail transaction.
 These commands, and a "250 OK" reply to one of them, confirm that
 both the SMTP client and the SMTP server are in the initial state,
 that is, there is no transaction in progress and all state tables and
 buffers are cleared.
 Syntax:
 ehlo           = "EHLO" SP ( Domain / address-literal ) CRLF
 helo           = "HELO" SP Domain CRLF
 Normally, the response to EHLO will be a multiline reply.  Each line
 of the response contains a keyword and, optionally, one or more
 parameters.  Following the normal syntax for multiline replies, these
 keywords follow the code (250) and a hyphen for all but the last
 line, and the code and a space for the last line.  The syntax for a
 positive response, using the ABNF notation and terminal symbols of
 RFC 5234 [7], is:
 ehlo-ok-rsp    = ( "250" SP Domain [ SP ehlo-greet ] CRLF )
                  / ( "250-" Domain [ SP ehlo-greet ] CRLF
                  *( "250-" ehlo-line CRLF )
                  "250" SP ehlo-line CRLF )

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 ehlo-greet     = 1*(%d0-9 / %d11-12 / %d14-127)
                  ; string of any characters other than CR or LF
 ehlo-line      = ehlo-keyword *( SP ehlo-param )
 ehlo-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                  ; additional syntax of ehlo-params depends on
                  ; ehlo-keyword
 ehlo-param     = 1*(%d33-126)
                  ; any CHAR excluding <SP> and all
                  ; control characters (US-ASCII 0-31 and 127
                  ; inclusive)
 Although EHLO keywords may be specified in upper, lower, or mixed
 case, they MUST always be recognized and processed in a case-
 insensitive manner.  This is simply an extension of practices
 specified in RFC 821 and Section 2.4.
 The EHLO response MUST contain keywords (and associated parameters if
 required) for all commands not listed as "required" in Section 4.5.1
 excepting only private-use commands as described in Section 4.1.5.
 Private-use commands MAY be listed.

4.1.1.2. MAIL (MAIL)

 This command is used to initiate a mail transaction in which the mail
 data is delivered to an SMTP server that may, in turn, deliver it to
 one or more mailboxes or pass it on to another system (possibly using
 SMTP).  The argument clause contains a reverse-path and may contain
 optional parameters.  In general, the MAIL command may be sent only
 when no mail transaction is in progress, see Section 4.1.4.
 The reverse-path consists of the sender mailbox.  Historically, that
 mailbox might optionally have been preceded by a list of hosts, but
 that behavior is now deprecated (see Appendix C).  In some types of
 reporting messages for which a reply is likely to cause a mail loop
 (for example, mail delivery and non-delivery notifications), the
 reverse-path may be null (see Section 3.6).
 This command clears the reverse-path buffer, the forward-path buffer,
 and the mail data buffer, and it inserts the reverse-path information
 from its argument clause into the reverse-path buffer.
 If service extensions were negotiated, the MAIL command may also
 carry parameters associated with a particular service extension.

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 Syntax:
 mail = "MAIL FROM:" Reverse-path
                                     [SP Mail-parameters] CRLF

4.1.1.3. RECIPIENT (RCPT)

 This command is used to identify an individual recipient of the mail
 data; multiple recipients are specified by multiple uses of this
 command.  The argument clause contains a forward-path and may contain
 optional parameters.
 The forward-path normally consists of the required destination
 mailbox.  Sending systems SHOULD NOT generate the optional list of
 hosts known as a source route.  Receiving systems MUST recognize
 source route syntax but SHOULD strip off the source route
 specification and utilize the domain name associated with the mailbox
 as if the source route had not been provided.
 Similarly, relay hosts SHOULD strip or ignore source routes, and
 names MUST NOT be copied into the reverse-path.  When mail reaches
 its ultimate destination (the forward-path contains only a
 destination mailbox), the SMTP server inserts it into the destination
 mailbox in accordance with its host mail conventions.
 This command appends its forward-path argument to the forward-path
 buffer; it does not change the reverse-path buffer nor the mail data
 buffer.
 For example, mail received at relay host xyz.com with envelope
 commands
    MAIL FROM:<userx@y.foo.org>
    RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
 will normally be sent directly on to host d.bar.org with envelope
 commands
    MAIL FROM:<userx@y.foo.org>
    RCPT TO:<userc@d.bar.org>
 As provided in Appendix C, xyz.com MAY also choose to relay the
 message to hosta.int, using the envelope commands
    MAIL FROM:<userx@y.foo.org>
    RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>

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 or to jkl.org, using the envelope commands
    MAIL FROM:<userx@y.foo.org>
    RCPT TO:<@jkl.org:userc@d.bar.org>
 Attempting to use relaying this way is now strongly discouraged.
 Since hosts are not required to relay mail at all, xyz.com MAY also
 reject the message entirely when the RCPT command is received, using
 a 550 code (since this is a "policy reason").
 If service extensions were negotiated, the RCPT command may also
 carry parameters associated with a particular service extension
 offered by the server.  The client MUST NOT transmit parameters other
 than those associated with a service extension offered by the server
 in its EHLO response.
 Syntax:
    rcpt = "RCPT TO:" ( "<Postmaster@" Domain ">" / "<Postmaster>" /
                Forward-path ) [SP Rcpt-parameters] CRLF
                Note that, in a departure from the usual rules for
                local-parts, the "Postmaster" string shown above is
                treated as case-insensitive.

4.1.1.4. DATA (DATA)

 The receiver normally sends a 354 response to DATA, and then treats
 the lines (strings ending in <CRLF> sequences, as described in
 Section 2.3.7) following the command as mail data from the sender.
 This command causes the mail data to be appended to the mail data
 buffer.  The mail data may contain any of the 128 ASCII character
 codes, although experience has indicated that use of control
 characters other than SP, HT, CR, and LF may cause problems and
 SHOULD be avoided when possible.
 The mail data are terminated by a line containing only a period, that
 is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is
 actually the terminator of the previous line (see Section 4.5.2).
 This is the end of mail data indication.  The first <CRLF> of this
 terminating sequence is also the <CRLF> that ends the final line of
 the data (message text) or, if there was no mail data, ends the DATA
 command itself (the "no mail data" case does not conform to this
 specification since it would require that neither the trace header
 fields required by this specification nor the message header section
 required by RFC 5322 [4] be transmitted).  An extra <CRLF> MUST NOT
 be added, as that would cause an empty line to be added to the
 message.  The only exception to this rule would arise if the message

Klensin Standards Track [Page 36] RFC 5321 SMTP October 2008

 body were passed to the originating SMTP-sender with a final "line"
 that did not end in <CRLF>; in that case, the originating SMTP system
 MUST either reject the message as invalid or add <CRLF> in order to
 have the receiving SMTP server recognize the "end of data" condition.
 The custom of accepting lines ending only in <LF>, as a concession to
 non-conforming behavior on the part of some UNIX systems, has proven
 to cause more interoperability problems than it solves, and SMTP
 server systems MUST NOT do this, even in the name of improved
 robustness.  In particular, the sequence "<LF>.<LF>" (bare line
 feeds, without carriage returns) MUST NOT be treated as equivalent to
 <CRLF>.<CRLF> as the end of mail data indication.
 Receipt of the end of mail data indication requires the server to
 process the stored mail transaction information.  This processing
 consumes the information in the reverse-path buffer, the forward-path
 buffer, and the mail data buffer, and on the completion of this
 command these buffers are cleared.  If the processing is successful,
 the receiver MUST send an OK reply.  If the processing fails, the
 receiver MUST send a failure reply.  The SMTP model does not allow
 for partial failures at this point: either the message is accepted by
 the server for delivery and a positive response is returned or it is
 not accepted and a failure reply is returned.  In sending a positive
 "250 OK" completion reply to the end of data indication, the receiver
 takes full responsibility for the message (see Section 6.1).  Errors
 that are diagnosed subsequently MUST be reported in a mail message,
 as discussed in Section 4.4.
 When the SMTP server accepts a message either for relaying or for
 final delivery, it inserts a trace record (also referred to
 interchangeably as a "time stamp line" or "Received" line) at the top
 of the mail data.  This trace record indicates the identity of the
 host that sent the message, the identity of the host that received
 the message (and is inserting this time stamp), and the date and time
 the message was received.  Relayed messages will have multiple time
 stamp lines.  Details for formation of these lines, including their
 syntax, is specified in Section 4.4.
 Additional discussion about the operation of the DATA command appears
 in Section 3.3.
 Syntax:
    data = "DATA" CRLF

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4.1.1.5. RESET (RSET)

 This command specifies that the current mail transaction will be
 aborted.  Any stored sender, recipients, and mail data MUST be
 discarded, and all buffers and state tables cleared.  The receiver
 MUST send a "250 OK" reply to a RSET command with no arguments.  A
 reset command may be issued by the client at any time.  It is
 effectively equivalent to a NOOP (i.e., it has no effect) if issued
 immediately after EHLO, before EHLO is issued in the session, after
 an end of data indicator has been sent and acknowledged, or
 immediately before a QUIT.  An SMTP server MUST NOT close the
 connection as the result of receiving a RSET; that action is reserved
 for QUIT (see Section 4.1.1.10).
 Since EHLO implies some additional processing and response by the
 server, RSET will normally be more efficient than reissuing that
 command, even though the formal semantics are the same.
 There are circumstances, contrary to the intent of this
 specification, in which an SMTP server may receive an indication that
 the underlying TCP connection has been closed or reset.  To preserve
 the robustness of the mail system, SMTP servers SHOULD be prepared
 for this condition and SHOULD treat it as if a QUIT had been received
 before the connection disappeared.
 Syntax:
    rset = "RSET" CRLF

4.1.1.6. VERIFY (VRFY)

 This command asks the receiver to confirm that the argument
 identifies a user or mailbox.  If it is a user name, information is
 returned as specified in Section 3.5.
 This command has no effect on the reverse-path buffer, the forward-
 path buffer, or the mail data buffer.
 Syntax:
    vrfy = "VRFY" SP String CRLF

Klensin Standards Track [Page 38] RFC 5321 SMTP October 2008

4.1.1.7. EXPAND (EXPN)

 This command asks the receiver to confirm that the argument
 identifies a mailing list, and if so, to return the membership of
 that list.  If the command is successful, a reply is returned
 containing information as described in Section 3.5.  This reply will
 have multiple lines except in the trivial case of a one-member list.
 This command has no effect on the reverse-path buffer, the forward-
 path buffer, or the mail data buffer, and it may be issued at any
 time.
 Syntax:
    expn = "EXPN" SP String CRLF

4.1.1.8. HELP (HELP)

 This command causes the server to send helpful information to the
 client.  The command MAY take an argument (e.g., any command name)
 and return more specific information as a response.
 This command has no effect on the reverse-path buffer, the forward-
 path buffer, or the mail data buffer, and it may be issued at any
 time.
 SMTP servers SHOULD support HELP without arguments and MAY support it
 with arguments.
 Syntax:
    help = "HELP" [ SP String ] CRLF

Klensin Standards Track [Page 39] RFC 5321 SMTP October 2008

4.1.1.9. NOOP (NOOP)

 This command does not affect any parameters or previously entered
 commands.  It specifies no action other than that the receiver send a
 "250 OK" reply.
 This command has no effect on the reverse-path buffer, the forward-
 path buffer, or the mail data buffer, and it may be issued at any
 time.  If a parameter string is specified, servers SHOULD ignore it.
 Syntax:
    noop = "NOOP" [ SP String ] CRLF

4.1.1.10. QUIT (QUIT)

 This command specifies that the receiver MUST send a "221 OK" reply,
 and then close the transmission channel.
 The receiver MUST NOT intentionally close the transmission channel
 until it receives and replies to a QUIT command (even if there was an
 error).  The sender MUST NOT intentionally close the transmission
 channel until it sends a QUIT command, and it SHOULD wait until it
 receives the reply (even if there was an error response to a previous
 command).  If the connection is closed prematurely due to violations
 of the above or system or network failure, the server MUST cancel any
 pending transaction, but not undo any previously completed
 transaction, and generally MUST act as if the command or transaction
 in progress had received a temporary error (i.e., a 4yz response).
 The QUIT command may be issued at any time.  Any current uncompleted
 mail transaction will be aborted.
 Syntax:
    quit = "QUIT" CRLF

4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses

 If the server SMTP does not recognize or cannot implement one or more
 of the parameters associated with a particular MAIL FROM or RCPT TO
 command, it will return code 555.
 If, for some reason, the server is temporarily unable to accommodate
 one or more of the parameters associated with a MAIL FROM or RCPT TO
 command, and if the definition of the specific parameter does not
 mandate the use of another code, it should return code 455.

Klensin Standards Track [Page 40] RFC 5321 SMTP October 2008

 Errors specific to particular parameters and their values will be
 specified in the parameter's defining RFC.

