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rfc:rfc2821

Network Working Group J. Klensin, Editor Request for Comments: 2821 AT&T Laboratories Obsoletes: 821, 974, 1869 April 2001 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.

Copyright Notice

 Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

 This document is a self-contained specification of the basic protocol
 for the Internet electronic mail transport.  It consolidates, updates
 and clarifies, but doesn't add new or change existing functionality
 of the following:
  1. the original SMTP (Simple Mail Transfer Protocol) specification of

RFC 821 [30],

  1. domain name system requirements and implications for mail

transport from RFC 1035 [22] and RFC 974 [27],

  1. the clarifications and applicability statements in RFC 1123 [2],

and

  1. material drawn from the SMTP Extension mechanisms [19].
 It obsoletes RFC 821, RFC 974, and updates RFC 1123 (replaces 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.

Klensin Standards Track [Page 1] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 POP [3, 26]
 and IMAP [6].  Additional submission issues are discussed in RFC 2476
 [15].
 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 [32] discusses message headers, message bodies
 and formats and structures for them, and their relationship.

Table of Contents

 1. Introduction ..................................................  4
 2. The SMTP Model ................................................  5
 2.1 Basic Structure ..............................................  5
 2.2 The Extension Model ..........................................  7
 2.2.1 Background .................................................  7
 2.2.2 Definition and Registration of Extensions ..................  8
 2.3 Terminology ..................................................  9
 2.3.1 Mail Objects ............................................... 10
 2.3.2 Senders and Receivers ...................................... 10
 2.3.3 Mail Agents and Message Stores ............................. 10
 2.3.4 Host ....................................................... 11
 2.3.5 Domain ..................................................... 11
 2.3.6 Buffer and State Table ..................................... 11
 2.3.7 Lines ...................................................... 12
 2.3.8 Originator, Delivery, Relay, and Gateway Systems ........... 12
 2.3.9 Message Content and Mail Data .............................. 13
 2.3.10 Mailbox and Address ....................................... 13
 2.3.11 Reply ..................................................... 13
 2.4 General Syntax Principles and Transaction Model .............. 13
 3. The SMTP Procedures: An Overview .............................. 15
 3.1 Session Initiation ........................................... 15
 3.2 Client Initiation ............................................ 16
 3.3 Mail Transactions ............................................ 16
 3.4 Forwarding for Address Correction or Updating ................ 19

Klensin Standards Track [Page 2] RFC 2821 Simple Mail Transfer Protocol April 2001

 3.5 Commands for Debugging Addresses ............................. 20
 3.5.1 Overview ................................................... 20
 3.5.2 VRFY Normal Response ....................................... 22
 3.5.3 Meaning of VRFY or EXPN Success Response ................... 22
 3.5.4 Semantics and Applications of EXPN ......................... 23
 3.6 Domains ...................................................... 23
 3.7 Relaying ..................................................... 24
 3.8 Mail Gatewaying .............................................. 25
 3.8.1 Header Fields in Gatewaying ................................ 26
 3.8.2 Received Lines in Gatewaying ............................... 26
 3.8.3 Addresses in Gatewaying .................................... 26
 3.8.4 Other Header Fields in Gatewaying .......................... 27
 3.8.5 Envelopes in Gatewaying .................................... 27
 3.9 Terminating Sessions and Connections ......................... 27
 3.10 Mailing Lists and Aliases ................................... 28
 3.10.1 Alias ..................................................... 28
 3.10.2 List ...................................................... 28
 4. The SMTP Specifications ....................................... 29
 4.1 SMTP Commands ................................................ 29
 4.1.1 Command Semantics and Syntax ............................... 29
 4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO) ................... 29
 4.1.1.2 MAIL (MAIL) .............................................. 31
 4.1.1.3 RECIPIENT (RCPT) ......................................... 31
 4.1.1.4 DATA (DATA) .............................................. 33
 4.1.1.5 RESET (RSET) ............................................. 34
 4.1.1.6 VERIFY (VRFY) ............................................ 35
 4.1.1.7 EXPAND (EXPN) ............................................ 35
 4.1.1.8 HELP (HELP) .............................................. 35
 4.1.1.9 NOOP (NOOP) .............................................. 35
 4.1.1.10 QUIT (QUIT) ............................................. 36
 4.1.2 Command Argument Syntax .................................... 36
 4.1.3 Address Literals ........................................... 38
 4.1.4 Order of Commands .......................................... 39
 4.1.5 Private-use Commands ....................................... 40
 4.2  SMTP Replies ................................................ 40
 4.2.1 Reply Code Severities and Theory ........................... 42
 4.2.2 Reply Codes by Function Groups ............................. 44
 4.2.3  Reply Codes in Numeric Order .............................. 45
 4.2.4 Reply Code 502 ............................................. 46
 4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF> .... 46
 4.3 Sequencing of Commands and Replies ........................... 47
 4.3.1 Sequencing Overview ........................................ 47
 4.3.2 Command-Reply Sequences .................................... 48
 4.4 Trace Information ............................................ 49
 4.5 Additional Implementation Issues ............................. 53
 4.5.1 Minimum Implementation ..................................... 53
 4.5.2 Transparency ............................................... 53
 4.5.3 Sizes and Timeouts ......................................... 54

Klensin Standards Track [Page 3] RFC 2821 Simple Mail Transfer Protocol April 2001

