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Table of Contents

[Note that this file is a concatenation of more than one RFC.]

 RFC 821
                                  
                                  
                                  
                                  
                                  
                   SIMPLE MAIL TRANSFER PROTOCOL
                                  
                                  
                                  
                         Jonathan B. Postel
                            August 1982
                                  
                                  
                                  
                   Information Sciences Institute
                 University of Southern California
                         4676 Admiralty Way
                 Marina del Rey, California  90291
                           (213) 822-1511

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
                         TABLE OF CONTENTS
 1.  INTRODUCTION .................................................. 1
 2.  THE SMTP MODEL ................................................ 2
 3.  THE SMTP PROCEDURE ............................................ 4
    3.1.  Mail ..................................................... 4
    3.2.  Forwarding ............................................... 7
    3.3.  Verifying and Expanding .................................. 8
    3.4.  Sending and Mailing ..................................... 11
    3.5.  Opening and Closing ..................................... 13
    3.6.  Relaying ................................................ 14
    3.7.  Domains ................................................. 17
    3.8.  Changing Roles .......................................... 18
 4.  THE SMTP SPECIFICATIONS ...................................... 19
    4.1.  SMTP Commands ........................................... 19
    4.1.1.  Command Semantics ..................................... 19
    4.1.2.  Command Syntax ........................................ 27
    4.2.  SMTP Replies ............................................ 34
    4.2.1.  Reply Codes by Function Group ......................... 35
    4.2.2.  Reply Codes in Numeric Order .......................... 36
    4.3.  Sequencing of Commands and Replies ...................... 37
    4.4.  State Diagrams .......................................... 39
    4.5.  Details ................................................. 41
    4.5.1.  Minimum Implementation ................................ 41
    4.5.2.  Transparency .......................................... 41
    4.5.3.  Sizes ................................................. 42
 APPENDIX A:  TCP ................................................. 44
 APPENDIX B:  NCP ................................................. 45
 APPENDIX C:  NITS ................................................ 46
 APPENDIX D:  X.25 ................................................ 47
 APPENDIX E:  Theory of Reply Codes ............................... 48
 APPENDIX F:  Scenarios ........................................... 51
 GLOSSARY ......................................................... 64
 REFERENCES ....................................................... 67

Network Working Group J. Postel Request for Comments: DRAFT ISI Replaces: RFC 788, 780, 772 August 1982

                   SIMPLE MAIL TRANSFER PROTOCOL

1. INTRODUCTION

 The objective of 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.  Appendices A,
 B, C, and D describe the use of SMTP with various transport services.
 A Glossary provides the definitions of terms as used in this
 document.
 An important feature of SMTP is its capability to relay mail across
 transport service environments.  A transport service provides an
 interprocess communication environment (IPCE).  An IPCE may cover one
 network, several networks, or a subset of a network.  It is important
 to realize that transport systems (or IPCEs) are not one-to-one with
 networks.  A process can communicate directly with another process
 through any mutually known IPCE.  Mail is an application or use of
 interprocess communication.  Mail can be communicated between
 processes in different IPCEs by relaying through a process connected
 to two (or more) IPCEs.  More specifically, mail can be relayed
 between hosts on different transport systems by a host on both
 transport systems.

Postel [Page 1]

August 1982 RFC 821 Simple Mail Transfer Protocol

2. THE SMTP MODEL

 The SMTP design is based on the following model of communication:  as
 the result of a user mail request, the sender-SMTP establishes a
 two-way transmission channel to a receiver-SMTP.  The receiver-SMTP
 may be either the ultimate destination or an intermediate.  SMTP
 commands are generated by the sender-SMTP and sent to the
 receiver-SMTP.  SMTP replies are sent from the receiver-SMTP to the
 sender-SMTP in response to the commands.
 Once the transmission channel is established, the SMTP-sender sends a
 MAIL command indicating the sender of the mail.  If the SMTP-receiver
 can accept mail it responds with an OK reply.  The SMTP-sender then
 sends a RCPT command identifying a recipient of the mail.  If the
 SMTP-receiver can accept mail for that recipient it responds with an
 OK reply; if not, it responds with a reply rejecting that recipient
 (but not the whole mail transaction).  The SMTP-sender and
 SMTP-receiver may negotiate several recipients.  When the recipients
 have been negotiated the SMTP-sender sends the mail data, terminating
 with a special sequence.  If the SMTP-receiver successfully processes
 the mail data it responds with an OK reply.  The dialog is purposely
 lock-step, one-at-a-time.
  1. ————————————————————
 