4.1.2. Command Argument Syntax

 The syntax of the argument clauses of the above commands (using the
 syntax specified in RFC 5234 [7] where applicable) is given below.
 Some of the productions given below are used only in conjunction with
 source routes as described in Appendix C.  Terminals not defined in
 this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined
 in the "core" syntax in Section 6 of RFC 5234 [7] or in the message
 format syntax in RFC 5322 [4].
 Reverse-path   = Path / "<>"
 Forward-path   = Path
 Path           = "<" [ A-d-l ":" ] Mailbox ">"
 A-d-l          = At-domain *( "," At-domain )
                ; Note that this form, the so-called "source
                ; route", MUST BE accepted, SHOULD NOT be
                ; generated, and SHOULD be ignored.
 At-domain      = "@" Domain
 Mail-parameters  = esmtp-param *(SP esmtp-param)
 Rcpt-parameters  = esmtp-param *(SP esmtp-param)
 esmtp-param    = esmtp-keyword ["=" esmtp-value]
 esmtp-keyword  = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
 esmtp-value    = 1*(%d33-60 / %d62-126)
                ; any CHAR excluding "=", SP, and control
                ; characters.  If this string is an email address,
                ; i.e., a Mailbox, then the "xtext" syntax [32]
                ; SHOULD be used.
 Keyword        = Ldh-str
 Argument       = Atom
 Domain         = sub-domain *("." sub-domain)

Klensin Standards Track [Page 41] RFC 5321 SMTP October 2008

 sub-domain     = Let-dig [Ldh-str]
 Let-dig        = ALPHA / DIGIT
 Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig
 address-literal  = "[" ( IPv4-address-literal /
                  IPv6-address-literal /
                  General-address-literal ) "]"
                  ; See Section 4.1.3
 Mailbox        = Local-part "@" ( Domain / address-literal )
 Local-part     = Dot-string / Quoted-string
                ; MAY be case-sensitive
 Dot-string     = Atom *("."  Atom)
 Atom           = 1*atext
 Quoted-string  = DQUOTE *QcontentSMTP DQUOTE
 QcontentSMTP   = qtextSMTP / quoted-pairSMTP
 quoted-pairSMTP  = %d92 %d32-126
                  ; i.e., backslash followed by any ASCII
                  ; graphic (including itself) or SPace
 qtextSMTP      = %d32-33 / %d35-91 / %d93-126
                ; i.e., within a quoted string, any
                ; ASCII graphic or space is permitted
                ; without blackslash-quoting except
                ; double-quote and the backslash itself.
 String         = Atom / Quoted-string
 While the above definition for Local-part is relatively permissive,
 for maximum interoperability, a host that expects to receive mail
 SHOULD avoid defining mailboxes where the Local-part requires (or
 uses) the Quoted-string form or where the Local-part is case-
 sensitive.  For any purposes that require generating or comparing
 Local-parts (e.g., to specific mailbox names), all quoted forms MUST
 be treated as equivalent, and the sending system SHOULD transmit the
 form that uses the minimum quoting possible.
 Systems MUST NOT define mailboxes in such a way as to require the use
 in SMTP of non-ASCII characters (octets with the high order bit set

Klensin Standards Track [Page 42] RFC 5321 SMTP October 2008

 to one) or ASCII "control characters" (decimal value 0-31 and 127).
 These characters MUST NOT be used in MAIL or RCPT commands or other
 commands that require mailbox names.
 Note that the backslash, "\", is a quote character, which is used to
 indicate that the next character is to be used literally (instead of
 its normal interpretation).  For example, "Joe\,Smith" indicates a
 single nine-character user name string with the comma being the
 fourth character of that string.
 To promote interoperability and consistent with long-standing
 guidance about conservative use of the DNS in naming and applications
 (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [2]),
 characters outside the set of alphabetic characters, digits, and
 hyphen MUST NOT appear in domain name labels for SMTP clients or
 servers.  In particular, the underscore character is not permitted.
 SMTP servers that receive a command in which invalid character codes
 have been employed, and for which there are no other reasons for
 rejection, MUST reject that command with a 501 response (this rule,
 like others, could be overridden by appropriate SMTP extensions).

4.1.3. Address Literals

 Sometimes a host is not known to the domain name system and
 communication (and, in particular, communication to report and repair
 the error) is blocked.  To bypass this barrier, a special literal
 form of the address is allowed as an alternative to a domain name.
 For IPv4 addresses, this form uses four small decimal integers
 separated by dots and enclosed by brackets such as [123.255.37.2],
 which indicates an (IPv4) Internet Address in sequence-of-octets
 form.  For IPv6 and other forms of addressing that might eventually
 be standardized, the form consists of a standardized "tag" that
 identifies the address syntax, a colon, and the address itself, in a
 format specified as part of the relevant standards (i.e., RFC 4291
 [8] for IPv6).
 Specifically:
 IPv4-address-literal  = Snum 3("."  Snum)
 IPv6-address-literal  = "IPv6:" IPv6-addr
 General-address-literal  = Standardized-tag ":" 1*dcontent
 Standardized-tag  = Ldh-str
                   ; Standardized-tag MUST be specified in a
                   ; Standards-Track RFC and registered with IANA

Klensin Standards Track [Page 43] RFC 5321 SMTP October 2008

 dcontent       = %d33-90 / ; Printable US-ASCII
                %d94-126 ; excl. "[", "\", "]"
 Snum           = 1*3DIGIT
                ; representing a decimal integer
                ; value in the range 0 through 255
 IPv6-addr      = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
 IPv6-hex       = 1*4HEXDIG
 IPv6-full      = IPv6-hex 7(":" IPv6-hex)
 IPv6-comp      = [IPv6-hex *5(":" IPv6-hex)] "::"
                [IPv6-hex *5(":" IPv6-hex)]
                ; The "::" represents at least 2 16-bit groups of
                ; zeros.  No more than 6 groups in addition to the
                ; "::" may be present.
 IPv6v4-full    = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
 IPv6v4-comp    = [IPv6-hex *3(":" IPv6-hex)] "::"
                [IPv6-hex *3(":" IPv6-hex) ":"]
                IPv4-address-literal
                ; The "::" represents at least 2 16-bit groups of
                ; zeros.  No more than 4 groups in addition to the
                ; "::" and IPv4-address-literal may be present.

4.1.4. Order of Commands

 There are restrictions on the order in which these commands may be
 used.
 A session that will contain mail transactions MUST first be
 initialized by the use of the EHLO command.  An SMTP server SHOULD
 accept commands for non-mail transactions (e.g., VRFY or EXPN)
 without this initialization.
 An EHLO command MAY be issued by a client later in the session.  If
 it is issued after the session begins and the EHLO command is
 acceptable to the SMTP server, the SMTP server MUST clear all buffers
 and reset the state exactly as if a RSET command had been issued.  In
 other words, the sequence of RSET followed immediately by EHLO is
 redundant, but not harmful other than in the performance cost of
 executing unnecessary commands.
 If the EHLO command is not acceptable to the SMTP server, 501, 500,
 502, or 550 failure replies MUST be returned as appropriate.  The

Klensin Standards Track [Page 44] RFC 5321 SMTP October 2008

 SMTP server MUST stay in the same state after transmitting these
 replies that it was in before the EHLO was received.
 The SMTP client MUST, if possible, ensure that the domain parameter
 to the EHLO command is a primary host name as specified for this
 command in Section 2.3.5.  If this is not possible (e.g., when the
 client's address is dynamically assigned and the client does not have
 an obvious name), an address literal SHOULD be substituted for the
 domain name.
 An SMTP server MAY verify that the domain name argument in the EHLO
 command actually corresponds to the IP address of the client.
 However, if the verification fails, the server MUST NOT refuse to
 accept a message on that basis.  Information captured in the
 verification attempt is for logging and tracing purposes.  Note that
 this prohibition applies to the matching of the parameter to its IP
 address only; see Section 7.9 for a more extensive discussion of
 rejecting incoming connections or mail messages.
 The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
 during a session, or without previously initializing a session.  SMTP
 servers SHOULD process these normally (that is, not return a 503
 code) even if no EHLO command has yet been received; clients SHOULD
 open a session with EHLO before sending these commands.
 If these rules are followed, the example in RFC 821 that shows "550
 access denied to you" in response to an EXPN command is incorrect
 unless an EHLO command precedes the EXPN or the denial of access is
 based on the client's IP address or other authentication or
 authorization-determining mechanisms.
 The MAIL command (or the obsolete SEND, SOML, or SAML commands)
 begins a mail transaction.  Once started, a mail transaction consists
 of a transaction beginning command, one or more RCPT commands, and a
 DATA command, in that order.  A mail transaction may be aborted by
 the RSET, a new EHLO, or the QUIT command.  There may be zero or more
 transactions in a session.  MAIL (or SEND, SOML, or SAML) MUST NOT be
 sent if a mail transaction is already open, i.e., it should be sent
 only if no mail transaction had been started in the session, or if
 the previous one successfully concluded with a successful DATA
 command, or if the previous one was aborted, e.g., with a RSET or new
 EHLO.
 If the transaction beginning command argument is not acceptable, a
 501 failure reply MUST be returned and the SMTP server MUST stay in
 the same state.  If the commands in a transaction are out of order to
 the degree that they cannot be processed by the server, a 503 failure

Klensin Standards Track [Page 45] RFC 5321 SMTP October 2008

 reply MUST be returned and the SMTP server MUST stay in the same
 state.
 The last command in a session MUST be the QUIT command.  The QUIT
 command SHOULD be used by the client SMTP to request connection
 closure, even when no session opening command was sent and accepted.

4.1.5. Private-Use Commands

 As specified in Section 2.2.2, commands starting in "X" may be used
 by bilateral agreement between the client (sending) and server
 (receiving) SMTP agents.  An SMTP server that does not recognize such
 a command is expected to reply with "500 Command not recognized".  An
 extended SMTP server MAY list the feature names associated with these
 private commands in the response to the EHLO command.
 Commands sent or accepted by SMTP systems that do not start with "X"
 MUST conform to the requirements of Section 2.2.2.

4.2. SMTP Replies

 Replies to SMTP commands serve to ensure the synchronization of
 requests and actions in the process of mail transfer and to guarantee
 that the SMTP client always knows the state of the SMTP server.
 Every command MUST generate exactly one reply.
 The details of the command-reply sequence are described in
 Section 4.3.
 An SMTP reply consists of a three digit number (transmitted as three
 numeric characters) followed by some text unless specified otherwise
 in this document.  The number is for use by automata to determine
 what state to enter next; the text is for the human user.  The three
 digits contain enough encoded information that the SMTP client need
 not examine the text and may either discard it or pass it on to the
 user, as appropriate.  Exceptions are as noted elsewhere in this
 document.  In particular, the 220, 221, 251, 421, and 551 reply codes
 are associated with message text that must be parsed and interpreted
 by machines.  In the general case, the text may be receiver dependent
 and context dependent, so there are likely to be varying texts for
 each reply code.  A discussion of the theory of reply codes is given
 in Section 4.2.1.  Formally, a reply is defined to be the sequence: a
 three-digit code, <SP>, one line of text, and <CRLF>, or a multiline
 reply (as defined in the same section).  Since, in violation of this
 specification, the text is sometimes not sent, clients that do not
 receive it SHOULD be prepared to process the code alone (with or
 without a trailing space character).  Only the EHLO, EXPN, and HELP
 commands are expected to result in multiline replies in normal

Klensin Standards Track [Page 46] RFC 5321 SMTP October 2008

 circumstances; however, multiline replies are allowed for any
 command.
 In ABNF, server responses are:
 Greeting       = ( "220 " (Domain / address-literal)
                [ SP textstring ] CRLF ) /
                ( "220-" (Domain / address-literal)
                [ SP textstring ] CRLF
                *( "220-" [ textstring ] CRLF )
                "220" [ SP textstring ] CRLF )
 textstring     = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII
 Reply-line     = *( Reply-code "-" [ textstring ] CRLF )
                Reply-code [ SP textstring ] CRLF
 Reply-code     = %x32-35 %x30-35 %x30-39
 where "Greeting" appears only in the 220 response that announces that
 the server is opening its part of the connection.  (Other possible
 server responses upon connection follow the syntax of Reply-line.)
 An SMTP server SHOULD send only the reply codes listed in this
 document.  An SMTP server SHOULD use the text shown in the examples
 whenever appropriate.
 An SMTP client MUST determine its actions only by the reply code, not
 by the text (except for the "change of address" 251 and 551 and, if
 necessary, 220, 221, and 421 replies); in the general case, any text,
 including no text at all (although senders SHOULD NOT send bare
 codes), MUST be acceptable.  The space (blank) following the reply
 code is considered part of the text.  Whenever possible, a receiver-
 SMTP SHOULD test the first digit (severity indication) of the reply
 code.
 The list of codes that appears below MUST NOT be construed as
 permanent.  While the addition of new codes should be a rare and
 significant activity, with supplemental information in the textual
 part of the response being preferred, new codes may be added as the
 result of new Standards or Standards-Track specifications.
 Consequently, a sender-SMTP MUST be prepared to handle codes not
 specified in this document and MUST do so by interpreting the first
 digit only.
 In the absence of extensions negotiated with the client, SMTP servers
 MUST NOT send reply codes whose first digits are other than 2, 3, 4,

Klensin Standards Track [Page 47] RFC 5321 SMTP October 2008

 or 5.  Clients that receive such out-of-range codes SHOULD normally
 treat them as fatal errors and terminate the mail transaction.