 4.5.3.1 Size limits and minimums ................................. 54
 4.5.3.2 Timeouts ................................................. 56
 4.5.4 Retry Strategies ........................................... 57
 4.5.4.1 Sending Strategy ......................................... 58
 4.5.4.2 Receiving Strategy ....................................... 59
 4.5.5 Messages with a null reverse-path .......................... 59
 5. Address Resolution and Mail Handling .......................... 60
 6. Problem Detection and Handling ................................ 62
 6.1 Reliable Delivery and Replies by Email ....................... 62
 6.2 Loop Detection ............................................... 63
 6.3 Compensating for Irregularities .............................. 63
 7. Security Considerations ....................................... 64
 7.1 Mail Security and Spoofing ................................... 64
 7.2 "Blind" Copies ............................................... 65
 7.3 VRFY, EXPN, and Security ..................................... 65
 7.4 Information Disclosure in Announcements ...................... 66
 7.5 Information Disclosure in Trace Fields ....................... 66
 7.6 Information Disclosure in Message Forwarding ................. 67
 7.7 Scope of Operation of SMTP Servers ........................... 67
 8. IANA Considerations ........................................... 67
 9. References .................................................... 68
 10. Editor's Address ............................................. 70
 11. Acknowledgments .............................................. 70
 Appendices ....................................................... 71
 A. TCP Transport Service ......................................... 71
 B. Generating SMTP Commands from RFC 822 Headers ................. 71
 C. Source Routes ................................................. 72
 D. Scenarios ..................................................... 73
 E. Other Gateway Issues .......................................... 76
 F. Deprecated Features of RFC 821 ................................ 76
 Full Copyright Statement ......................................... 79

1. Introduction

 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 describe some of them.
 An important feature of SMTP is its capability to transport mail
 across networks, usually referred to as "SMTP mail relaying" (see
 section 3.8).  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

Klensin Standards Track [Page 4] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 [22, 27] (and
 section 5 of this document) are used to identify the appropriate
 next-hop destination for a message being transported.

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 domain name(s) to which mail messages
 are to be transferred is a local matter, and is not addressed by this
 document.  In some cases, the domain name(s) transferred to, or
 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 [3, 26] or IMAP [6]
 protocols, or when the SMTP client is inside an isolated transport
 service environment, the domain name 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 domain names 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

Klensin Standards Track [Page 5] RFC 2821 Simple Mail Transfer Protocol April 2001

 ones, and their destinations, are expected to support all of the
 queuing, retrying, and alternate address functions discussed in this
 specification.
 The means by which an SMTP client, once it has determined a target
 domain name, 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, a formal handoff of responsibility for the message occurs: the
 protocol requires that a server accept responsibility for either
 delivering a message or properly reporting the failure to do so.
 Once the transmission channel is established and initial handshaking
 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 headers 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 extension requests such as command
 pipelining [13].

Klensin Standards Track [Page 6] RFC 2821 Simple Mail Transfer Protocol April 2001

 Once a given mail message has been transmitted, the client may either
 request that the connection be shut down or may initiate other mail
 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.  This transmission normally occurs 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.  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 appendices C and
 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
 added, it must be done in a way that permits older implementations to
 continue working acceptably.  The extension framework consists of:

Klensin Standards Track [Page 7] RFC 2821 Simple Mail Transfer Protocol April 2001

  1. The SMTP command EHLO, superseding the earlier HELO,
  1. a registry of SMTP service extensions,
  1. additional parameters to the SMTP MAIL and RCPT commands, and
  1. optional replacements for commands defined in this protocol, such

as for DATA in non-ASCII transmissions [33].

 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:
  1. the textual name of the SMTP service extension;
  1. the EHLO keyword value associated with the extension;
  1. the syntax and possible values of parameters associated with the

EHLO keyword value;

  1. 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);
  1. any new parameters the extension associates with the MAIL or RCPT

verbs;

  1. a description of how support for the extension affects the

behavior of a server and client SMTP; and,

  1. the increment by which the extension is increasing the maximum

length of the commands MAIL and/or RCPT, over that specified in

    this standard.

Klensin Standards Track [Page 8] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.

2.3 Terminology

 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 below.
 1. MUST   This word, or the terms "REQUIRED" or "SHALL", mean that
    the definition is an absolute requirement of the specification.
 2. MUST NOT   This phrase, or the phrase "SHALL NOT", mean that the
    definition is an absolute prohibition of the specification.
 3. SHOULD   This word, or the adjective "RECOMMENDED", mean that
    there may exist valid reasons in particular circumstances to
    ignore a particular item, but the full implications must be
    understood and carefully weighed before choosing a different
    course.
 4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED" mean
    that there may exist valid reasons in particular circumstances
    when the particular behavior is acceptable or even useful, but the
    full implications should be understood and the case carefully
    weighed before implementing any behavior described with this
    label.
 5. MAY   This word, or the adjective "OPTIONAL", mean that an item is
    truly optional.  One vendor may choose to include the item because
    a particular marketplace requires it or because the vendor feels
    that it enhances the product while another vendor may omit the
    same item.  An implementation which does not include a particular
    option MUST be prepared to interoperate with another
    implementation which does include the option, though perhaps with
    reduced functionality.  In the same vein an implementation which

Klensin Standards Track [Page 9] RFC 2821 Simple Mail Transfer Protocol April 2001

    does include a particular option MUST be prepared to interoperate
    with another implementation which does not include the option
    (except, of course, for the feature the option provides.)

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
 which error reports should be directed); one or more recipient
 addresses; and optional protocol extension material.  Historically,
 variations on the recipient address specification command (RCPT TO)
 could be used to specify alternate delivery modes, such as immediate
 display; those variations have now been deprecated (see appendix F,
 section F.6).
 The SMTP content is sent in the SMTP DATA protocol unit and has two
 parts:  the headers and the body.  If the content conforms to other
 contemporary standards, the headers form a collection of field/value
 pairs structured as in the message format specification [32]; the
 body, if structured, is defined according to MIME [12].  The content
 is textual in nature, expressed using the US-ASCII repertoire [1].
 Although SMTP extensions (such as "8BITMIME" [20]) may relax this
 restriction for the content body, the content headers are always
 encoded using the US-ASCII repertoire.  A MIME extension [23] defines
 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

Klensin Standards Track [Page 10] RFC 2821 Simple Mail Transfer Protocol April 2001

 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
 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 "domain"); identifying them by numerical address is discouraged.