             +----------+                +----------+
 +------+    |          |                |          |
 | User |<-->|          |      SMTP      |          |
 +------+    |  Sender- |Commands/Replies| Receiver-|
 +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
 | File |<-->|          |    and Mail    |          |<-->| File |
 |System|    |          |                |          |    |System|
 +------+    +----------+                +----------+    +------+
 
              Sender-SMTP                Receiver-SMTP
                         Model for SMTP Use
                              Figure 1
  1. ————————————————————
 The SMTP provides mechanisms for the transmission of mail; directly
 from the sending user's host to the receiving user's host when the

[Page 2] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 two host are connected to the same transport service, or via one or
 more relay SMTP-servers when the source and destination hosts are not
 connected to the same transport service.
 To be able to provide the relay capability the SMTP-server must be
 supplied with the name of the ultimate destination host as well as
 the destination mailbox name.
 The argument to the MAIL command is a reverse-path, which specifies
 who the mail is from.  The argument to the RCPT command is a
 forward-path, which specifies who the mail is to.  The forward-path
 is a source route, while the reverse-path is a return route (which
 may be used to return a message to the sender when an error occurs
 with a relayed message).
 When the same message is sent to multiple recipients the SMTP
 encourages the transmission of only one copy of the data for all the
 recipients at the same destination host.
 The mail commands and replies have a rigid syntax.  Replies also have
 a numeric code.  In the following, examples appear which use actual
 commands and replies.  The complete lists of commands and replies
 appears in Section 4 on specifications.
 Commands and replies are not case sensitive.  That is, a command or
 reply word may be upper case, lower case, or any mixture of upper and
 lower case.  Note that this is not true of mailbox user names.  For
 some hosts the user name is case sensitive, and SMTP implementations
 must take case to preserve the case of user names as they appear in
 mailbox arguments.  Host names are not case sensitive.
 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.
 When specifying the general form of a command or reply, an argument
 (or special symbol) will be denoted by a meta-linguistic variable (or
 constant), for example, "<string>" or "<reverse-path>".  Here the
 angle brackets indicate these are meta-linguistic variables.
 However, some arguments use the angle brackets literally.  For
 example, an actual reverse-path is enclosed in angle brackets, i.e.,
 "<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the
 angle brackets are actually transmitted in the command or reply).

Postel [Page 3]

August 1982 RFC 821 Simple Mail Transfer Protocol

3. THE SMTP PROCEDURES

 This section presents the procedures used in SMTP in several parts.
 First comes the basic mail procedure defined as a mail transaction.
 Following this are descriptions of forwarding mail, verifying mailbox
 names and expanding mailing lists, sending to terminals instead of or
 in combination with mailboxes, and the opening and closing exchanges.
 At the end of this section are comments on relaying, a note on mail
 domains, and a discussion of changing roles.  Throughout this section
 are examples of partial command and reply sequences, several complete
 scenarios are presented in Appendix F.
 3.1.  MAIL
    There are three steps to SMTP mail transactions.  The transaction
    is started with a MAIL command which gives the sender
    identification.  A series of one or more RCPT commands follows
    giving the receiver information.  Then a DATA command gives the
    mail data.  And finally, the end of mail data indicator confirms
    the transaction.
       The first step in the procedure is the MAIL command.  The
       <reverse-path> contains the source mailbox.
          MAIL <SP> FROM:<reverse-path> <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.  It gives the
       reverse-path which can be used to report errors.  If accepted,
       the receiver-SMTP returns a 250 OK reply.
       The <reverse-path> can contain more than just a mailbox.  The
       <reverse-path> is a reverse source routing list of hosts and
       source mailbox.  The first host in the <reverse-path> should be
       the host sending this command.
       The second step in the procedure is the RCPT command.
          RCPT <SP> TO:<forward-path> <CRLF>
       This command gives a forward-path identifying one recipient.
       If accepted, the receiver-SMTP returns a 250 OK reply, and
       stores the forward-path.  If the recipient is unknown the
       receiver-SMTP returns a 550 Failure reply.  This second step of
       the procedure can be repeated any number of times.