4.2.1. Reply Code Severities and Theory

 The three digits of the reply each have a special significance.  The
 first digit denotes whether the response is good, bad, or incomplete.
 An unsophisticated SMTP client, or one that receives an unexpected
 code, will be able to determine its next action (proceed as planned,
 redo, retrench, etc.) by examining this first digit.  An SMTP client
 that wants to know approximately what kind of error occurred (e.g.,
 mail system error, command syntax error) may examine the second
 digit.  The third digit and any supplemental information that may be
 present is reserved for the finest gradation of information.
 There are four values for the first digit of the reply code:
 2yz  Positive Completion reply
    The requested action has been successfully completed.  A new
    request may be initiated.
 3yz  Positive Intermediate reply
    The command has been accepted, but the requested action is being
    held in abeyance, pending receipt of further information.  The
    SMTP client should send another command specifying this
    information.  This reply is used in command sequence groups (i.e.,
    in DATA).
 4yz  Transient Negative Completion reply
    The command was not accepted, and the requested action did not
    occur.  However, the error condition is temporary, and the action
    may be requested again.  The sender should return to the beginning
    of the command sequence (if any).  It is difficult to assign a
    meaning to "transient" when two different sites (receiver- and
    sender-SMTP agents) must agree on the interpretation.  Each reply
    in this category might have a different time value, but the SMTP
    client SHOULD try again.  A rule of thumb to determine whether a
    reply fits into the 4yz or the 5yz category (see below) is that
    replies are 4yz if they can be successful if repeated without any
    change in command form or in properties of the sender or receiver
    (that is, the command is repeated identically and the receiver
    does not put up a new implementation).
 5yz  Permanent Negative Completion reply
    The command was not accepted and the requested action did not
    occur.  The SMTP client SHOULD NOT repeat the exact request (in
    the same sequence).  Even some "permanent" error conditions can be
    corrected, so the human user may want to direct the SMTP client to

Klensin Standards Track [Page 48] RFC 5321 SMTP October 2008

    reinitiate the command sequence by direct action at some point in
    the future (e.g., after the spelling has been changed, or the user
    has altered the account status).
 It is worth noting that the file transfer protocol (FTP) [34] uses a
 very similar code architecture and that the SMTP codes are based on
 the FTP model.  However, SMTP uses a one-command, one-response model
 (while FTP is asynchronous) and FTP's 1yz codes are not part of the
 SMTP model.
 The second digit encodes responses in specific categories:
 x0z  Syntax: These replies refer to syntax errors, syntactically
    correct commands that do not fit any functional category, and
    unimplemented or superfluous commands.
 x1z  Information: These are replies to requests for information, such
    as status or help.
 x2z  Connections: These are replies referring to the transmission
    channel.
 x3z  Unspecified.
 x4z  Unspecified.
 x5z  Mail system: These replies indicate the status of the receiver
    mail system vis-a-vis the requested transfer or other mail system
    action.
 The third digit gives a finer gradation of meaning in each category
 specified by the second digit.  The list of replies illustrates this.
 Each reply text is recommended rather than mandatory, and may even
 change according to the command with which it is associated.  On the
 other hand, the reply codes must strictly follow the specifications
 in this section.  Receiver implementations should not invent new
 codes for slightly different situations from the ones described here,
 but rather adapt codes already defined.
 For example, a command such as NOOP, whose successful execution does
 not offer the SMTP client any new information, will return a 250
 reply.  The reply is 502 when the command requests an unimplemented
 non-site-specific action.  A refinement of that is the 504 reply for
 a command that is implemented, but that requests an unimplemented
 parameter.

Klensin Standards Track [Page 49] RFC 5321 SMTP October 2008

 The reply text may be longer than a single line; in these cases the
 complete text must be marked so the SMTP client knows when it can
 stop reading the reply.  This requires a special format to indicate a
 multiple line reply.
 The format for multiline replies requires that every line, except the
 last, begin with the reply code, followed immediately by a hyphen,
 "-" (also known as minus), followed by text.  The last line will
 begin with the reply code, followed immediately by <SP>, optionally
 some text, and <CRLF>.  As noted above, servers SHOULD send the <SP>
 if subsequent text is not sent, but clients MUST be prepared for it
 to be omitted.
 For example:
    250-First line
    250-Second line
    250-234 Text beginning with numbers
    250 The last line
 In a multiline reply, the reply code on each of the lines MUST be the
 same.  It is reasonable for the client to rely on this, so it can
 make processing decisions based on the code in any line, assuming
 that all others will be the same.  In a few cases, there is important
 data for the client in the reply "text".  The client will be able to
 identify these cases from the current context.

4.2.2. Reply Codes by Function Groups

 500  Syntax error, command unrecognized (This may include errors such
    as command line too long)
 501  Syntax error in parameters or arguments
 502  Command not implemented (see Section 4.2.4)
 503  Bad sequence of commands
 504  Command parameter not implemented
 211  System status, or system help reply
 214  Help message (Information on how to use the receiver or the
    meaning of a particular non-standard command; this reply is useful
    only to the human user)

Klensin Standards Track [Page 50] RFC 5321 SMTP October 2008

 220  <domain> Service ready
 221  <domain> Service closing transmission channel
 421  <domain> Service not available, closing transmission channel
    (This may be a reply to any command if the service knows it must
    shut down)
 250  Requested mail action okay, completed
 251  User not local; will forward to <forward-path> (See Section 3.4)
 252  Cannot VRFY user, but will accept message and attempt delivery
    (See Section 3.5.3)
 455  Server unable to accommodate parameters
 555  MAIL FROM/RCPT TO parameters not recognized or not implemented
 450  Requested mail action not taken: mailbox unavailable (e.g.,
    mailbox busy or temporarily blocked for policy reasons)
 550  Requested action not taken: mailbox unavailable (e.g., mailbox
    not found, no access, or command rejected for policy reasons)
 451  Requested action aborted: error in processing
 551  User not local; please try <forward-path> (See Section 3.4)
 452  Requested action not taken: insufficient system storage
 552  Requested mail action aborted: exceeded storage allocation
 553  Requested action not taken: mailbox name not allowed (e.g.,
    mailbox syntax incorrect)
 354  Start mail input; end with <CRLF>.<CRLF>
 554  Transaction failed (Or, in the case of a connection-opening
    response, "No SMTP service here")

Klensin Standards Track [Page 51] RFC 5321 SMTP October 2008

4.2.3. Reply Codes in Numeric Order

 211  System status, or system help reply
 214  Help message (Information on how to use the receiver or the
    meaning of a particular non-standard command; this reply is useful
    only to the human user)
 220  <domain> Service ready
 221  <domain> Service closing transmission channel
 250  Requested mail action okay, completed
 251  User not local; will forward to <forward-path> (See Section 3.4)
 252  Cannot VRFY user, but will accept message and attempt delivery
    (See Section 3.5.3)
 354  Start mail input; end with <CRLF>.<CRLF>
 421  <domain> Service not available, closing transmission channel
    (This may be a reply to any command if the service knows it must
    shut down)
 450  Requested mail action not taken: mailbox unavailable (e.g.,
    mailbox busy or temporarily blocked for policy reasons)
 451  Requested action aborted: local error in processing
 452  Requested action not taken: insufficient system storage
 455  Server unable to accommodate parameters
 500  Syntax error, command unrecognized (This may include errors such
    as command line too long)
 501  Syntax error in parameters or arguments
 502  Command not implemented (see Section 4.2.4)
 503  Bad sequence of commands
 504  Command parameter not implemented
 550  Requested action not taken: mailbox unavailable (e.g., mailbox
    not found, no access, or command rejected for policy reasons)

Klensin Standards Track [Page 52] RFC 5321 SMTP October 2008

 551  User not local; please try <forward-path> (See Section 3.4)
 552  Requested mail action aborted: exceeded storage allocation
 553  Requested action not taken: mailbox name not allowed (e.g.,
    mailbox syntax incorrect)
 554  Transaction failed (Or, in the case of a connection-opening
    response, "No SMTP service here")
 555  MAIL FROM/RCPT TO parameters not recognized or not implemented

4.2.4. Reply Code 502

 Questions have been raised as to when reply code 502 (Command not
 implemented) SHOULD be returned in preference to other codes. 502
 SHOULD be used when the command is actually recognized by the SMTP
 server, but not implemented.  If the command is not recognized, code
 500 SHOULD be returned.  Extended SMTP systems MUST NOT list
 capabilities in response to EHLO for which they will return 502 (or
 500) replies.

4.2.5. Reply Codes after DATA and the Subsequent <CRLF>.<CRLF>

 When an SMTP server returns a positive completion status (2yz code)
 after the DATA command is completed with <CRLF>.<CRLF>, it accepts
 responsibility for:
 o  delivering the message (if the recipient mailbox exists), or
 o  if attempts to deliver the message fail due to transient
    conditions, retrying delivery some reasonable number of times at
    intervals as specified in Section 4.5.4.
 o  if attempts to deliver the message fail due to permanent
    conditions, or if repeated attempts to deliver the message fail
    due to transient conditions, returning appropriate notification to
    the sender of the original message (using the address in the SMTP
    MAIL command).
 When an SMTP server returns a temporary error status (4yz) code after
 the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make a
 subsequent attempt to deliver that message.  The SMTP client retains
 responsibility for the delivery of that message and may either return
 it to the user or requeue it for a subsequent attempt (see
 Section 4.5.4.1).

Klensin Standards Track [Page 53] RFC 5321 SMTP October 2008

 The user who originated the message SHOULD be able to interpret the
 return of a transient failure status (by mail message or otherwise)
 as a non-delivery indication, just as a permanent failure would be
 interpreted.  If the client SMTP successfully handles these
 conditions, the user will not receive such a reply.
 When an SMTP server returns a permanent error status (5yz) code after
 the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
 any subsequent attempt to deliver the message.  As with temporary
 error status codes, the SMTP client retains responsibility for the
 message, but SHOULD not again attempt delivery to the same server
 without user review of the message and response and appropriate
 intervention.

4.3. Sequencing of Commands and Replies

4.3.1. Sequencing Overview

 The communication between the sender and receiver is an alternating
 dialogue, controlled by the sender.  As such, the sender issues a
 command and the receiver responds with a reply.  Unless other
 arrangements are negotiated through service extensions, the sender
 MUST wait for this response before sending further commands.  One
 important reply is the connection greeting.  Normally, a receiver
 will send a 220 "Service ready" reply when the connection is
 completed.  The sender SHOULD wait for this greeting message before
 sending any commands.
 Note: all the greeting-type replies have the official name (the
 fully-qualified primary domain name) of the server host as the first
 word following the reply code.  Sometimes the host will have no
 meaningful name.  See Section 4.1.3 for a discussion of alternatives
 in these situations.
 For example,
    220 ISIF.USC.EDU Service ready
 or
    220 mail.example.com SuperSMTP v 6.1.2 Service ready
 or
    220 [10.0.0.1] Clueless host service ready
 The table below lists alternative success and failure replies for
 each command.  These SHOULD be strictly adhered to.  A receiver MAY

Klensin Standards Track [Page 54] RFC 5321 SMTP October 2008

 substitute text in the replies, but the meanings and actions implied
 by the code numbers and by the specific command reply sequence MUST
 be preserved.

4.3.2. Command-Reply Sequences

 Each command is listed with its usual possible replies.  The prefixes
 used before the possible replies are "I" for intermediate, "S" for
 success, and "E" for error.  Since some servers may generate other
 replies under special circumstances, and to allow for future
 extension, SMTP clients SHOULD, when possible, interpret only the
 first digit of the reply and MUST be prepared to deal with
 unrecognized reply codes by interpreting the first digit only.
 Unless extended using the mechanisms described in Section 2.2, SMTP
 servers MUST NOT transmit reply codes to an SMTP client that are
 other than three digits or that do not start in a digit between 2 and
 5 inclusive.
 These sequencing rules and, in principle, the codes themselves, can
 be extended or modified by SMTP extensions offered by the server and
 accepted (requested) by the client.  However, if the target is more
 precise granularity in the codes, rather than codes for completely
 new purposes, the system described in RFC 3463 [25] SHOULD be used in
 preference to the invention of new codes.
 In addition to the codes listed below, any SMTP command can return
 any of the following codes if the corresponding unusual circumstances
 are encountered:
 500  For the "command line too long" case or if the command name was
    not recognized.  Note that producing a "command not recognized"
    error in response to the required subset of these commands is a
    violation of this specification.  Similarly, producing a "command
    too long" message for a command line shorter than 512 characters
    would violate the provisions of Section 4.5.3.1.4.
 501  Syntax error in command or arguments.  In order to provide for
    future extensions, commands that are specified in this document as
    not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501
    message if arguments are supplied in the absence of EHLO-
    advertised extensions.
 421  Service shutting down and closing transmission channel

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 Specific sequences are:
    CONNECTION ESTABLISHMENT
       S: 220
       E: 554
    EHLO or HELO
       S: 250
       E: 504 (a conforming implementation could return this code only
       in fairly obscure cases), 550, 502 (permitted only with an old-
       style server that does not support EHLO)
    MAIL
       S: 250
       E: 552, 451, 452, 550, 553, 503, 455, 555
    RCPT
       S: 250, 251 (but see Section 3.4 for discussion of 251 and 551)
       E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555
    DATA
       I: 354 -> data -> S: 250
                         E: 552, 554, 451, 452
                         E: 450, 550 (rejections for policy reasons)
       E: 503, 554
    RSET
       S: 250
    VRFY
       S: 250, 251, 252
       E: 550, 551, 553, 502, 504
    EXPN
       S: 250, 252
       E: 550, 500, 502, 504

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    HELP
       S: 211, 214
       E: 502, 504
    NOOP
       S: 250
    QUIT
       S: 221

4.4. Trace Information

 When an SMTP server receives a message for delivery or further
 processing, it MUST insert trace ("time stamp" or "Received")
 information at the beginning of the message content, as discussed in
 Section 4.1.1.4.
 This line MUST be structured as follows:
 o  The FROM clause, which MUST be supplied in an SMTP environment,
    SHOULD contain both (1) the name of the source host as presented
    in the EHLO command and (2) an address literal containing the IP
    address of the source, determined from the TCP connection.
 o  The ID clause MAY contain an "@" as suggested in RFC 822, but this
    is not required.
 o  If the FOR clause appears, it MUST contain exactly one <path>
    entry, even when multiple RCPT commands have been given.  Multiple
    <path>s raise some security issues and have been deprecated, see
    Section 7.2.
 An Internet mail program MUST NOT change or delete a Received: line
 that was previously added to the message header section.  SMTP
 servers MUST prepend Received lines to messages; they MUST NOT change
 the order of existing lines or insert Received lines in any other
 location.
 As the Internet grows, comparability of Received header fields is
 important for detecting problems, especially slow relays.  SMTP
 servers that create Received header fields SHOULD use explicit
 offsets in the dates (e.g., -0800), rather than time zone names of
 any type.  Local time (with an offset) SHOULD be used rather than UT
 when feasible.  This formulation allows slightly more information
 about local circumstances to be specified.  If UT is needed, the