2.3.5 Domain

 A domain (or domain name) consists of one or more dot-separated
 components.  These components ("labels" in DNS terminology [22]) are
 restricted for SMTP purposes to consist of a sequence of letters,
 digits, and hyphens drawn from the ASCII character set [1].  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 [22] and
 section 5 of this specification.
 The domain name, as described in this document and in [22], 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.

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 which
 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.

Klensin Standards Track [Page 11] RFC 2821 Simple Mail Transfer Protocol April 2001

2.3.7 Lines

 SMTP commands and, unless altered by a service extension, message
 data, are transmitted in "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 4.5.3).
 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.

2.3.8 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
 which 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 [11]).

Klensin Standards Track [Page 12] RFC 2821 Simple Mail Transfer Protocol April 2001

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 message headers and the
 possibly-structured message body.  The MIME specification [12]
 provides the standard mechanisms for structured message bodies.

2.3.10 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
 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.

2.3.11 Reply

 An SMTP reply is an acknowledgment (positive or negative) sent 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.  Recent work [34] has specified further structuring
 of the reply strings, including the use of supplemental and more
 specific completion codes.

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 MUST follow 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.

Klensin Standards Track [Page 13] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.  This
 is NOT true of a mailbox local-part.  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.  Mailbox
 domains are not case sensitive.  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.
 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 field 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 [1].  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 seven bit
 transport only.  An originating SMTP client which has not
 successfully negotiated an appropriate extension with a particular
 server 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 ultimate delivery of a
 severely garbled message to the recipient.  Delivery SMTP systems MAY
 reject ("bounce") such messages rather than deliver them.  No sending
 SMTP system is permitted to send envelope commands in any character

Klensin Standards Track [Page 14] RFC 2821 Simple Mail Transfer Protocol April 2001

 set other than US-ASCII; receiving systems SHOULD reject such
 commands, normally using "500 syntax error - invalid character"
 replies.
 Eight-bit message content transmission MAY be requested of the server
 by a client using extended SMTP facilities, notably the "8BITMIME"
 extension [20].  8BITMIME SHOULD be supported by SMTP servers.
 However, it MUST not be construed as authorization to transmit
 unrestricted eight bit material.  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 [8].  Metalanguage terms used in running text are
 surrounded by pointed brackets (e.g., <CRLF>) for clarity.

3. The SMTP Procedures: An Overview

 This section contains descriptions of the procedures used in SMTP:
 session initiation, the mail transaction, forwarding mail, verifying
 mailbox names and expanding mailing lists, and the 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.

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 4.5.1).
 The SMTP protocol allows a server to formally reject a transaction
 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

Klensin Standards Track [Page 15] RFC 2821 Simple Mail Transfer Protocol April 2001

 "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 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 which are unable to
 support service extensions and contemporary clients which 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").

3.3 Mail Transactions

 There are three steps to SMTP mail transactions.  The transaction
 starts with a MAIL command which 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

Klensin Standards Track [Page 16] RFC 2821 Simple Mail Transfer Protocol April 2001

 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> can 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.
    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 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).
 This step of the procedure can be repeated any number of times.
 The <forward-path> can contain more than just a mailbox.
 Historically, the <forward-path> can be 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 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).
 The third step in the procedure is the DATA command (or some
 alternative specified in a service extension).

Klensin Standards Track [Page 17] RFC 2821 Simple Mail Transfer Protocol April 2001

    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
 data.  If accepted, the SMTP server returns a 250 OK reply.  The DATA
 command can fail at only two points in the protocol exchange:
  1. 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.
  1. 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), or 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.  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 RFC 822 format [7, 32] is being used, the mail data include the
 memo header items such as Date, Subject, To, Cc, From.  Server SMTP
 systems SHOULD NOT reject messages based on perceived defects in the
 RFC 822 or MIME [12] message header or message body.  In particular,

Klensin Standards Track [Page 18] RFC 2821 Simple Mail Transfer Protocol April 2001

 they MUST NOT reject messages in which the numbers of Resent-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 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
 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.
 In particular:
  • 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.  But, 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,
  • Servers MAY reject or bounce messages when they are not

deliverable when 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.  But, 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 are strongly encouraged to provide configuration mechanisms so
 that sites which conclude that they would undesirably disclose
 information can disable or restrict their use.

Klensin Standards Track [Page 19] RFC 2821 Simple Mail Transfer Protocol April 2001

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 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
 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
 response 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 [34],
 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

Klensin Standards Track [Page 20] RFC 2821 Simple Mail Transfer Protocol April 2001

 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
 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.

Klensin Standards Track [Page 21] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 normally includes the mailbox name, i.e.,
 "<local-part@domain>", where "domain" is a fully qualified domain
 name, MUST appear in the syntax.  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
 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.  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, they MUST be listed as
 service extensions in an EHLO response, if they are supported.

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.

Klensin Standards Track [Page 22] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 discussed 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 bounced
 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.

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 Domains

 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 A RRs (as discussed in section 5) are
 permitted, as are CNAME RRs whose targets can be resolved, in turn,
 to MX or A RRs.  Local nicknames or unqualified names MUST NOT be
 used.  There are two exceptions to the rule requiring FQDNs:
  1. The domain name given in the EHLO command MUST BE either a primary

host name (a domain name that resolves to an A RR) or, if the host

    has no name, an address literal as described in section 4.1.1.1.
  1. 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.

Klensin Standards Track [Page 23] RFC 2821 Simple Mail Transfer Protocol April 2001

3.7 Relaying

 In general, the availability of Mail eXchanger records in the domain
 name system [22, 27] makes the use of explicit source routes in the
 Internet mail system unnecessary.  Many historical problems with
 their interpretation 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 are also permitted to 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.
 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
 relay mail to a particular address for policy reasons, a 550 response
 SHOULD be returned.
 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, and work is underway on standardized mail
 submission protocols that might eventually supercede the current
 practices.  In any event, because these arrangements are private and
 fall outside the scope of this specification, they are not described
 here.