[Page 4] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
       The <forward-path> can contain more than just a mailbox.  The
       <forward-path> is a source routing list of hosts and the
       destination mailbox.  The first host in the <forward-path>
       should be the host receiving this command.
       The third step in the procedure is the DATA command.
          DATA <CRLF>
       If accepted, the receiver-SMTP returns a 354 Intermediate reply
       and considers all succeeding lines to be the message text.
       When the end of text is received and stored the SMTP-receiver
       sends a 250 OK reply.
       Since the mail data is sent on the transmission channel the end
       of the 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.  A
       transparency procedure is used to prevent this from interfering
       with the user's text (see Section 4.5.2).
          Please note that the mail data includes the memo header
          items such as Date, Subject, To, Cc, From [2].
       The end of mail data indicator also confirms the mail
       transaction and tells the receiver-SMTP to now process the
       stored recipients and mail data.  If accepted, the
       receiver-SMTP returns a 250 OK reply.  The DATA command should
       fail only if the mail transaction was incomplete (for example,
       no recipients), or if resources are not available.
    The above procedure is an example of a mail transaction.  These
    commands must be used only in the order discussed above.
    Example 1 (below) illustrates the use of these commands in a mail
    transaction.

Postel [Page 5]

August 1982 RFC 821 Simple Mail Transfer Protocol

  1. ————————————————————
                   Example of the SMTP Procedure
       This SMTP example shows mail sent by Smith at host Alpha.ARPA,
       to Jones, Green, and Brown at host Beta.ARPA.  Here we assume
       that host Alpha contacts host Beta directly.
          S: MAIL FROM:<Smith@Alpha.ARPA>
          R: 250 OK
          S: RCPT TO:<Jones@Beta.ARPA>
          R: 250 OK
          S: RCPT TO:<Green@Beta.ARPA>
          R: 550 No such user here
          S: RCPT TO:<Brown@Beta.ARPA>
          R: 250 OK
          S: DATA
          R: 354 Start mail input; end with <CRLF>.<CRLF>
          S: Blah blah blah...
          S: ...etc. etc. etc.
          S: <CRLF>.<CRLF>
          R: 250 OK
       The mail has now been accepted for Jones and Brown.  Green did
       not have a mailbox at host Beta.
                             Example 1
  1. ————————————————————

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RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 3.2.  FORWARDING
    There are some cases where the destination information in the
    <forward-path> is incorrect, but the receiver-SMTP knows the
    correct destination.  In such cases, one of the following replies
    should be used to allow the sender to contact the correct
    destination.
       251 User not local; will forward to <forward-path>
          This reply indicates that the receiver-SMTP knows the user's
          mailbox is on another host and indicates the correct
          forward-path to use in the future.  Note that either the
          host or user or both may be different.  The receiver takes
          responsibility for delivering the message.
       551 User not local; please try <forward-path>
          This reply indicates that the receiver-SMTP knows the user's
          mailbox is on another host and indicates the correct
          forward-path to use.  Note that either the host or user or
          both may be different.  The receiver refuses to accept mail
          for this user, and the sender must either redirect the mail
          according to the information provided or return an error
          response to the originating user.
    Example 2 illustrates the use of these responses.
  1. ————————————————————
                       Example of Forwarding
    Either
    S: RCPT TO:<Postel@USC-ISI.ARPA>
    R: 251 User not local; will forward to <Postel@USC-ISIF.ARPA>
    Or
    S: RCPT TO:<Paul@USC-ISIB.ARPA>
    R: 551 User not local; please try <Mockapetris@USC-ISIF.ARPA>
                             Example 2
  1. ————————————————————