Klensin Standards Track [Page 57] RFC 5321 SMTP October 2008

 receiver need merely do some simple arithmetic to convert the values.
 Use of UT loses information about the time zone-location of the
 server.  If it is desired to supply a time zone name, it SHOULD be
 included in a comment.
 When the delivery SMTP server makes the "final delivery" of a
 message, it inserts a return-path line at the beginning of the mail
 data.  This use of return-path is required; mail systems MUST support
 it.  The return-path line preserves the information in the <reverse-
 path> from the MAIL command.  Here, final delivery means the message
 has left the SMTP environment.  Normally, this would mean it had been
 delivered to the destination user or an associated mail drop, but in
 some cases it may be further processed and transmitted by another
 mail system.
 It is possible for the mailbox in the return path to be different
 from the actual sender's mailbox, for example, if error responses are
 to be delivered to a special error handling mailbox rather than to
 the message sender.  When mailing lists are involved, this
 arrangement is common and useful as a means of directing errors to
 the list maintainer rather than the message originator.
 The text above implies that the final mail data will begin with a
 return path line, followed by one or more time stamp lines.  These
 lines will be followed by the rest of the mail data: first the
 balance of the mail header section and then the body (RFC 5322 [4]).
 It is sometimes difficult for an SMTP server to determine whether or
 not it is making final delivery since forwarding or other operations
 may occur after the message is accepted for delivery.  Consequently,
 any further (forwarding, gateway, or relay) systems MAY remove the
 return path and rebuild the MAIL command as needed to ensure that
 exactly one such line appears in a delivered message.
 A message-originating SMTP system SHOULD NOT send a message that
 already contains a Return-path header field.  SMTP servers performing
 a relay function MUST NOT inspect the message data, and especially
 not to the extent needed to determine if Return-path header fields
 are present.  SMTP servers making final delivery MAY remove Return-
 path header fields before adding their own.
 The primary purpose of the Return-path is to designate the address to
 which messages indicating non-delivery or other mail system failures
 are to be sent.  For this to be unambiguous, exactly one return path
 SHOULD be present when the message is delivered.  Systems using RFC
 822 syntax with non-SMTP transports SHOULD designate an unambiguous
 address, associated with the transport envelope, to which error
 reports (e.g., non-delivery messages) should be sent.

Klensin Standards Track [Page 58] RFC 5321 SMTP October 2008

 Historical note: Text in RFC 822 that appears to contradict the use
 of the Return-path header field (or the envelope reverse-path address
 from the MAIL command) as the destination for error messages is not
 applicable on the Internet.  The reverse-path address (as copied into
 the Return-path) MUST be used as the target of any mail containing
 delivery error messages.
 In particular:
 o  a gateway from SMTP -> elsewhere SHOULD insert a return-path
    header field, unless it is known that the "elsewhere" transport
    also uses Internet domain addresses and maintains the envelope
    sender address separately.
 o  a gateway from elsewhere -> SMTP SHOULD delete any return-path
    header field present in the message, and either copy that
    information to the SMTP envelope or combine it with information
    present in the envelope of the other transport system to construct
    the reverse-path argument to the MAIL command in the SMTP
    envelope.
 The server must give special treatment to cases in which the
 processing following the end of mail data indication is only
 partially successful.  This could happen if, after accepting several
 recipients and the mail data, the SMTP server finds that the mail
 data could be successfully delivered to some, but not all, of the
 recipients.  In such cases, the response to the DATA command MUST be
 an OK reply.  However, the SMTP server MUST compose and send an
 "undeliverable mail" notification message to the originator of the
 message.
 A single notification listing all of the failed recipients or
 separate notification messages MUST be sent for each failed
 recipient.  For economy of processing by the sender, the former
 SHOULD be used when possible.  Note that the key difference between
 handling aliases (Section 3.9.1) and forwarding (this subsection) is
 the change to the backward-pointing address in this case.  All
 notification messages about undeliverable mail MUST be sent using the
 MAIL command (even if they result from processing the obsolete SEND,
 SOML, or SAML commands) and MUST use a null return path as discussed
 in Section 3.6.
 The time stamp line and the return path line are formally defined as
 follows (the definitions for "FWS" and "CFWS" appear in RFC 5322
 [4]):
 Return-path-line  = "Return-Path:" FWS Reverse-path <CRLF>
 Time-stamp-line  = "Received:" FWS Stamp <CRLF>

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 Stamp          = From-domain By-domain Opt-info [CFWS] ";"
                FWS date-time
                ; where "date-time" is as defined in RFC 5322 [4]
                ; but the "obs-" forms, especially two-digit
                ; years, are prohibited in SMTP and MUST NOT be used.
 From-domain    = "FROM" FWS Extended-Domain
 By-domain      = CFWS "BY" FWS Extended-Domain
 Extended-Domain  = Domain /
                  ( Domain FWS "(" TCP-info ")" ) /
                  ( address-literal FWS "(" TCP-info ")" )
 TCP-info       = address-literal / ( Domain FWS address-literal )
                ; Information derived by server from TCP connection
                ; not client EHLO.
 Opt-info       = [Via] [With] [ID] [For]
                [Additional-Registered-Clauses]
 Via            = CFWS "VIA" FWS Link
 With           = CFWS "WITH" FWS Protocol
 ID             = CFWS "ID" FWS ( Atom / msg-id )
                ; msg-id is defined in RFC 5322 [4]
 For            = CFWS "FOR" FWS ( Path / Mailbox )
 Additional-Registered-Clauses  = CFWS Atom FWS String
                                ; Additional standard clauses may be
                                added in this
                                ; location by future standards and
                                registration with
                                ; IANA.  SMTP servers SHOULD NOT use
                                unregistered
                                ; names.  See Section 8.
 Link           = "TCP" / Addtl-Link
 Addtl-Link     = Atom
                ; Additional standard names for links are
                ; registered with the Internet Assigned Numbers
                ; Authority (IANA).  "Via" is primarily of value
                ; with non-Internet transports.  SMTP servers
                ; SHOULD NOT use unregistered names.

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 Protocol       = "ESMTP" / "SMTP" / Attdl-Protocol
 Attdl-Protocol = Atom
                ; Additional standard names for protocols are
                ; registered with the Internet Assigned Numbers
                ; Authority (IANA) in the "mail parameters"
                ; registry [9].  SMTP servers SHOULD NOT
                ; use unregistered names.

4.5. Additional Implementation Issues

4.5.1. Minimum Implementation

 In order to make SMTP workable, the following minimum implementation
 MUST be provided by all receivers.  The following commands MUST be
 supported to conform to this specification:
    EHLO
    HELO
    MAIL
    RCPT
    DATA
    RSET
    NOOP
    QUIT
    VRFY
 Any system that includes an SMTP server supporting mail relaying or
 delivery MUST support the reserved mailbox "postmaster" as a case-
 insensitive local name.  This postmaster address is not strictly
 necessary if the server always returns 554 on connection opening (as
 described in Section 3.1).  The requirement to accept mail for
 postmaster implies that RCPT commands that specify a mailbox for
 postmaster at any of the domains for which the SMTP server provides
 mail service, as well as the special case of "RCPT TO:<Postmaster>"
 (with no domain specification), MUST be supported.
 SMTP systems are expected to make every reasonable effort to accept
 mail directed to Postmaster from any other system on the Internet.
 In extreme cases -- such as to contain a denial of service attack or
 other breach of security -- an SMTP server may block mail directed to
 Postmaster.  However, such arrangements SHOULD be narrowly tailored
 so as to avoid blocking messages that are not part of such attacks.

Klensin Standards Track [Page 61] RFC 5321 SMTP October 2008

4.5.2. Transparency

 Without some provision for data transparency, the character sequence
 "<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.
 In general, users are not aware of such "forbidden" sequences.  To
 allow all user composed text to be transmitted transparently, the
 following procedures are used:
 o  Before sending a line of mail text, the SMTP client checks the
    first character of the line.  If it is a period, one additional
    period is inserted at the beginning of the line.
 o  When a line of mail text is received by the SMTP server, it checks
    the line.  If the line is composed of a single period, it is
    treated as the end of mail indicator.  If the first character is a
    period and there are other characters on the line, the first
    character is deleted.
 The mail data may contain any of the 128 ASCII characters.  All
 characters are to be delivered to the recipient's mailbox, including
 spaces, vertical and horizontal tabs, and other control characters.
 If the transmission channel provides an 8-bit byte (octet) data
 stream, the 7-bit ASCII codes are transmitted, right justified, in
 the octets, with the high-order bits cleared to zero.  See
 Section 3.6 for special treatment of these conditions in SMTP systems
 serving a relay function.
 In some systems, it may be necessary to transform the data as it is
 received and stored.  This may be necessary for hosts that use a
 different character set than ASCII as their local character set, that
 store data in records rather than strings, or which use special
 character sequences as delimiters inside mailboxes.  If such
 transformations are necessary, they MUST be reversible, especially if
 they are applied to mail being relayed.

4.5.3. Sizes and Timeouts

4.5.3.1. Size Limits and Minimums

 There are several objects that have required minimum/maximum sizes.
 Every implementation MUST be able to receive objects of at least
 these sizes.  Objects larger than these sizes SHOULD be avoided when
 possible.  However, some Internet mail constructs such as encoded
 X.400 addresses (RFC 2156 [35]) will often require larger objects.
 Clients MAY attempt to transmit these, but MUST be prepared for a
 server to reject them if they cannot be handled by it.  To the
 maximum extent possible, implementation techniques that impose no
 limits on the length of these objects should be used.

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 Extensions to SMTP may involve the use of characters that occupy more
 than a single octet each.  This section therefore specifies lengths
 in octets where absolute lengths, rather than character counts, are
 intended.

4.5.3.1.1. Local-part

 The maximum total length of a user name or other local-part is 64
 octets.

4.5.3.1.2. Domain

 The maximum total length of a domain name or number is 255 octets.

4.5.3.1.3. Path

 The maximum total length of a reverse-path or forward-path is 256
 octets (including the punctuation and element separators).

4.5.3.1.4. Command Line

 The maximum total length of a command line including the command word
 and the <CRLF> is 512 octets.  SMTP extensions may be used to
 increase this limit.

4.5.3.1.5. Reply Line

 The maximum total length of a reply line including the reply code and
 the <CRLF> is 512 octets.  More information may be conveyed through
 multiple-line replies.

4.5.3.1.6. Text Line

 The maximum total length of a text line including the <CRLF> is 1000
 octets (not counting the leading dot duplicated for transparency).
 This number may be increased by the use of SMTP Service Extensions.

4.5.3.1.7. Message Content

 The maximum total length of a message content (including any message
 header section as well as the message body) MUST BE at least 64K
 octets.  Since the introduction of Internet Standards for multimedia
 mail (RFC 2045 [21]), message lengths on the Internet have grown
 dramatically, and message size restrictions should be avoided if at
 all possible.  SMTP server systems that must impose restrictions
 SHOULD implement the "SIZE" service extension of RFC 1870 [10], and
 SMTP client systems that will send large messages SHOULD utilize it
 when possible.

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4.5.3.1.8. Recipients Buffer

 The minimum total number of recipients that MUST be buffered is 100
 recipients.  Rejection of messages (for excessive recipients) with
 fewer than 100 RCPT commands is a violation of this specification.
 The general principle that relaying SMTP server MUST NOT, and
 delivery SMTP servers SHOULD NOT, perform validation tests on message
 header fields suggests that messages SHOULD NOT be rejected based on
 the total number of recipients shown in header fields.  A server that
 imposes a limit on the number of recipients MUST behave in an orderly
 fashion, such as rejecting additional addresses over its limit rather
 than silently discarding addresses previously accepted.  A client
 that needs to deliver a message containing over 100 RCPT commands
 SHOULD be prepared to transmit in 100-recipient "chunks" if the
 server declines to accept more than 100 recipients in a single
 message.

4.5.3.1.9. Treatment When Limits Exceeded

 Errors due to exceeding these limits may be reported by using the
 reply codes.  Some examples of reply codes are:
    500 Line too long.
 or
    501 Path too long
 or
    452 Too many recipients (see below)
 or
    552 Too much mail data.

4.5.3.1.10. Too Many Recipients Code

 RFC 821 [1] incorrectly listed the error where an SMTP server
 exhausts its implementation limit on the number of RCPT commands
 ("too many recipients") as having reply code 552.  The correct reply
 code for this condition is 452.  Clients SHOULD treat a 552 code in
 this case as a temporary, rather than permanent, failure so the logic
 below works.
 When a conforming SMTP server encounters this condition, it has at
 least 100 successful RCPT commands in its recipients buffer.  If the
 server is able to accept the message, then at least these 100

Klensin Standards Track [Page 64] RFC 5321 SMTP October 2008

 addresses will be removed from the SMTP client's queue.  When the
 client attempts retransmission of those addresses that received 452
 responses, at least 100 of these will be able to fit in the SMTP
 server's recipients buffer.  Each retransmission attempt that is able
 to deliver anything will be able to dispose of at least 100 of these
 recipients.
 If an SMTP server has an implementation limit on the number of RCPT
 commands and this limit is exhausted, it MUST use a response code of
 452 (but the client SHOULD also be prepared for a 552, as noted
 above).  If the server has a configured site-policy limitation on the
 number of RCPT commands, it MAY instead use a 5yz response code.  In
 particular, if the intent is to prohibit messages with more than a
 site-specified number of recipients, rather than merely limit the
 number of recipients in a given mail transaction, it would be
 reasonable to return a 503 response to any DATA command received
 subsequent to the 452 (or 552) code or to simply return the 503 after
 DATA without returning any previous negative response.