Klensin Standards Track [Page 24] RFC 2821 Simple Mail Transfer Protocol April 2001

 It is important to note that MX records can point to SMTP servers
 which act as gateways into other environments, not just SMTP relays
 and final delivery systems; see sections 3.8 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, [24, 25]) 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 section
 4.5.5 for additional discussion).  A MAIL command with a null
 reverse-path appears as follows:
    MAIL FROM:<>
 As discussed in section 2.4.1, a relay SMTP has no need to inspect or
 act upon the headers or body of the message data and MUST NOT do so
 except to add its own "Received:" header (section 4.4) and,
 optionally, to attempt to detect looping in the mail system (see
 section 6.2).

3.8 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 2.3.8, 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.

Klensin Standards Track [Page 25] RFC 2821 Simple Mail Transfer Protocol April 2001

3.8.1 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 2.4.1.
 Other mail systems gatewayed to the Internet often use a subset of
 RFC 822 headers 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.  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).

3.8.2 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.
 "Received:" 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
 fields arising in non-SMTP environments, receiving systems MUST NOT
 reject mail based on the format of a trace field and SHOULD be
 extremely robust in the light of unexpected information or formats in
 those fields.
 The gateway SHOULD indicate the environment and protocol in the "via"
 clauses of Received field(s) that it supplies.

3.8.3 Addresses in Gatewaying

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

Klensin Standards Track [Page 26] RFC 2821 Simple Mail Transfer Protocol April 2001

3.8.4 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., fields MUST be transformed (if necessary) to satisfy RFC
 822 syntax, 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 return path from the
 SMTP envelope, not to the sender listed in the "From:" field (or
 other fields) of the RFC 822 message.

3.8.5 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.

3.9 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 except:
  1. After receiving a QUIT command and responding with a 221 reply.
  1. 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).
 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.

Klensin Standards Track [Page 27] RFC 2821 Simple Mail Transfer Protocol April 2001

 An SMTP server which 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.

3.10 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 [32] MUST be left
 unchanged; in particular, the "From" field of the message header 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.

3.10.1 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.

3.10.2 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 all of the expanded

Klensin Standards Track [Page 28] RFC 2821 Simple Mail Transfer Protocol April 2001

 addresses.  The return 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.

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 are encouraged to 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 which 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 which 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 field 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 29] RFC 2821 Simple Mail Transfer Protocol April 2001

 available), the client SHOULD send an address literal (see section
 4.1.3), optionally followed by information that will help to identify
 the client system.  y 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 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
 keyworks 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
 [8], is:
    ehlo-ok-rsp  =    ( "250"    domain [ SP ehlo-greet ] CRLF )
                 / (    "250-"   domain [ SP ehlo-greet ] CRLF
                     *( "250-"   ehlo-line                CRLF )
                        "250"    SP ehlo-line             CRLF  )
    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

Klensin Standards Track [Page 30] RFC 2821 Simple Mail Transfer Protocol April 2001

    ehlo-param   = 1*(%d33-127)
                 ; any CHAR excluding <SP> and all
                 ; control characters (US-ASCII 0-31 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 2.4.1.

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 which may, in turn, deliver it to
 one or more mailboxes or pass it on to another system (possibly using
 SMTP).  The argument field 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 nondelivery notifications), the
 reverse-path may be null (see section 3.7).
 This command clears the reverse-path buffer, the forward-path buffer,
 and the mail data buffer; and inserts the reverse-path information
 from this command into the reverse-path buffer.
 If service extensions were negotiated, the MAIL command may also
 carry parameters associated with a particular service extension.
 Syntax:
    "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 use of this
 command.  The argument field 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

Klensin Standards Track [Page 31] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.
 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>
 or to jkl.org, using the envelope commands
    MAIL FROM:<userx@y.foo.org>
    RCPT TO:<@jkl.org:userc@d.bar.org>
 Of course, 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 TO:" ("<Postmaster@" domain ">" / "<Postmaster>" / Forward-Path)
                  [SP Rcpt-parameters] CRLF

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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 is terminated by a line containing only a period, that
 is, the character sequence "<CRLF>.<CRLF>" (see section 4.5.2).  This
 is the end of mail data indication.  Note that 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 data, ends the DATA
 command itself.  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 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
 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.

Klensin Standards Track [Page 33] RFC 2821 Simple Mail Transfer Protocol April 2001

 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" CRLF

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., if 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" CRLF

Klensin Standards Track [Page 34] RFC 2821 Simple Mail Transfer Protocol April 2001

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" SP String CRLF

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 may be issued at any time.
 Syntax:
    "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 may be issued at any time.
 SMTP servers SHOULD support HELP without arguments and MAY support it
 with arguments.
 Syntax:
    "HELP" [ SP String ] CRLF

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
 an OK reply.

Klensin Standards Track [Page 35] RFC 2821 Simple Mail Transfer Protocol April 2001

 This command has no effect on the reverse-path buffer, the forward-
 path buffer, or the mail data buffer and may be issued at any time.
 If a parameter string is specified, servers SHOULD ignore it.
 Syntax:
    "NOOP" [ SP String ] CRLF

4.1.1.10 QUIT (QUIT)

 This command specifies that the receiver MUST send an 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 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.
 Syntax:
    "QUIT" CRLF

4.1.2 Command Argument Syntax

 The syntax of the argument fields of the above commands (using the
 syntax specified in [8] 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 [8 (section 6)] or in the message format syntax
 [32].
    Reverse-path = Path
    Forward-path = Path
    Path = "<" [ A-d-l ":" ] Mailbox ">"
    A-d-l = At-domain *( "," A-d-l )
          ; 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)

Klensin Standards Track [Page 36] RFC 2821 Simple Mail Transfer Protocol April 2001

    esmtp-param     = esmtp-keyword ["=" esmtp-value]
    esmtp-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
    esmtp-value     = 1*(%d33-60 / %d62-127)
          ; any CHAR excluding "=", SP, and control characters
    Keyword  = Ldh-str
    Argument = Atom
    Domain = (sub-domain 1*("." sub-domain)) / address-literal
    sub-domain = Let-dig [Ldh-str]
    address-literal = "[" IPv4-address-literal /
                          IPv6-address-literal /
                          General-address-literal "]"
          ; See section 4.1.3
    Mailbox = Local-part "@" Domain
    Local-part = Dot-string / Quoted-string
          ; MAY be case-sensitive
    Dot-string = Atom *("." Atom)
    Atom = 1*atext
    Quoted-string = DQUOTE *qcontent DQUOTE
    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
 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 field with the comma being the fourth
 character of the field.