Postel [Page 7]

August 1982 RFC 821 Simple Mail Transfer Protocol

 3.3.  VERIFYING AND EXPANDING
    SMTP provides as additional features, commands to verify a user
    name or expand a mailing list.  This is done with the VRFY and
    EXPN commands, which have character string arguments.  For the
    VRFY command, the string is a user name, and the response may
    include the full name of the user and must include the mailbox of
    the user.  For the EXPN command, the string identifies a mailing
    list, and the multiline response may include the full name of the
    users and must give the mailboxes on the mailing list.
    "User name" is a fuzzy term and used purposely.  If a host
    implements the VRFY or EXPN commands then at least local mailboxes
    must be recognized as "user names".  If a host chooses to
    recognize other strings as "user names" that is allowed.
    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 of one mailbox.  If a request is made to verify a mailing
    list a positive response can be given if on receipt of a message
    so addressed it will be delivered to everyone on the list,
    otherwise an error should be reported (e.g., "550 That is a
    mailing list, not a user").  If a request is made to expand a user
    name a positive response can be formed by returning a list
    containing one name, or an error can be reported (e.g., "550 That
    is a user name, not a mailing list").
    In the case of a multiline reply (normal for EXPN) exactly one
    mailbox is to be specified on each line of the reply.  In the case
    of an ambiguous request, for example, "VRFY Smith", where there
    are two Smith's the response must be "553 User ambiguous".
    The case of verifying a user name is straightforward as shown in
    example 3.

[Page 8] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
  1. ————————————————————
                  Example of Verifying a User Name
       Either
          S: VRFY Smith
          R: 250 Fred Smith <Smith@USC-ISIF.ARPA>
       Or
          S: VRFY Smith
          R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>
       Or
          S: VRFY Jones
          R: 550 String does not match anything.
       Or
          S: VRFY Jones
          R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>
       Or
          S: VRFY Gourzenkyinplatz
          R: 553 User ambiguous.
                             Example 3
  1. ————————————————————

Postel [Page 9]

August 1982 RFC 821 Simple Mail Transfer Protocol

    The case of expanding a mailbox list requires a multiline reply as
    shown in example 4.
  1. ————————————————————
                Example of Expanding a Mailing List
       Either
          S: EXPN Example-People
          R: 250-Jon Postel <Postel@USC-ISIF.ARPA>
          R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
          R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA>
          R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
          R: 250-<joe@foo-unix.ARPA>
          R: 250 <xyz@bar-unix.ARPA>
       Or
          S: EXPN Executive-Washroom-List
          R: 550 Access Denied to You.
                             Example 4
  1. ————————————————————
    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 is
    a variety of file naming conventions in the Internet.
    The VRFY and EXPN commands are not included in the minimum
    implementation (Section 4.5.1), and are not required to work
    across relays when they are implemented.

[Page 10] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 3.4.  SENDING AND MAILING
    The main purpose of SMTP is to deliver messages to user's
    mailboxes.  A very similar service provided by some hosts is to
    deliver messages to user's terminals (provided the user is active
    on the host).  The delivery to the user's mailbox is called
    "mailing", the delivery to the user's terminal is called
    "sending".  Because in many hosts the implementation of sending is
    nearly identical to the implementation of mailing these two
    functions are combined in SMTP.  However the sending commands are
    not included in the required minimum implementation
    (Section 4.5.1).  Users should have the ability to control the
    writing of messages on their terminals.  Most hosts permit the
    users to accept or refuse such messages.
    The following three command are defined to support the sending
    options.  These are used in the mail transaction instead of the
    MAIL command and inform the receiver-SMTP of the special semantics
    of this transaction:
       SEND <SP> FROM:<reverse-path> <CRLF>
          The SEND command requires that the mail data be delivered to
          the user's terminal.  If the user is not active (or not
          accepting terminal messages) on the host a 450 reply may
          returned to a RCPT command.  The mail transaction is
          successful if the message is delivered the terminal.
       SOML <SP> FROM:<reverse-path> <CRLF>
          The Send Or MaiL command requires that the mail data be
          delivered to the user's terminal if the user is active (and
          accepting terminal messages) on the host.  If the user is
          not active (or not accepting terminal messages) then the
          mail data is entered into the user's mailbox.  The mail
          transaction is successful if the message is delivered either
          to the terminal or the mailbox.
       SAML <SP> FROM:<reverse-path> <CRLF>
          The Send And MaiL command requires that the mail data be
          delivered to the user's terminal if the user is active (and
          accepting terminal messages) on the host.  In any case the
          mail data is entered into the user's mailbox.  The mail
          transaction is successful if the message is delivered the
          mailbox.