4.5.3.2. Timeouts

 An SMTP client MUST provide a timeout mechanism.  It MUST use per-
 command timeouts rather than somehow trying to time the entire mail
 transaction.  Timeouts SHOULD be easily reconfigurable, preferably
 without recompiling the SMTP code.  To implement this, a timer is set
 for each SMTP command and for each buffer of the data transfer.  The
 latter means that the overall timeout is inherently proportional to
 the size of the message.
 Based on extensive experience with busy mail-relay hosts, the minimum
 per-command timeout values SHOULD be as follows:

4.5.3.2.1. Initial 220 Message: 5 Minutes

 An SMTP client process needs to distinguish between a failed TCP
 connection and a delay in receiving the initial 220 greeting message.
 Many SMTP servers accept a TCP connection but delay delivery of the
 220 message until their system load permits more mail to be
 processed.

4.5.3.2.2. MAIL Command: 5 Minutes

4.5.3.2.3. RCPT Command: 5 Minutes

 A longer timeout is required if processing of mailing lists and
 aliases is not deferred until after the message was accepted.

Klensin Standards Track [Page 65] RFC 5321 SMTP October 2008

4.5.3.2.4. DATA Initiation: 2 Minutes

 This is while awaiting the "354 Start Input" reply to a DATA command.

4.5.3.2.5. Data Block: 3 Minutes

 This is while awaiting the completion of each TCP SEND call
 transmitting a chunk of data.

4.5.3.2.6. DATA Termination: 10 Minutes.

 This is while awaiting the "250 OK" reply.  When the receiver gets
 the final period terminating the message data, it typically performs
 processing to deliver the message to a user mailbox.  A spurious
 timeout at this point would be very wasteful and would typically
 result in delivery of multiple copies of the message, since it has
 been successfully sent and the server has accepted responsibility for
 delivery.  See Section 6.1 for additional discussion.

4.5.3.2.7. Server Timeout: 5 Minutes.

 An SMTP server SHOULD have a timeout of at least 5 minutes while it
 is awaiting the next command from the sender.

4.5.4. Retry Strategies

 The common structure of a host SMTP implementation includes user
 mailboxes, one or more areas for queuing messages in transit, and one
 or more daemon processes for sending and receiving mail.  The exact
 structure will vary depending on the needs of the users on the host
 and the number and size of mailing lists supported by the host.  We
 describe several optimizations that have proved helpful, particularly
 for mailers supporting high traffic levels.
 Any queuing strategy MUST include timeouts on all activities on a
 per-command basis.  A queuing strategy MUST NOT send error messages
 in response to error messages under any circumstances.

4.5.4.1. Sending Strategy

 The general model for an SMTP client is one or more processes that
 periodically attempt to transmit outgoing mail.  In a typical system,
 the program that composes a message has some method for requesting
 immediate attention for a new piece of outgoing mail, while mail that
 cannot be transmitted immediately MUST be queued and periodically
 retried by the sender.  A mail queue entry will include not only the
 message itself but also the envelope information.

Klensin Standards Track [Page 66] RFC 5321 SMTP October 2008

 The sender MUST delay retrying a particular destination after one
 attempt has failed.  In general, the retry interval SHOULD be at
 least 30 minutes; however, more sophisticated and variable strategies
 will be beneficial when the SMTP client can determine the reason for
 non-delivery.
 Retries continue until the message is transmitted or the sender gives
 up; the give-up time generally needs to be at least 4-5 days.  It MAY
 be appropriate to set a shorter maximum number of retries for non-
 delivery notifications and equivalent error messages than for
 standard messages.  The parameters to the retry algorithm MUST be
 configurable.
 A client SHOULD keep a list of hosts it cannot reach and
 corresponding connection timeouts, rather than just retrying queued
 mail items.
 Experience suggests that failures are typically transient (the target
 system or its connection has crashed), favoring a policy of two
 connection attempts in the first hour the message is in the queue,
 and then backing off to one every two or three hours.
 The SMTP client can shorten the queuing delay in cooperation with the
 SMTP server.  For example, if mail is received from a particular
 address, it is likely that mail queued for that host can now be sent.
 Application of this principle may, in many cases, eliminate the
 requirement for an explicit "send queues now" function such as ETRN,
 RFC 1985 [36].
 The strategy may be further modified as a result of multiple
 addresses per host (see below) to optimize delivery time versus
 resource usage.
 An SMTP client may have a large queue of messages for each
 unavailable destination host.  If all of these messages were retried
 in every retry cycle, there would be excessive Internet overhead and
 the sending system would be blocked for a long period.  Note that an
 SMTP client can generally determine that a delivery attempt has
 failed only after a timeout of several minutes, and even a one-minute
 timeout per connection will result in a very large delay if retries
 are repeated for dozens, or even hundreds, of queued messages to the
 same host.
 At the same time, SMTP clients SHOULD use great care in caching
 negative responses from servers.  In an extreme case, if EHLO is
 issued multiple times during the same SMTP connection, different
 answers may be returned by the server.  More significantly, 5yz
 responses to the MAIL command MUST NOT be cached.

Klensin Standards Track [Page 67] RFC 5321 SMTP October 2008

 When a mail message is to be delivered to multiple recipients, and
 the SMTP server to which a copy of the message is to be sent is the
 same for multiple recipients, then only one copy of the message
 SHOULD be transmitted.  That is, the SMTP client SHOULD use the
 command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the
 sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA.  However, if there
 are very many addresses, a limit on the number of RCPT commands per
 MAIL command MAY be imposed.  This efficiency feature SHOULD be
 implemented.
 Similarly, to achieve timely delivery, the SMTP client MAY support
 multiple concurrent outgoing mail transactions.  However, some limit
 may be appropriate to protect the host from devoting all its
 resources to mail.

4.5.4.2. Receiving Strategy

 The SMTP server SHOULD attempt to keep a pending listen on the SMTP
 port (specified by IANA as port 25) at all times.  This requires the
 support of multiple incoming TCP connections for SMTP.  Some limit
 MAY be imposed, but servers that cannot handle more than one SMTP
 transaction at a time are not in conformance with the intent of this
 specification.
 As discussed above, when the SMTP server receives mail from a
 particular host address, it could activate its own SMTP queuing
 mechanisms to retry any mail pending for that host address.

4.5.5. Messages with a Null Reverse-Path

 There are several types of notification messages that are required by
 existing and proposed Standards to be sent with a null reverse-path,
 namely non-delivery notifications as discussed in Section 3.7, other
 kinds of Delivery Status Notifications (DSNs, RFC 3461 [32]), and
 Message Disposition Notifications (MDNs, RFC 3798 [37]).  All of
 these kinds of messages are notifications about a previous message,
 and they are sent to the reverse-path of the previous mail message.
 (If the delivery of such a notification message fails, that usually
 indicates a problem with the mail system of the host to which the
 notification message is addressed.  For this reason, at some hosts
 the MTA is set up to forward such failed notification messages to
 someone who is able to fix problems with the mail system, e.g., via
 the postmaster alias.)
 All other types of messages (i.e., any message which is not required
 by a Standards-Track RFC to have a null reverse-path) SHOULD be sent
 with a valid, non-null reverse-path.

Klensin Standards Track [Page 68] RFC 5321 SMTP October 2008

 Implementers of automated email processors should be careful to make
 sure that the various kinds of messages with a null reverse-path are
 handled correctly.  In particular, such systems SHOULD NOT reply to
 messages with a null reverse-path, and they SHOULD NOT add a non-null
 reverse-path, or change a null reverse-path to a non-null one, to
 such messages when forwarding.

5. Address Resolution and Mail Handling

5.1. Locating the Target Host

 Once an SMTP client lexically identifies a domain to which mail will
 be delivered for processing (as described in Sections 2.3.5 and 3.6),
 a DNS lookup MUST be performed to resolve the domain name (RFC 1035
 [2]).  The names are expected to be fully-qualified domain names
 (FQDNs): mechanisms for inferring FQDNs from partial names or local
 aliases are outside of this specification.  Due to a history of
 problems, SMTP servers used for initial submission of messages SHOULD
 NOT make such inferences (Message Submission Servers [18] have
 somewhat more flexibility) and intermediate (relay) SMTP servers MUST
 NOT make them.
 The lookup first attempts to locate an MX record associated with the
 name.  If a CNAME record is found, the resulting name is processed as
 if it were the initial name.  If a non-existent domain error is
 returned, this situation MUST be reported as an error.  If a
 temporary error is returned, the message MUST be queued and retried
 later (see Section 4.5.4.1).  If an empty list of MXs is returned,
 the address is treated as if it was associated with an implicit MX
 RR, with a preference of 0, pointing to that host.  If MX records are
 present, but none of them are usable, or the implicit MX is unusable,
 this situation MUST be reported as an error.
 If one or more MX RRs are found for a given name, SMTP systems MUST
 NOT utilize any address RRs associated with that name unless they are
 located using the MX RRs; the "implicit MX" rule above applies only
 if there are no MX records present.  If MX records are present, but
 none of them are usable, this situation MUST be reported as an error.
 When a domain name associated with an MX RR is looked up and the
 associated data field obtained, the data field of that response MUST
 contain a domain name.  That domain name, when queried, MUST return
 at least one address record (e.g., A or AAAA RR) that gives the IP
 address of the SMTP server to which the message should be directed.
 Any other response, specifically including a value that will return a
 CNAME record when queried, lies outside the scope of this Standard.
 The prohibition on labels in the data that resolve to CNAMEs is
 discussed in more detail in RFC 2181, Section 10.3 [38].

Klensin Standards Track [Page 69] RFC 5321 SMTP October 2008

 When the lookup succeeds, the mapping can result in a list of
 alternative delivery addresses rather than a single address, because
 of multiple MX records, multihoming, or both.  To provide reliable
 mail transmission, the SMTP client MUST be able to try (and retry)
 each of the relevant addresses in this list in order, until a
 delivery attempt succeeds.  However, there MAY also be a configurable
 limit on the number of alternate addresses that can be tried.  In any
 case, the SMTP client SHOULD try at least two addresses.
 Two types of information are used to rank the host addresses:
 multiple MX records, and multihomed hosts.
 MX records contain a preference indication that MUST be used in
 sorting if more than one such record appears (see below).  Lower
 numbers are more preferred than higher ones.  If there are multiple
 destinations with the same preference and there is no clear reason to
 favor one (e.g., by recognition of an easily reached address), then
 the sender-SMTP MUST randomize them to spread the load across
 multiple mail exchangers for a specific organization.
 The destination host (perhaps taken from the preferred MX record) may
 be multihomed, in which case the domain name resolver will return a
 list of alternative IP addresses.  It is the responsibility of the
 domain name resolver interface to have ordered this list by
 decreasing preference if necessary, and the SMTP sender MUST try them
 in the order presented.
 Although the capability to try multiple alternative addresses is
 required, specific installations may want to limit or disable the use
 of alternative addresses.  The question of whether a sender should
 attempt retries using the different addresses of a multihomed host
 has been controversial.  The main argument for using the multiple
 addresses is that it maximizes the probability of timely delivery,
 and indeed sometimes the probability of any delivery; the counter-
 argument is that it may result in unnecessary resource use.  Note
 that resource use is also strongly determined by the sending strategy
 discussed in Section 4.5.4.1.
 If an SMTP server receives a message with a destination for which it
 is a designated Mail eXchanger, it MAY relay the message (potentially
 after having rewritten the MAIL FROM and/or RCPT TO addresses), make
 final delivery of the message, or hand it off using some mechanism
 outside the SMTP-provided transport environment.  Of course, neither
 of the latter require that the list of MX records be examined
 further.
 If it determines that it should relay the message without rewriting
 the address, it MUST sort the MX records to determine candidates for

Klensin Standards Track [Page 70] RFC 5321 SMTP October 2008

 delivery.  The records are first ordered by preference, with the
 lowest-numbered records being most preferred.  The relay host MUST
 then inspect the list for any of the names or addresses by which it
 might be known in mail transactions.  If a matching record is found,
 all records at that preference level and higher-numbered ones MUST be
 discarded from consideration.  If there are no records left at that
 point, it is an error condition, and the message MUST be returned as
 undeliverable.  If records do remain, they SHOULD be tried, best
 preference first, as described above.

5.2. IPv6 and MX Records

 In the contemporary Internet, SMTP clients and servers may be hosted
 on IPv4 systems, IPv6 systems, or dual-stack systems that are
 compatible with either version of the Internet Protocol.  The host
 domains to which MX records point may, consequently, contain "A RR"s
 (IPv4), "AAAA RR"s (IPv6), or any combination of them.  While RFC
 3974 [39] discusses some operational experience in mixed
 environments, it was not comprehensive enough to justify
 standardization, and some of its recommendations appear to be
 inconsistent with this specification.  The appropriate actions to be
 taken either will depend on local circumstances, such as performance
 of the relevant networks and any conversions that might be necessary,
 or will be obvious (e.g., an IPv6-only client need not attempt to
 look up A RRs or attempt to reach IPv4-only servers).  Designers of
 SMTP implementations that might run in IPv6 or dual-stack
 environments should study the procedures above, especially the
 comments about multihomed hosts, and, preferably, provide mechanisms
 to facilitate operational tuning and mail interoperability between
 IPv4 and IPv6 systems while considering local circumstances.