Klensin Standards Track [Page 37] RFC 2821 Simple Mail Transfer Protocol April 2001

 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, RFC1035 [22]),
 characters outside the set of alphas, 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.

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 IPv6 standards [17].
 Specifically:
    IPv4-address-literal = Snum 3("." Snum)
    IPv6-address-literal = "IPv6:" IPv6-addr
    General-address-literal = Standardized-tag ":" 1*dcontent
    Standardized-tag = Ldh-str
          ; MUST be specified in a standards-track RFC
          ; and registered with IANA
    Snum = 1*3DIGIT  ; representing a decimal integer
          ; value in the range 0 through 255
    Let-dig = ALPHA / DIGIT
    Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
    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)] "::"

Klensin Standards Track [Page 38] RFC 2821 Simple Mail Transfer Protocol April 2001

                 [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, 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,
 or 502 failure replies MUST be returned as appropriate.  The 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 valid principal host name (not a CNAME or MX
 name) for its host.  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 and supplemental information provided that will assist in
 identifying the client.
 An SMTP server MAY verify that the domain name parameter in the EHLO
 command actually corresponds to the IP address of the client.
 However, the server MUST NOT refuse to accept a message for this
 reason if the verification fails: the information about verification
 failure is for logging and tracing only.
 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.

Klensin Standards Track [Page 39] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 (or a new EHLO) 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 it
 the previous one successfully concluded with a successful DATA
 command, or if the previous one was aborted with a RSET.
 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
 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 cannot be used at any other time in a session, but 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.

Klensin Standards Track [Page 40] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 section 4.2.1).  Since, in violation of this
 specification, the text is sometimes not sent, clients which 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
 circumstances, however, multiline replies are allowed for any
 command.
 In ABNF, server responses are:
    Greeting = "220 " Domain [ SP text ] CRLF
    Reply-line = Reply-code [ SP text ] CRLF
 where "Greeting" appears only in the 220 response that announces that
 the server is opening its part of the connection.
 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.

Klensin Standards Track [Page 41] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.

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 five values for the first digit of the reply code:
 1yz   Positive Preliminary reply
    The command has been accepted, but the requested action is being
    held in abeyance, pending confirmation of the information in this
    reply.  The SMTP client should send another command specifying
    whether to continue or abort the action.  Note: unextended SMTP
    does not have any commands that allow this type of reply, and so
    does not have continue or abort commands.
 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

Klensin Standards Track [Page 42] RFC 2821 Simple Mail Transfer Protocol April 2001

    sender-SMTP agents) must agree on the interpretation.  Each reply
    in this category might have a different time value, but the SMTP
    client is encouraged to 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 is discouraged from repeating 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 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).
 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.

Klensin Standards Track [Page 43] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.
 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:
    123-First line
    123-Second line
    123-234 text beginning with numbers
    123 The last line
 In many cases the SMTP client then simply needs to search for a line
 beginning with the reply code followed by <SP> or <CRLF> and ignore
 all preceding lines.  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 44] RFC 2821 Simple Mail Transfer Protocol April 2001

    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)
    450 Requested mail action not taken: mailbox unavailable
       (e.g., mailbox busy)
    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")

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>

Klensin Standards Track [Page 45] RFC 2821 Simple Mail Transfer Protocol April 2001

    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)
    451 Requested action aborted: local error in processing
    452 Requested action not taken: insufficient system storage
    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)
    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")

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:
  1. delivering the message (if the recipient mailbox exists), or
  1. 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.

Klensin Standards Track [Page 46] RFC 2821 Simple Mail Transfer Protocol April 2001

  1. 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 permanent error status (5yz) code after
 the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
 any subsequent attempt to deliver that message.  The SMTP client
 retains responsibility for 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).
 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.  I.e., 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 completely 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 and intervention of the message.

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 4.1.3 for a discussion of alternatives in these
 situations.

Klensin Standards Track [Page 47] RFC 2821 Simple Mail Transfer Protocol April 2001

 For example,
    220 ISIF.USC.EDU Service ready
 or
    220 mail.foo.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
 substitute text in the replies, but the meaning and action implied by
 the code numbers and by the specific command reply sequence cannot be
 altered.

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.
 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.
 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

Klensin Standards Track [Page 48] RFC 2821 Simple Mail Transfer Protocol April 2001

 Specific sequences are:
 CONNECTION ESTABLISHMENT
    S: 220
    E: 554
 EHLO or HELO
    S: 250
    E: 504, 550
 MAIL
    S: 250
    E: 552, 451, 452, 550, 553, 503
 RCPT
    S: 250, 251 (but see section 3.4 for discussion of 251 and 551)
    E: 550, 551, 552, 553, 450, 451, 452, 503, 550
 DATA
    I: 354 -> data -> S: 250
                      E: 552, 554, 451, 452
    E: 451, 554, 503
 RSET
    S: 250
 VRFY
    S: 250, 251, 252
    E: 550, 551, 553, 502, 504
 EXPN
    S: 250, 252
    E: 550, 500, 502, 504
 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:
  1. The FROM field, 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.

Klensin Standards Track [Page 49] RFC 2821 Simple Mail Transfer Protocol April 2001

  1. The ID field MAY contain an "@" as suggested in RFC 822, but this

is not required.