Postel [Page 11]

August 1982 RFC 821 Simple Mail Transfer Protocol

    The same reply codes that are used for the MAIL commands are used
    for these commands.

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RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 3.5.  OPENING AND CLOSING
    At the time the transmission channel is opened there is an
    exchange to ensure that the hosts are communicating with the hosts
    they think they are.
    The following two commands are used in transmission channel
    opening and closing:
       HELO <SP> <domain> <CRLF>
       QUIT <CRLF>
    In the HELO command the host sending the command identifies
    itself; the command may be interpreted as saying "Hello, I am
    <domain>".
  1. ————————————————————
                   Example of Connection Opening
       R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
       S: HELO USC-ISIF.ARPA
       R: 250 BBN-UNIX.ARPA
                             Example 5
  1. ————————————————————
  1. ————————————————————
                   Example of Connection Closing
       S: QUIT
       R: 221 BBN-UNIX.ARPA Service closing transmission channel
                             Example 6
  1. ————————————————————

Postel [Page 13]

August 1982 RFC 821 Simple Mail Transfer Protocol

 3.6.  RELAYING
    The forward-path may be a source route of the form
    "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts.  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.
    Conceptually the elements of the forward-path are moved to the
    reverse-path as the message is relayed from one server-SMTP to
    another.  The reverse-path is a reverse source route, (i.e., a
    source route from the current location of the message to the
    originator of the message).  When a server-SMTP deletes its
    identifier from the forward-path and inserts it into the
    reverse-path, it must use the name it is known by in the
    environment it is sending into, not the environment the mail came
    from, in case the server-SMTP is known by different names in
    different environments.
    If when the message arrives at an SMTP the first element of the
    forward-path is not the identifier of that SMTP the element is not
    deleted from the forward-path and is used to determine the next
    SMTP to send the message to.  In any case, the SMTP adds its own
    identifier to the reverse-path.
    Using source routing the receiver-SMTP receives mail to be relayed
    to another server-SMTP  The receiver-SMTP may accept or reject the
    task of relaying the mail in the same way it accepts or rejects
    mail for a local user.  The receiver-SMTP transforms the command
    arguments by moving its own identifier from the forward-path to
    the beginning of the reverse-path.  The receiver-SMTP then becomes
    a sender-SMTP, establishes a transmission channel to the next SMTP
    in the forward-path, and sends it the mail.
    The first host in the reverse-path should be the host sending the
    SMTP commands, and the first host in the forward-path should be
    the host receiving the SMTP commands.
    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.
    If a server-SMTP has accepted the task of relaying the mail and

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                                         Simple Mail Transfer Protocol
    later finds that the forward-path is incorrect or that the mail
    cannot be delivered for whatever 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).
    This notification message must be from the server-SMTP at this
    host.  Of course, server-SMTPs should not send notification
    messages about problems with 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 relayed it is permissible to leave the reverse-path
    null.  A MAIL command with a null reverse-path appears as follows:
       MAIL FROM:<>
    An undeliverable mail notification message is shown in example 7.
    This notification is in response to a message originated by JOE at
    HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
    to HOSTZ.  What we see in the example is the transaction between
    HOSTY and HOSTX, which is the first step in the return of the
    notification message.