6. Problem Detection and Handling

6.1. Reliable Delivery and Replies by Email

 When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
 message in response to DATA), it is accepting responsibility for
 delivering or relaying the message.  It must take this responsibility
 seriously.  It MUST NOT lose the message for frivolous reasons, such
 as because the host later crashes or because of a predictable
 resource shortage.  Some reasons that are not considered frivolous
 are discussed in the next subsection and in Section 7.8.
 If there is a delivery failure after acceptance of a message, the
 receiver-SMTP MUST formulate and mail a notification message.  This
 notification MUST be sent using a null ("<>") reverse-path in the
 envelope.  The recipient of this notification MUST be the address
 from the envelope return path (or the Return-Path: line).  However,

Klensin Standards Track [Page 71] RFC 5321 SMTP October 2008

 if this address is null ("<>"), the receiver-SMTP MUST NOT send a
 notification.  Obviously, nothing in this section can or should
 prohibit local decisions (i.e., as part of the same system
 environment as the receiver-SMTP) to log or otherwise transmit
 information about null address events locally if that is desired.  If
 the address is an explicit source route, it MUST be stripped down to
 its final hop.
 For example, suppose that an error notification must be sent for a
 message that arrived with:
    MAIL FROM:<@a,@b:user@d>
 The notification message MUST be sent using:
    RCPT TO:<user@d>
 Some delivery failures after the message is accepted by SMTP will be
 unavoidable.  For example, it may be impossible for the receiving
 SMTP server to validate all the delivery addresses in RCPT command(s)
 due to a "soft" domain system error, because the target is a mailing
 list (see earlier discussion of RCPT), or because the server is
 acting as a relay and has no immediate access to the delivering
 system.
 To avoid receiving duplicate messages as the result of timeouts, a
 receiver-SMTP MUST seek to minimize the time required to respond to
 the final <CRLF>.<CRLF> end of data indicator.  See RFC 1047 [40] for
 a discussion of this problem.

6.2. Unwanted, Unsolicited, and "Attack" Messages

 Utility and predictability of the Internet mail system requires that
 messages that can be delivered should be delivered, regardless of any
 syntax or other faults associated with those messages and regardless
 of their content.  If they cannot be delivered, and cannot be
 rejected by the SMTP server during the SMTP transaction, they should
 be "bounced" (returned with non-delivery notification messages) as
 described above.  In today's world, in which many SMTP server
 operators have discovered that the quantity of undesirable bulk email
 vastly exceeds the quantity of desired mail and in which accepting a
 message may trigger additional undesirable traffic by providing
 verification of the address, those principles may not be practical.
 As discussed in Section 7.8 and Section 7.9 below, dropping mail
 without notification of the sender is permitted in practice.
 However, it is extremely dangerous and violates a long tradition and
 community expectations that mail is either delivered or returned.  If

Klensin Standards Track [Page 72] RFC 5321 SMTP October 2008

 silent message-dropping is misused, it could easily undermine
 confidence in the reliability of the Internet's mail systems.  So
 silent dropping of messages should be considered only in those cases
 where there is very high confidence that the messages are seriously
 fraudulent or otherwise inappropriate.
 To stretch the principle of delivery if possible even further, it may
 be a rational policy to not deliver mail that has an invalid return
 address, although the history of the network is that users are
 typically better served by delivering any message that can be
 delivered.  Reliably determining that a return address is invalid can
 be a difficult and time-consuming process, especially if the putative
 sending system is not directly accessible or does not fully and
 accurately support VRFY and, even if a "drop messages with invalid
 return addresses" policy is adopted, it SHOULD be applied only when
 there is near-certainty that the return addresses are, in fact,
 invalid.
 Conversely, if a message is rejected because it is found to contain
 hostile content (a decision that is outside the scope of an SMTP
 server as defined in this document), rejection ("bounce") messages
 SHOULD NOT be sent unless the receiving site is confident that those
 messages will be usefully delivered.  The preference and default in
 these cases is to avoid sending non-delivery messages when the
 incoming message is determined to contain hostile content.

6.3. Loop Detection

 Simple counting of the number of "Received:" header fields in a
 message has proven to be an effective, although rarely optimal,
 method of detecting loops in mail systems.  SMTP servers using this
 technique SHOULD use a large rejection threshold, normally at least
 100 Received entries.  Whatever mechanisms are used, servers MUST
 contain provisions for detecting and stopping trivial loops.

6.4. Compensating for Irregularities

 Unfortunately, variations, creative interpretations, and outright
 violations of Internet mail protocols do occur; some would suggest
 that they occur quite frequently.  The debate as to whether a well-
 behaved SMTP receiver or relay should reject a malformed message,
 attempt to pass it on unchanged, or attempt to repair it to increase
 the odds of successful delivery (or subsequent reply) began almost
 with the dawn of structured network mail and shows no signs of
 abating.  Advocates of rejection claim that attempted repairs are
 rarely completely adequate and that rejection of bad messages is the
 only way to get the offending software repaired.  Advocates of
 "repair" or "deliver no matter what" argue that users prefer that

Klensin Standards Track [Page 73] RFC 5321 SMTP October 2008

 mail go through it if at all possible and that there are significant
 market pressures in that direction.  In practice, these market
 pressures may be more important to particular vendors than strict
 conformance to the standards, regardless of the preference of the
 actual developers.
 The problems associated with ill-formed messages were exacerbated by
 the introduction of the split-UA mail reading protocols (Post Office
 Protocol (POP) version 2 [15], Post Office Protocol (POP) version 3
 [16], IMAP version 2 [41], and PCMAIL [42]).  These protocols
 encouraged the use of SMTP as a posting (message submission)
 protocol, and SMTP servers as relay systems for these client hosts
 (which are often only intermittently connected to the Internet).
 Historically, many of those client machines lacked some of the
 mechanisms and information assumed by SMTP (and indeed, by the mail
 format protocol, RFC 822 [28]).  Some could not keep adequate track
 of time; others had no concept of time zones; still others could not
 identify their own names or addresses; and, of course, none could
 satisfy the assumptions that underlay RFC 822's conception of
 authenticated addresses.
 In response to these weak SMTP clients, many SMTP systems now
 complete messages that are delivered to them in incomplete or
 incorrect form.  This strategy is generally considered appropriate
 when the server can identify or authenticate the client, and there
 are prior agreements between them.  By contrast, there is at best
 great concern about fixes applied by a relay or delivery SMTP server
 that has little or no knowledge of the user or client machine.  Many
 of these issues are addressed by using a separate protocol, such as
 that defined in RFC 4409 [18], for message submission, rather than
 using originating SMTP servers for that purpose.
 The following changes to a message being processed MAY be applied
 when necessary by an originating SMTP server, or one used as the
 target of SMTP as an initial posting (message submission) protocol:
 o  Addition of a message-id field when none appears
 o  Addition of a date, time, or time zone when none appears
 o  Correction of addresses to proper FQDN format
 The less information the server has about the client, the less likely
 these changes are to be correct and the more caution and conservatism
 should be applied when considering whether or not to perform fixes
 and how.  These changes MUST NOT be applied by an SMTP server that
 provides an intermediate relay function.

Klensin Standards Track [Page 74] RFC 5321 SMTP October 2008

 In all cases, properly operating clients supplying correct
 information are preferred to corrections by the SMTP server.  In all
 cases, documentation SHOULD be provided in trace header fields and/or
 header field comments for actions performed by the servers.

7. Security Considerations

7.1. Mail Security and Spoofing

 SMTP mail is inherently insecure in that it is feasible for even
 fairly casual users to negotiate directly with receiving and relaying
 SMTP servers and create messages that will trick a naive recipient
 into believing that they came from somewhere else.  Constructing such
 a message so that the "spoofed" behavior cannot be detected by an
 expert is somewhat more difficult, but not sufficiently so as to be a
 deterrent to someone who is determined and knowledgeable.
 Consequently, as knowledge of Internet mail increases, so does the
 knowledge that SMTP mail inherently cannot be authenticated, or
 integrity checks provided, at the transport level.  Real mail
 security lies only in end-to-end methods involving the message
 bodies, such as those that use digital signatures (see RFC 1847 [43]
 and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [44] or Secure/
 Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [45]).
 Various protocol extensions and configuration options that provide
 authentication at the transport level (e.g., from an SMTP client to
 an SMTP server) improve somewhat on the traditional situation
 described above.  However, in general, they only authenticate one
 server to another rather than a chain of relays and servers, much
 less authenticating users or user machines.  Consequently, unless
 they are accompanied by careful handoffs of responsibility in a
 carefully designed trust environment, they remain inherently weaker
 than end-to-end mechanisms that use digitally signed messages rather
 than depending on the integrity of the transport system.
 Efforts to make it more difficult for users to set envelope return
 path and header "From" fields to point to valid addresses other than
 their own are largely misguided: they frustrate legitimate
 applications in which mail is sent by one user on behalf of another,
 in which error (or normal) replies should be directed to a special
 address, or in which a single message is sent to multiple recipients
 on different hosts.  (Systems that provide convenient ways for users
 to alter these header fields on a per-message basis should attempt to
 establish a primary and permanent mailbox address for the user so
 that Sender header fields within the message data can be generated
 sensibly.)

Klensin Standards Track [Page 75] RFC 5321 SMTP October 2008

 This specification does not further address the authentication issues
 associated with SMTP other than to advocate that useful functionality
 not be disabled in the hope of providing some small margin of
 protection against a user who is trying to fake mail.

7.2. "Blind" Copies

 Addresses that do not appear in the message header section may appear
 in the RCPT commands to an SMTP server for a number of reasons.  The
 two most common involve the use of a mailing address as a "list
 exploder" (a single address that resolves into multiple addresses)
 and the appearance of "blind copies".  Especially when more than one
 RCPT command is present, and in order to avoid defeating some of the
 purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
 the full set of RCPT command arguments into the header section,
 either as part of trace header fields or as informational or private-
 extension header fields.  Since this rule is often violated in
 practice, and cannot be enforced, sending SMTP systems that are aware
 of "bcc" use MAY find it helpful to send each blind copy as a
 separate message transaction containing only a single RCPT command.
 There is no inherent relationship between either "reverse" (from
 MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
 transaction ("envelope") and the addresses in the header section.
 Receiving systems SHOULD NOT attempt to deduce such relationships and
 use them to alter the header section of the message for delivery.
 The popular "Apparently-to" header field is a violation of this
 principle as well as a common source of unintended information
 disclosure and SHOULD NOT be used.

7.3. VRFY, EXPN, and Security

 As discussed in Section 3.5, individual sites may want to disable
 either or both of VRFY or EXPN for security reasons (see below).  As
 a corollary to the above, implementations that permit this MUST NOT
 appear to have verified addresses that are not, in fact, verified.
 If a site disables these commands for security reasons, the SMTP
 server MUST return a 252 response, rather than a code that could be
 confused with successful or unsuccessful verification.
 Returning a 250 reply code with the address listed in the VRFY
 command after having checked it only for syntax violates this rule.
 Of course, an implementation that "supports" VRFY by always returning
 550 whether or not the address is valid is equally not in
 conformance.
 On the public Internet, the contents of mailing lists have become
 popular as an address information source for so-called "spammers."

Klensin Standards Track [Page 76] RFC 5321 SMTP October 2008

 The use of EXPN to "harvest" addresses has increased as list
 administrators have installed protections against inappropriate uses
 of the lists themselves.  However, VRFY and EXPN are still useful for
 authenticated users and within an administrative domain.  For
 example, VRFY and EXPN are useful for performing internal audits of
 how email gets routed to check and to make sure no one is
 automatically forwarding sensitive mail outside the organization.
 Sites implementing SMTP authentication may choose to make VRFY and
 EXPN available only to authenticated requestors.  Implementations
 SHOULD still provide support for EXPN, but sites SHOULD carefully
 evaluate the tradeoffs.
 Whether disabling VRFY provides any real marginal security depends on
 a series of other conditions.  In many cases, RCPT commands can be
 used to obtain the same information about address validity.  On the
 other hand, especially in situations where determination of address
 validity for RCPT commands is deferred until after the DATA command
 is received, RCPT may return no information at all, while VRFY is
 expected to make a serious attempt to determine validity before
 generating a response code (see discussion above).

7.4. Mail Rerouting Based on the 251 and 551 Response Codes

 Before a client uses the 251 or 551 reply codes from a RCPT command
 to automatically update its future behavior (e.g., updating the
 user's address book), it should be certain of the server's
 authenticity.  If it does not, it may be subject to a man in the
 middle attack.

7.5. Information Disclosure in Announcements

 There has been an ongoing debate about the tradeoffs between the
 debugging advantages of announcing server type and version (and,
 sometimes, even server domain name) in the greeting response or in
 response to the HELP command and the disadvantages of exposing
 information that might be useful in a potential hostile attack.  The
 utility of the debugging information is beyond doubt.  Those who
 argue for making it available point out that it is far better to
 actually secure an SMTP server rather than hope that trying to
 conceal known vulnerabilities by hiding the server's precise identity
 will provide more protection.  Sites are encouraged to evaluate the
 tradeoff with that issue in mind; implementations SHOULD minimally
 provide for making type and version information available in some way
 to other network hosts.

Klensin Standards Track [Page 77] RFC 5321 SMTP October 2008

7.6. Information Disclosure in Trace Fields

 In some circumstances, such as when mail originates from within a LAN
 whose hosts are not directly on the public Internet, trace
 ("Received") header fields produced in conformance with this
 specification may disclose host names and similar information that
 would not normally be available.  This ordinarily does not pose a
 problem, but sites with special concerns about name disclosure should
 be aware of it.  Also, the optional FOR clause should be supplied
 with caution or not at all when multiple recipients are involved lest
 it inadvertently disclose the identities of "blind copy" recipients
 to others.