  1. The FOR field MAY contain a list of <path> entries when multiple

RCPT commands have been given. This may raise some security

    issues and is usually not desirable; see section 7.2.
 An Internet mail program MUST NOT change a Received: line that was
 previously added to the message header.  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 fields is important
 for detecting problems, especially slow relays.  SMTP servers that
 create Received fields SHOULD use explicit offsets in the dates
 (e.g., -0800), rather than time zone names of any type.  Local time
 (with an offset) is preferred to UT when feasible.  This formulation
 allows slightly more information about local circumstances to be
 specified.  If UT is needed, the 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 mail data headers and body [32].
 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,

Klensin Standards Track [Page 50] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.  SMTP servers performing a
 relay function MUST NOT inspect the message data, and especially not
 to the extent needed to determine if Return-path headers are present.
 SMTP servers making final delivery MAY remove Return-path headers
 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.
 Historical note: Text in RFC 822 that appears to contradict the use
 of the Return-path header (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:
  1. a gateway from SMTP→elsewhere SHOULD insert a return-path header,

unless it is known that the "elsewhere" transport also uses

    Internet domain addresses and maintains the envelope sender
    address separately.
  1. a gateway from elsewhere→SMTP SHOULD delete any return-path

header 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.

Klensin Standards Track [Page 51] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 is
 preferred when possible.  All undeliverable mail notification
 messages are sent using the MAIL command (even if they result from
 processing the obsolete SEND, SOML, or SAML commands) and use a null
 return path as discussed in section 3.7.
 The time stamp line and the return path line are formally defined as
 follows:

Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>

Time-stamp-line = "Received:" FWS Stamp <CRLF>

Stamp = From-domain By-domain Opt-info ";" FWS date-time

    ; where "date-time" is as defined in [32]
    ; but the "obs-" forms, especially two-digit
    ; years, are prohibited in SMTP and MUST NOT be used.

From-domain = "FROM" FWS Extended-Domain CFWS

By-domain = "BY" FWS Extended-Domain CFWS

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]

Via = "VIA" FWS Link CFWS

With = "WITH" FWS Protocol CFWS

ID = "ID" FWS String / msg-id CFWS

For = "FOR" FWS 1*( Path / Mailbox ) CFWS

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

Klensin Standards Track [Page 52] RFC 2821 Simple Mail Transfer Protocol April 2001

       ; servers SHOULD NOT use unregistered names.

Protocol = "ESMTP" / "SMTP" / Attdl-Protocol Attdl-Protocol = Atom

    ; Additional standard names for protocols are registered with the
       ; Internet Assigned Numbers Authority (IANA).  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
 is required for 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 which 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 which are not part of such attacks.

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

Klensin Standards Track [Page 53] RFC 2821 Simple Mail Transfer Protocol April 2001

 allow all user composed text to be transmitted transparently, the
 following procedures are used:
  1. 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.
  1. 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 3.7 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 [16] 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 which impose no limits on the
 length of these objects should be used.
 local-part
    The maximum total length of a user name or other local-part is 64
    characters.

Klensin Standards Track [Page 54] RFC 2821 Simple Mail Transfer Protocol April 2001

 domain
    The maximum total length of a domain name or number is 255
    characters.
 path
    The maximum total length of a reverse-path or forward-path is 256
    characters (including the punctuation and element separators).
 command line
    The maximum total length of a command line including the command
    word and the <CRLF> is 512 characters.  SMTP extensions may be
    used to increase this limit.
 reply line
    The maximum total length of a reply line including the reply code
    and the <CRLF> is 512 characters.  More information may be
    conveyed through multiple-line replies.
 text line
    The maximum total length of a text line including the <CRLF> is
    1000 characters (not counting the leading dot duplicated for
    transparency).  This number may be increased by the use of SMTP
    Service Extensions.
 message content
    The maximum total length of a message content (including any
    message headers as well as the message body) MUST BE at least 64K
    octets.  Since the introduction of Internet standards for
    multimedia mail [12], 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 [18],
    and SMTP client systems that will send large messages SHOULD
    utilize it when possible.
 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 servers
    MUST NOT, and delivery SMTP servers SHOULD NOT, perform validation
    tests on message headers suggests that rejecting a message based
    on the total number of recipients shown in header fields is to be
    discouraged.  A server which imposes a limit on the number of
    recipients MUST behave in an orderly fashion,  such as to reject
    additional addresses over its limit rather than silently
    discarding addresses previously accepted.  A client that needs to

Klensin Standards Track [Page 55] RFC 2821 Simple Mail Transfer Protocol April 2001

    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.
 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.
 RFC 821 [30] 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
 addresses will be removed from the SMTP client's queue.  When the
 client attempts retransmission of those addresses which received 452
 responses, at least 100 of these will be able to fit in the SMTP
 server's recipients buffer.  Each retransmission attempt which 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 5XX response code.
 This would be most appropriate if the policy limitation was intended
 to apply if the total recipient count for a particular message body
 were enforced even if that message body was sent in multiple mail
 transactions.

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

Klensin Standards Track [Page 56] RFC 2821 Simple Mail Transfer Protocol April 2001

 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:
 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.
 MAIL Command: 5 minutes
 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.
 DATA Initiation: 2 minutes
    This is while awaiting the "354 Start Input" reply to a DATA
    command.
 Data Block: 3 minutes
    This is while awaiting the completion of each TCP SEND call
    transmitting a chunk of data.
 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.
 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

Klensin Standards Track [Page 57] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.
 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.  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
 [9].
 The strategy may be further modified as a result of multiple
 addresses per host (see below) to optimize delivery time vs. resource
 usage.

Klensin Standards Track [Page 58] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.
 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.  Implementation of this efficiency
 feature is strongly encouraged.
 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 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 which 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,

Klensin Standards Track [Page 59] RFC 2821 Simple Mail Transfer Protocol April 2001

 other kinds of Delivery Status Notifications (DSNs) [24], and also
 Message Disposition Notifications (MDNs) [10].  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 with a valid, non-null reverse-path.
 Implementors of automated email processors should be careful to make
 sure that the various kinds of messages with null reverse-path are
 handled correctly, in particular such systems SHOULD NOT reply to
 messages with null reverse-path.