7.7. Information Disclosure in Message Forwarding

 As discussed in Section 3.4, use of the 251 or 551 reply codes to
 identify the replacement address associated with a mailbox may
 inadvertently disclose sensitive information.  Sites that are
 concerned about those issues should ensure that they select and
 configure servers appropriately.

7.8. Resistance to Attacks

 In recent years, there has been an increase of attacks on SMTP
 servers, either in conjunction with attempts to discover addresses
 for sending unsolicited messages or simply to make the servers
 inaccessible to others (i.e., as an application-level denial of
 service attack).  While the means of doing so are beyond the scope of
 this Standard, rational operational behavior requires that servers be
 permitted to detect such attacks and take action to defend
 themselves.  For example, if a server determines that a large number
 of RCPT TO commands are being sent, most or all with invalid
 addresses, as part of such an attack, it would be reasonable for the
 server to close the connection after generating an appropriate number
 of 5yz (normally 550) replies.

7.9. Scope of Operation of SMTP Servers

 It is a well-established principle that an SMTP server may refuse to
 accept mail for any operational or technical reason that makes sense
 to the site providing the server.  However, cooperation among sites
 and installations makes the Internet possible.  If sites take
 excessive advantage of the right to reject traffic, the ubiquity of
 email availability (one of the strengths of the Internet) will be
 threatened; considerable care should be taken and balance maintained
 if a site decides to be selective about the traffic it will accept
 and process.

Klensin Standards Track [Page 78] RFC 5321 SMTP October 2008

 In recent years, use of the relay function through arbitrary sites
 has been used as part of hostile efforts to hide the actual origins
 of mail.  Some sites have decided to limit the use of the relay
 function to known or identifiable sources, and implementations SHOULD
 provide the capability to perform this type of filtering.  When mail
 is rejected for these or other policy reasons, a 550 code SHOULD be
 used in response to EHLO (or HELO), MAIL, or RCPT as appropriate.

8. IANA Considerations

 IANA maintains three registries in support of this specification, all
 of which were created for RFC 2821 or earlier.  This document expands
 the third one as specified below.  The registry references listed are
 as of the time of publication; IANA does not guarantee the locations
 associated with the URLs.  The registries are as follows:
 o  The first, "Simple Mail Transfer Protocol (SMTP) Service
    Extensions" [46], consists of SMTP service extensions with the
    associated keywords, and, as needed, parameters and verbs.  As
    specified in Section 2.2.2, no entry may be made in this registry
    that starts in an "X".  Entries may be made only for service
    extensions (and associated keywords, parameters, or verbs) that
    are defined in Standards-Track or Experimental RFCs specifically
    approved by the IESG for this purpose.
 o  The second registry, "Address Literal Tags" [47], consists of
    "tags" that identify forms of domain literals other than those for
    IPv4 addresses (specified in RFC 821 and in this document).  The
    initial entry in that registry is for IPv6 addresses (specified in
    this document).  Additional literal types require standardization
    before being used; none are anticipated at this time.
 o  The third, "Mail Transmission Types" [46], established by RFC 821
    and renewed by this specification, is a registry of link and
    protocol identifiers to be used with the "via" and "with"
    subclauses of the time stamp ("Received:" header field) described
    in Section 4.4.  Link and protocol identifiers in addition to
    those specified in this document may be registered only by
    standardization or by way of an RFC-documented, IESG-approved,
    Experimental protocol extension.  This name space is for
    identification and not limited in size: the IESG is encouraged to
    approve on the basis of clear documentation and a distinct method
    rather than preferences about the properties of the method itself.
    An additional subsection has been added to the "VIA link types"
    and "WITH protocol types" subsections of this registry to contain
    registrations of "Additional-registered-clauses" as described
    above.  The registry will contain clause names, a description, a

Klensin Standards Track [Page 79] RFC 5321 SMTP October 2008

    summary of the syntax of the associated String, and a reference.
    As new clauses are defined, they may, in principle, specify
    creation of their own registries if the Strings consist of
    reserved terms or keywords rather than less restricted strings.
    As with link and protocol identifiers, additional clauses may be
    registered only by standardization or by way of an RFC-documented,
    IESG-approved, Experimental protocol extension.  The additional
    clause name space is for identification and is not limited in
    size: the IESG is encouraged to approve on the basis of clear
    documentation, actual use or strong signs that the clause will be
    used, and a distinct requirement rather than preferences about the
    properties of the clause itself.
 In addition, if additional trace header fields (i.e., in addition to
 Return-path and Received) are ever created, those trace fields MUST
 be added to the IANA registry established by BCP 90 (RFC 3864) [11]
 for use with RFC 5322 [4].

9. Acknowledgments

 Many people contributed to the development of RFC 2821.  That
 document should be consulted for those acknowledgments.  For the
 present document, the editor and the community owe thanks to Dawn
 Mann and Tony Hansen who assisted in the very painful process of
 editing and converting the internal format of the document from one
 system to another.
 Neither this document nor RFC 2821 would have been possible without
 the many contribution and insights of the late Jon Postel.  Those
 contributions of course include the original specification of SMTP in
 RFC 821.  A considerable quantity of text from RFC 821 still appears
 in this document as do several of Jon's original examples that have
 been updated only as needed to reflect other changes in the
 specification.
 Many people made comments or suggestions on the mailing list or in
 notes to the author.  Important corrections or clarifications were
 suggested by several people, including Matti Aarnio, Glenn Anderson,
 Derek J. Balling, Alex van den Bogaerdt, Stephane Bortzmeyer, Vint
 Cerf, Jutta Degener, Steve Dorner, Lisa Dusseault, Frank Ellerman,
 Ned Freed, Randy Gellens, Sabahattin Gucukoglu, Philip Guenther, Arnt
 Gulbrandsen, Eric Hall, Richard O. Hammer, Tony Hansen, Peter J.
 Holzer, Kari Hurtta, Bryon Roche Kain, Valdis Kletnieks, Mathias
 Koerber, John Leslie, Bruce Lilly, Jeff Macdonald, Mark E. Mallett,
 Mark Martinec, S. Moonesamy, Lyndon Nerenberg, Chris Newman, Douglas
 Otis, Pete Resnick, Robert A. Rosenberg, Vince Sabio, Hector Santos,
 David F. Skoll, Paul Smith, and Brett Watson.

Klensin Standards Track [Page 80] RFC 5321 SMTP October 2008

 The efforts of the Area Directors -- Lisa Dusseault, Ted Hardie, and
 Chris Newman -- to get this effort restarted and keep it moving, and
 of an ad hoc committee with the same purpose, are gratefully
 acknowledged.  The members of that committee were (in alphabetical
 order) Dave Crocker, Cyrus Daboo, Tony Finch, Ned Freed, Randall
 Gellens, Tony Hansen, the author, and Alexey Melnikov.  Tony Hansen
 also acted as ad hoc chair on the mailing list reviewing this
 document; without his efforts, sense of balance and fairness, and
 patience, it clearly would not have been possible.

10. References

10.1. Normative References

 [1]   Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
       August 1982.
 [2]   Mockapetris, P., "Domain names - implementation and
       specification", STD 13, RFC 1035, November 1987.
 [3]   Braden, R., "Requirements for Internet Hosts - Application and
       Support", STD 3, RFC 1123, October 1989.
 [4]   Resnick, P., "Internet Message Format", RFC 5322, October 2008.
 [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [6]   American National Standards Institute (formerly United States
       of America Standards Institute), "USA Code for Information
       Interchange", ANSI X3.4-1968, 1968.
       ANSI X3.4-1968 has been replaced by newer versions with slight
       modifications, but the 1968 version remains definitive for the
       Internet.
 [7]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [8]   Hinden, R. and S. Deering, "IP Version 6 Addressing
       Architecture", RFC 4291, February 2006.
 [9]   Newman, C., "ESMTP and LMTP Transmission Types Registration",
       RFC 3848, July 2004.
 [10]  Klensin, J., Freed, N., and K. Moore, "SMTP Service Extension
       for Message Size Declaration", STD 10, RFC 1870, November 1995.

Klensin Standards Track [Page 81] RFC 5321 SMTP October 2008

 [11]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
       Procedures for Message Header Fields", BCP 90, RFC 3864,
       September 2004.

10.2. Informative References

 [12]  Partridge, C., "Mail routing and the domain system", RFC 974,
       January 1986.
 [13]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
       Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
       November 1995.
 [14]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
       April 2001.
 [15]  Butler, M., Postel, J., Chase, D., Goldberger, J., and J.
       Reynolds, "Post Office Protocol: Version 2", RFC 937,
       February 1985.
 [16]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
       STD 53, RFC 1939, May 1996.
 [17]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
       4rev1", RFC 3501, March 2003.
 [18]  Gellens, R. and J. Klensin, "Message Submission for Mail",
       RFC 4409, April 2006.
 [19]  Freed, N., "SMTP Service Extension for Command Pipelining",
       STD 60, RFC 2920, September 2000.
 [20]  Vaudreuil, G., "SMTP Service Extensions for Transmission of
       Large and Binary MIME Messages", RFC 3030, December 2000.
 [21]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
       Extensions (MIME) Part One: Format of Internet Message Bodies",
       RFC 2045, November 1996.
 [22]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
       Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
       RFC 1652, July 1994.
 [23]  Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
       Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
       November 1996.

Klensin Standards Track [Page 82] RFC 5321 SMTP October 2008

 [24]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
       Extensions: Character Sets, Languages, and Continuations",
       RFC 2231, November 1997.
 [25]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463,
       January 2003.
 [26]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced Mail
       System Status Codes", BCP 138, RFC 5248, June 2008.
 [27]  Freed, N., "Behavior of and Requirements for Internet
       Firewalls", RFC 2979, October 2000.
 [28]  Crocker, D., "Standard for the format of ARPA Internet text
       messages", STD 11, RFC 822, August 1982.
 [29]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for
       Authorizing Use of Domains in E-Mail, Version 1", RFC 4408,
       April 2006.
 [30]  Fenton, J., "Analysis of Threats Motivating DomainKeys
       Identified Mail (DKIM)", RFC 4686, September 2006.
 [31]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and
       M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures",
       RFC 4871, May 2007.
 [32]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
       Extension for Delivery Status Notifications (DSNs)", RFC 3461,
       January 2003.
 [33]  Moore, K. and G. Vaudreuil, "An Extensible Message Format for
       Delivery Status Notifications", RFC 3464, January 2003.
 [34]  Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
       RFC 959, October 1985.
 [35]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
       between X.400 and RFC 822/MIME", RFC 2156, January 1998.
 [36]  De Winter, J., "SMTP Service Extension for Remote Message Queue
       Starting", RFC 1985, August 1996.
 [37]  Hansen, T. and G. Vaudreuil, "Message Disposition
       Notification", RFC 3798, May 2004.
 [38]  Elz, R. and R. Bush, "Clarifications to the DNS Specification",
       RFC 2181, July 1997.

Klensin Standards Track [Page 83] RFC 5321 SMTP October 2008

 [39]  Nakamura, M. and J. Hagino, "SMTP Operational Experience in
       Mixed IPv4/v6 Environments", RFC 3974, January 2005.
 [40]  Partridge, C., "Duplicate messages and SMTP", RFC 1047,
       February 1988.
 [41]  Crispin, M., "Interactive Mail Access Protocol: Version 2",
       RFC 1176, August 1990.
 [42]  Lambert, M., "PCMAIL: A distributed mail system for personal
       computers", RFC 1056, June 1988.
 [43]  Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security
       Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
       RFC 1847, October 1995.
 [44]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
       Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
 [45]  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
       (S/MIME) Version 3.1 Message Specification", RFC 3851,
       July 2004.
 [46]  Internet Assigned Number Authority (IANA), "IANA Mail
       Parameters", 2007,
       <http://www.iana.org/assignments/mail-parameters>.
 [47]  Internet Assigned Number Authority (IANA), "Address Literal
       Tags", 2007,
       <http://www.iana.org/assignments/address-literal-tags>.

Klensin Standards Track [Page 84] RFC 5321 SMTP October 2008

Appendix A. TCP Transport Service

 The TCP connection supports the transmission of 8-bit bytes.  The
 SMTP data is 7-bit ASCII characters.  Each character is transmitted
 as an 8-bit byte with the high-order bit cleared to zero.  Service
 extensions may modify this rule to permit transmission of full 8-bit
 data bytes as part of the message body, or, if specifically designed
 to do so, in SMTP commands or responses.

Appendix B. Generating SMTP Commands from RFC 822 Header Fields

 Some systems use an RFC 822 header section (only) in a mail
 submission protocol, or otherwise generate SMTP commands from RFC 822
 header fields when such a message is handed to an MTA from a UA.
 While the MTA-UA protocol is a private matter, not covered by any
 Internet Standard, there are problems with this approach.  For
 example, there have been repeated problems with proper handling of
 "bcc" copies and redistribution lists when information that
 conceptually belongs to the mail envelope is not separated early in
 processing from header field information (and kept separate).
 It is recommended that the UA provide its initial ("submission
 client") MTA with an envelope separate from the message itself.
 However, if the envelope is not supplied, SMTP commands SHOULD be
 generated as follows:
 1.  Each recipient address from a TO, CC, or BCC header field SHOULD
     be copied to a RCPT command (generating multiple message copies
     if that is required for queuing or delivery).  This includes any
     addresses listed in a RFC 822 "group".  Any BCC header fields
     SHOULD then be removed from the header section.  Once this
     process is completed, the remaining header fields SHOULD be
     checked to verify that at least one TO, CC, or BCC header field
     remains.  If none do, then a BCC header field with no additional
     information SHOULD be inserted as specified in [4].
 2.  The return address in the MAIL command SHOULD, if possible, be
     derived from the system's identity for the submitting (local)
     user, and the "From:" header field otherwise.  If there is a
     system identity available, it SHOULD also be copied to the Sender
     header field if it is different from the address in the From
     header field.  (Any Sender header field that was already there
     SHOULD be removed.)  Systems may provide a way for submitters to
     override the envelope return address, but may want to restrict
     its use to privileged users.  This will not prevent mail forgery,
     but may lessen its incidence; see Section 7.1.

Klensin Standards Track [Page 85] RFC 5321 SMTP October 2008

 When an MTA is being used in this way, it bears responsibility for
 ensuring that the message being transmitted is valid.  The mechanisms
 for checking that validity, and for handling (or returning) messages
 that are not valid at the time of arrival, are part of the MUA-MTA
 interface and not covered by this specification.
 A submission protocol based on Standard RFC 822 information alone
 MUST NOT be used to gateway a message from a foreign (non-SMTP) mail
 system into an SMTP environment.  Additional information to construct
 an envelope must come from some source in the other environment,
 whether supplemental header fields or the foreign system's envelope.
 Attempts to gateway messages using only their header "To" and "Cc"
 fields have repeatedly caused mail loops and other behavior adverse
 to the proper functioning of the Internet mail environment.  These
 problems have been especially common when the message originates from
 an Internet mailing list and is distributed into the foreign
 environment using envelope information.  When these messages are then
 processed by a header-section-only remailer, loops back to the
 Internet environment (and the mailing list) are almost inevitable.

Appendix C. Source Routes

 Historically, the <reverse-path> was a reverse source routing list of
 hosts and a source mailbox.  The first host in the <reverse-path> was
 historically the host sending the MAIL command; today, source routes
 SHOULD NOT appear in the reverse-path.  Similarly, the <forward-path>
 may be a source routing lists of hosts and a destination mailbox.
 However, in general, the <forward-path> SHOULD contain only a mailbox
 and domain name, relying on the domain name system to supply routing
 information if required.  The use of source routes is deprecated (see
 Appendix F.2); while servers MUST be prepared to receive and handle
 them as discussed in Section 3.3 and Appendix F.2, clients SHOULD NOT
 transmit them and this section is included in the current
 specification only to provide context.  It has been modified somewhat
 from the material in RFC 821 to prevent server actions that might
 confuse clients or subsequent servers that do not expect a full
 source route implementation.
 For relay purposes, the forward-path may be a source route of the
 form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST be fully-
 qualified domain names.  This form is used to emphasize the
 distinction between an address and a route.  The mailbox (here, JOE@
 THREE) is an absolute address, and the route is information about how
 to get there.  The two concepts should not be confused.
 If source routes are used, RFC 821 and the text below should be
 consulted for the mechanisms for constructing and updating the

Klensin Standards Track [Page 86] RFC 5321 SMTP October 2008

 forward-path.  A server that is reached by means of a source route
 (e.g., its domain name appears first in the list in the forward-path)
 MUST remove its domain name from any forward-paths in which that
 domain name appears before forwarding the message and MAY remove all
 other source routing information.  The reverse-path SHOULD NOT be
 updated by servers conforming to this specification.
 Notice that the forward-path and reverse-path appear in the SMTP
 commands and replies, but not necessarily in the message.  That is,
 there is no need for these paths and especially this syntax to appear
 in the "To:" , "From:", "CC:", etc. fields of the message header
 section.  Conversely, SMTP servers MUST NOT derive final message
 routing information from message header fields.
 When the list of hosts is present despite the recommendations above,
 it is a "reverse" source route and indicates that the mail was
 relayed through each host on the list (the first host in the list was
 the most recent relay).  This list is used as a source route to
 return non-delivery notices to the sender.  If, contrary to the
 recommendations here, a relay host adds itself to the beginning of
 the list, it MUST use its name as known in the transport environment
 to which it is relaying the mail rather than that of the transport
 environment from which the mail came (if they are different).  Note
 that a situation could easily arise in which some relay hosts add
 their names to the reverse source route and others do not, generating
 discontinuities in the routing list.  This is another reason why
 servers needing to return a message SHOULD ignore the source route
 entirely and simply use the domain as specified in the Mailbox.

Appendix D. Scenarios

 This section presents complete scenarios of several types of SMTP
 sessions.  In the examples, "C:" indicates what is said by the SMTP
 client, and "S:" indicates what is said by the SMTP server.

Klensin Standards Track [Page 87] RFC 5321 SMTP October 2008

D.1. A Typical SMTP Transaction Scenario

 This SMTP example shows mail sent by Smith at host bar.com, and to
 Jones, Green, and Brown at host foo.com.  Here we assume that host
 bar.com contacts host foo.com directly.  The mail is accepted for
 Jones and Brown.  Green does not have a mailbox at host foo.com.
    S: 220 foo.com Simple Mail Transfer Service Ready
    C: EHLO bar.com
    S: 250-foo.com greets bar.com
    S: 250-8BITMIME
    S: 250-SIZE
    S: 250-DSN
    S: 250 HELP
    C: MAIL FROM:<Smith@bar.com>
    S: 250 OK
    C: RCPT TO:<Jones@foo.com>
    S: 250 OK
    C: RCPT TO:<Green@foo.com>
    S: 550 No such user here
    C: RCPT TO:<Brown@foo.com>
    S: 250 OK
    C: DATA
    S: 354 Start mail input; end with <CRLF>.<CRLF>
    C: Blah blah blah...
    C: ...etc. etc. etc.
    C: .
    S: 250 OK
    C: QUIT
    S: 221 foo.com Service closing transmission channel

Klensin Standards Track [Page 88] RFC 5321 SMTP October 2008

D.2. Aborted SMTP Transaction Scenario

    S: 220 foo.com Simple Mail Transfer Service Ready
    C: EHLO bar.com
    S: 250-foo.com greets bar.com
    S: 250-8BITMIME
    S: 250-SIZE
    S: 250-DSN
    S: 250 HELP
    C: MAIL FROM:<Smith@bar.com>
    S: 250 OK
    C: RCPT TO:<Jones@foo.com>
    S: 250 OK
    C: RCPT TO:<Green@foo.com>
    S: 550 No such user here
    C: RSET
    S: 250 OK
    C: QUIT
    S: 221 foo.com Service closing transmission channel

Klensin Standards Track [Page 89] RFC 5321 SMTP October 2008

D.3. Relayed Mail Scenario

 Step 1 -- Source Host to Relay Host
 The source host performs a DNS lookup on XYZ.COM (the destination
 address) and finds DNS MX records specifying xyz.com as the best
 preference and foo.com as a lower preference.  It attempts to open a
 connection to xyz.com and fails.  It then opens a connection to
 foo.com, with the following dialogue:
    S: 220 foo.com Simple Mail Transfer Service Ready
    C: EHLO bar.com
    S: 250-foo.com greets bar.com
    S: 250-8BITMIME
    S: 250-SIZE
    S: 250-DSN
    S: 250 HELP
    C: MAIL FROM:<JQP@bar.com>
    S: 250 OK
    C: RCPT TO:<Jones@XYZ.COM>
    S: 250 OK
    C: DATA
    S: 354 Start mail input; end with <CRLF>.<CRLF>
    C: Date: Thu, 21 May 1998 05:33:29 -0700
    C: From: John Q. Public <JQP@bar.com>
    C: Subject: The Next Meeting of the Board
    C: To: Jones@xyz.com
    C:
    C: Bill:
    C: The next meeting of the board of directors will be
    C: on Tuesday.
    C: John.
    C: .
    S: 250 OK
    C: QUIT
    S: 221 foo.com Service closing transmission channel

Klensin Standards Track [Page 90] RFC 5321 SMTP October 2008

 Step 2 -- Relay Host to Destination Host
 foo.com, having received the message, now does a DNS lookup on
 xyz.com.  It finds the same set of MX records, but cannot use the one
 that points to itself (or to any other host as a worse preference).
 It tries to open a connection to xyz.com itself and succeeds.  Then
 we have:
         S: 220 xyz.com Simple Mail Transfer Service Ready
         C: EHLO foo.com
         S: 250 xyz.com is on the air
         C: MAIL FROM:<JQP@bar.com>
         S: 250 OK
         C: RCPT TO:<Jones@XYZ.COM>
         S: 250 OK
         C: DATA
         S: 354 Start mail input; end with <CRLF>.<CRLF>
         C: Received: from bar.com by foo.com ; Thu, 21 May 1998
         C:     05:33:29 -0700
         C: Date: Thu, 21 May 1998 05:33:22 -0700
         C: From: John Q. Public <JQP@bar.com>
         C: Subject:  The Next Meeting of the Board
         C: To: Jones@xyz.com
         C:
         C: Bill:
         C: The next meeting of the board of directors will be
         C: on Tuesday.
         C:                         John.
         C: .
         S: 250 OK
         C: QUIT
         S: 221 foo.com Service closing transmission channel

Klensin Standards Track [Page 91] RFC 5321 SMTP October 2008

D.4. Verifying and Sending Scenario

    S: 220 foo.com Simple Mail Transfer Service Ready
    C: EHLO bar.com
    S: 250-foo.com greets bar.com
    S: 250-8BITMIME
    S: 250-SIZE
    S: 250-DSN
    S: 250-VRFY
    S: 250 HELP
    C: VRFY Crispin
    S: 250 Mark Crispin <Admin.MRC@foo.com>
    C: MAIL FROM:<EAK@bar.com>
    S: 250 OK
    C: RCPT TO:<Admin.MRC@foo.com>
    S: 250 OK
    C: DATA
    S: 354 Start mail input; end with <CRLF>.<CRLF>
    C: Blah blah blah...
    C: ...etc. etc. etc.
    C: .
    S: 250 OK
    C: QUIT
    S: 221 foo.com Service closing transmission channel

Appendix E. Other Gateway Issues

 In general, gateways between the Internet and other mail systems
 SHOULD attempt to preserve any layering semantics across the
 boundaries between the two mail systems involved.  Gateway-
 translation approaches that attempt to take shortcuts by mapping
 (such as mapping envelope information from one system to the message
 header section or body of another) have generally proven to be
 inadequate in important ways.  Systems translating between
 environments that do not support both envelopes and a header section
 and Internet mail must be written with the understanding that some
 information loss is almost inevitable.

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Appendix F. Deprecated Features of RFC 821

 A few features of RFC 821 have proven to be problematic and SHOULD
 NOT be used in Internet mail.

F.1. TURN

 This command, described in RFC 821, raises important security issues
 since, in the absence of strong authentication of the host requesting
 that the client and server switch roles, it can easily be used to
 divert mail from its correct destination.  Its use is deprecated;
 SMTP systems SHOULD NOT use it unless the server can authenticate the
 client.

F.2. Source Routing

 RFC 821 utilized the concept of explicit source routing to get mail
 from one host to another via a series of relays.  The requirement to
 utilize source routes in regular mail traffic was eliminated by the
 introduction of the domain name system "MX" record and the last
 significant justification for them was eliminated by the
 introduction, in RFC 1123, of a clear requirement that addresses
 following an "@" must all be fully-qualified domain names.
 Consequently, the only remaining justifications for the use of source
 routes are support for very old SMTP clients or MUAs and in mail
 system debugging.  They can, however, still be useful in the latter
 circumstance and for routing mail around serious, but temporary,
 problems such as problems with the relevant DNS records.
 SMTP servers MUST continue to accept source route syntax as specified
 in the main body of this document and in RFC 1123.  They MAY, if
 necessary, ignore the routes and utilize only the target domain in
 the address.  If they do utilize the source route, the message MUST
 be sent to the first domain shown in the address.  In particular, a
 server MUST NOT guess at shortcuts within the source route.
 Clients SHOULD NOT utilize explicit source routing except under
 unusual circumstances, such as debugging or potentially relaying
 around firewall or mail system configuration errors.

F.3. HELO

 As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather
 than HELO when the server will accept the former.  Servers MUST
 continue to accept and process HELO in order to support older
 clients.

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F.4. #-literals

 RFC 821 provided for specifying an Internet address as a decimal
 integer host number prefixed by a pound sign, "#".  In practice, that
 form has been obsolete since the introduction of TCP/IP.  It is
 deprecated and MUST NOT be used.

F.5. Dates and Years

 When dates are inserted into messages by SMTP clients or servers
 (e.g., in trace header fields), four-digit years MUST BE used.  Two-
 digit years are deprecated; three-digit years were never permitted in
 the Internet mail system.

F.6. Sending versus Mailing

 In addition to specifying a mechanism for delivering messages to
 user's mailboxes, RFC 821 provided additional, optional, commands to
 deliver messages directly to the user's terminal screen.  These
 commands (SEND, SAML, SOML) were rarely implemented, and changes in
 workstation technology and the introduction of other protocols may
 have rendered them obsolete even where they are implemented.
 Clients SHOULD NOT provide SEND, SAML, or SOML as services.  Servers
 MAY implement them.  If they are implemented by servers, the
 implementation model specified in RFC 821 MUST be used and the
 command names MUST be published in the response to the EHLO command.

Author's Address

 John C. Klensin
 1770 Massachusetts Ave, Suite 322
 Cambridge, MA  02140
 USA
 EMail: john+smtp@jck.com

Klensin Standards Track [Page 94] RFC 5321 SMTP October 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
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 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
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