5. Address Resolution and Mail Handling

 Once an SMTP client lexically identifies a domain to which mail will
 be delivered for processing (as described in sections 3.6 and 3.7), a
 DNS lookup MUST be performed to resolve the domain name [22].  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 and, due to a history of problems,
 are generally discouraged.  The lookup first attempts to locate an MX
 record associated with the name.  If a CNAME record is found instead,
 the resulting name is processed as if it were the initial name.  If
 no MX records are found, but an A RR is found, the A RR is treated as
 if it was associated with an implicit MX RR, with a preference of 0,
 pointing to that host.  If one or more MX RRs are found for a given
 name, SMTP systems MUST NOT utilize any A 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 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.

Klensin Standards Track [Page 60] RFC 2821 Simple Mail Transfer Protocol April 2001

 Two types of information is used to rank the host addresses: multiple
 MX records, and multihomed hosts.
 Multiple MX records contain a preference indication that MUST be used
 in sorting (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 SMTP 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
 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.

Klensin Standards Track [Page 61] RFC 2821 Simple Mail Transfer Protocol April 2001

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.
 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,
 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 [28] for
 a discussion of this problem.

Klensin Standards Track [Page 62] RFC 2821 Simple Mail Transfer Protocol April 2001

6.2 Loop Detection

 Simple counting of the number of "Received:" headers 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.

6.3 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
 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 [3, 26, 5,
 21].  These protocols have encouraged the use of SMTP as a posting
 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 [7]).  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.

Klensin Standards Track [Page 63] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 protocol:
  1. Addition of a message-id field when none appears
  1. Addition of a date, time or time zone when none appears
  1. 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.
 In all cases, properly-operating clients supplying correct
 information are preferred to corrections by the SMTP server.  In all
 cases, documentation of actions performed by the servers (in trace
 fields and/or header comments) is strongly encouraged.

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 which use digital signatures (see [14] and,
 e.g., PGP [4] or S/MIME [31]).
 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, unless they are accompanied by careful
 handoffs of responsibility in a carefully-designed trust environment,
 they remain inherently weaker than end-to-end mechanisms which use
 digitally signed messages rather than depending on the integrity of
 the transport system.

Klensin Standards Track [Page 64] RFC 2821 Simple Mail Transfer Protocol April 2001

 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
 or in which error (or normal) replies should be directed to a special
 address.  (Systems that provide convenient ways for users to alter
 these fields on a per-message basis should attempt to establish a
 primary and permanent mailbox address for the user so that Sender
 fields within the message data can be generated sensibly.)
 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 an ignorant user who is trying to fake mail.

7.2 "Blind" Copies

 Addresses that do not appear in the message headers 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 headers, either as
 part of trace headers or as informational or private-extension
 headers.  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 headers.  Receiving
 systems SHOULD NOT attempt to deduce such relationships and use them
 to alter the headers of the message for delivery.  The popular
 "Apparently-to" header 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.  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

Klensin Standards Track [Page 65] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.
 Within the last few years, the contents of mailing lists have become
 popular as an address information source for so-called "spammers."
 The use of EXPN to "harvest" addresses has increased as list
 administrators have installed protections against inappropriate uses
 of the lists themselves.  Implementations SHOULD still provide
 support for EXPN, but sites SHOULD carefully evaluate the tradeoffs.
 As authentication mechanisms are introduced into SMTP, some sites may
 choose to make EXPN available only to authenticated requestors.

7.4 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 are strongly
 encouraged to minimally provide for making type and version
 information available in some way to other network hosts.

7.5 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") 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.

Klensin Standards Track [Page 66] RFC 2821 Simple Mail Transfer Protocol April 2001

7.6 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.

7.7 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.
 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, MAIL, or RCPT as appropriate.

8. IANA Considerations

 IANA will maintain three registries in support of this specification.
 The first 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.
 The second registry consists of "tags" that identify forms of domain
 literals other than those for IPv4 addresses (specified in RFC 821
 and in this document) and IPv6 addresses (specified in this
 document).  Additional literal types require standardization before
 being used; none are anticipated at this time.
 The third, 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")

Klensin Standards Track [Page 67] RFC 2821 Simple Mail Transfer Protocol April 2001

 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.

9. References

 [1]  American National Standards Institute (formerly United States of
      America Standards Institute), X3.4, 1968, "USA Code for
      Information Interchange". ANSI X3.4-1968 has been replaced by
      newer versions with slight modifications, but the 1968 version
      remains definitive for the Internet.
 [2]  Braden, R., "Requirements for Internet hosts - application and
      support", STD 3, RFC 1123, October 1989.
 [3]  Butler, M., Chase, D., Goldberger, J., Postel, J. and J.
      Reynolds, "Post Office Protocol - version 2", RFC 937, February
      1985.
 [4]  Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
      Message Format", RFC 2440, November 1998.
 [5]  Crispin, M., "Interactive Mail Access Protocol - Version 2", RFC
      1176, August 1990.
 [6]  Crispin, M., "Internet Message Access Protocol - Version 4", RFC
      2060, December 1996.
 [7]  Crocker, D., "Standard for the Format of ARPA Internet Text
      Messages", RFC 822, August 1982.
 [8]  Crocker, D. and P. Overell, Eds., "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.
 [9]  De Winter, J., "SMTP Service Extension for Remote Message Queue
      Starting", RFC 1985, August 1996.
 [10] Fajman, R., "An Extensible Message Format for Message
      Disposition Notifications", RFC 2298, March 1998.
 [11] Freed, N, "Behavior of and Requirements for Internet Firewalls",
      RFC 2979, October 2000.
 [12] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message Bodies",
      RFC 2045, December 1996.

Klensin Standards Track [Page 68] RFC 2821 Simple Mail Transfer Protocol April 2001

 [13] Freed, N., "SMTP Service Extension for Command Pipelining", RFC
      2920, September 2000.
 [14] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security
      Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
      RFC 1847, October 1995.
 [15] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
      December 1998.
 [16] Kille, S., "Mapping between X.400 and RFC822/MIME", RFC 2156,
      January 1998.
 [17] Hinden, R and S. Deering, Eds. "IP Version 6 Addressing
      Architecture", RFC 2373, July 1998.
 [18] Klensin, J., Freed, N. and K. Moore, "SMTP Service Extension for
      Message Size Declaration", STD 10, RFC 1870, November 1995.
 [19] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
      "SMTP Service Extensions", STD 10, RFC 1869, November 1995.
 [20] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
      "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July
      1994.
 [21] Lambert, M., "PCMAIL: A distributed mail system for personal
      computers", RFC 1056, July 1988.
 [22] Mockapetris, P., "Domain names - implementation and
      specification", STD 13, RFC 1035, November 1987.
      Mockapetris, P., "Domain names - concepts and facilities", STD
      13, RFC 1034, November 1987.
 [23] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
      Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
      December 1996.
 [24] Moore, K., "SMTP Service Extension for Delivery Status
      Notifications", RFC 1891, January 1996.
 [25] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
      Delivery Status Notifications", RFC 1894, January 1996.
 [26] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
      53, RFC 1939, May 1996.

Klensin Standards Track [Page 69] RFC 2821 Simple Mail Transfer Protocol April 2001

 [27] Partridge, C., "Mail routing and the domain system", RFC 974,
      January 1986.
 [28] Partridge, C., "Duplicate messages and SMTP", RFC 1047, February
      1988.
 [29] Postel, J., ed., "Transmission Control Protocol - DARPA Internet
      Program Protocol Specification", STD 7, RFC 793, September 1981.
 [30] Postel, J., "Simple Mail Transfer Protocol", RFC 821, August
      1982.
 [31] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC
      2633, June 1999.
 [32] Resnick, P., Ed., "Internet Message Format", RFC 2822, April
      2001.
 [33] Vaudreuil, G., "SMTP Service Extensions for Transmission of
      Large and Binary MIME Messages", RFC 1830, August 1995.
 [34] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893,
      January 1996.

10. Editor's Address

 John C. Klensin
 AT&T Laboratories
 99 Bedford St
 Boston, MA 02111 USA
 Phone: 617-574-3076
 EMail: klensin@research.att.com

11. Acknowledgments

 Many people worked long and hard on the many iterations of this
 document.  There was wide-ranging debate in the IETF DRUMS Working
 Group, both on its mailing list and in face to face discussions,
 about many technical issues and the role of a revised standard for
 Internet mail transport, and many contributors helped form the
 wording in this specification.  The hundreds of participants in the
 many discussions since RFC 821 was produced are too numerous to
 mention, but they all helped this document become what it is.

Klensin Standards Track [Page 70] RFC 2821 Simple Mail Transfer Protocol April 2001

APPENDICES

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, but not in SMTP commands or
 responses.

B. Generating SMTP Commands from RFC 822 Headers

 Some systems use RFC 822 headers (only) in a mail submission
 protocol, or otherwise generate SMTP commands from RFC 822 headers
 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 a
 mail envelopes is not separated early in processing from header
 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 fields SHOULD then
    be removed from the headers.  Once this process is completed, the
    remaining headers SHOULD be checked to verify that at least one
    To:, Cc:, or Bcc: header remains.  If none do, then a bcc: header
    with no additional information SHOULD be inserted as specified in
    [32].
 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 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 71] RFC 2821 Simple Mail Transfer Protocol April 2001

 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 headers 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-only remailer, loops back to the Internet
 environment (and the mailing list) are almost inevitable.

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>
 SHOULD be the host sending the MAIL command.  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; while servers MUST be prepared to receive and
 handle them as discussed in section 3.3 and F.2, clients SHOULD NOT
 transmit them and this section was included only to provide context.
 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 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
 forward- and reverse-paths.

Klensin Standards Track [Page 72] RFC 2821 Simple Mail Transfer Protocol April 2001

 The SMTP server transforms the command arguments by moving its own
 identifier (its domain name or that of any domain for which it is
 acting as a mail exchanger), if it appears, from the forward-path to
 the beginning of the reverse-path.
 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.
 Conversely, SMTP servers MUST NOT derive final message delivery
 information from message header fields.
 When the list of hosts is present, 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.
 As each 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).

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.

D.1 A Typical SMTP Transaction Scenario

 This SMTP example shows mail sent by Smith at host bar.com, 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>

Klensin Standards Track [Page 73] RFC 2821 Simple Mail Transfer Protocol April 2001

    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

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

D.3 Relayed Mail Scenario

 Step 1  --  Source Host to Relay Host
    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:<@foo.com: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

Klensin Standards Track [Page 74] RFC 2821 Simple Mail Transfer Protocol April 2001

    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
 Step 2  --  Relay Host to Destination Host
    S: 220 xyz.com Simple Mail Transfer Service Ready
    C: EHLO foo.com
    S: 250 xyz.com is on the air
    C: MAIL FROM:<@foo.com: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

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

Klensin Standards Track [Page 75] RFC 2821 Simple Mail Transfer Protocol April 2001

    S: 250-VRFY
    S: 250 HELP
    C: VRFY Crispin
    S: 250 Mark Crispin <Admin.MRC@foo.com>
    C: SEND 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

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 envelope information from one system to the message headers
 or body of another) have generally proven to be inadequate in
 important ways.  Systems translating between environments that do not
 support both envelopes and headers and Internet mail must be written
 with the understanding that some information loss is almost
 inevitable.

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.

Klensin Standards Track [Page 76] RFC 2821 Simple Mail Transfer Protocol April 2001

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 is strongly preferred to
 HELO when the server will accept the former.  Servers must continue
 to accept and process HELO in order to support older clients.

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 fields), four-digit years MUST BE used.  Two-digit
 years are deprecated; three-digit years were never permitted in the
 Internet mail system.

Klensin Standards Track [Page 77] RFC 2821 Simple Mail Transfer Protocol April 2001

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.

Klensin Standards Track [Page 78] RFC 2821 Simple Mail Transfer Protocol April 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

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