Postel [Page 15]

August 1982 RFC 821 Simple Mail Transfer Protocol

  1. ————————————————————
          Example Undeliverable Mail Notification Message
       S: MAIL FROM:<>
       R: 250 ok
       S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>
       R: 250 ok
       S: DATA
       R: 354 send the mail data, end with .
       S: Date: 23 Oct 81 11:22:33
       S: From: SMTP@HOSTY.ARPA
       S: To: JOE@HOSTW.ARPA
       S: Subject: Mail System Problem
       S:
       S:   Sorry JOE, your message to SAM@HOSTZ.ARPA lost.
       S:   HOSTZ.ARPA said this:
       S:    "550 No Such User"
       S: .
       R: 250 ok
                             Example 7
  1. ————————————————————

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RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 3.7.  DOMAINS
    Domains are a recently introduced concept in the ARPA Internet
    mail system.  The use of domains changes the address space from a
    flat global space of simple character string host names to a
    hierarchically structured rooted tree of global addresses.  The
    host name is replaced by a domain and host designator which is a
    sequence of domain element strings separated by periods with the
    understanding that the domain elements are ordered from the most
    specific to the most general.
    For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
    "PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.
    Whenever domain names are used in SMTP only the official names are
    used, the use of nicknames or aliases is not allowed.

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August 1982 RFC 821 Simple Mail Transfer Protocol

 3.8.  CHANGING ROLES
    The TURN command may be used to reverse the roles of the two
    programs communicating over the transmission channel.
    If program-A is currently the sender-SMTP and it sends the TURN
    command and receives an ok reply (250) then program-A becomes the
    receiver-SMTP.
    If program-B is currently the receiver-SMTP and it receives the
    TURN command and sends an ok reply (250) then program-B becomes
    the sender-SMTP.
    To refuse to change roles the receiver sends the 502 reply.
    Please note that this command is optional.  It would not normally
    be used in situations where the transmission channel is TCP.
    However, when the cost of establishing the transmission channel is
    high, this command may be quite useful.  For example, this command
    may be useful in supporting be mail exchange using the public
    switched telephone system as a transmission channel, especially if
    some hosts poll other hosts for mail exchanges.

[Page 18] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol

4. THE SMTP SPECIFICATIONS

 4.1.  SMTP COMMANDS
    4.1.1.  COMMAND SEMANTICS
       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 command codes themselves are
       alphabetic characters terminated by <SP> if parameters follow
       and <CRLF> otherwise.  The syntax of mailboxes must conform to
       receiver site conventions.  The SMTP commands are discussed
       below.  The SMTP replies are discussed in the 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.
       HELLO (HELO)
          This command is used to identify the sender-SMTP to the
          receiver-SMTP.  The argument field contains the host name of
          the sender-SMTP.
          The receiver-SMTP identifies itself to the sender-SMTP in
          the connection greeting reply, and in the response to this
          command.
          This command and an OK reply to it confirm that both the
          sender-SMTP and the receiver-SMTP are in the initial state,
          that is, there is no transaction in progress and all state
          tables and buffers are cleared.

Postel [Page 19]

August 1982 RFC 821 Simple Mail Transfer Protocol

       MAIL (MAIL)
          This command is used to initiate a mail transaction in which
          the mail data is delivered to one or more mailboxes.  The
          argument field contains a reverse-path.
          The reverse-path consists of an optional list of hosts and
          the sender mailbox.  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 IPCE to which it is
          relaying the mail rather than the IPCE from which the mail
          came (if they are different).  In some types of error
          reporting messages (for example, undeliverable mail
          notifications) the reverse-path may be null (see Example 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.
       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 forward-path consists of an optional list of hosts and a
          required destination mailbox.  When the list of hosts is
          present, it is a source route and indicates that the mail
          must be relayed to the next host on the list.  If the
          receiver-SMTP does not implement the relay function it may
          user the same reply it would for an unknown local user
          (550).
          When mail is relayed, the relay host must remove itself from
          the beginning forward-path and put itself at the beginning
          of the reverse-path.  When mail reaches its ultimate
          destination (the forward-path contains only a destination
          mailbox), the receiver-SMTP inserts it into the destination
          mailbox in accordance with its host mail conventions.

[Page 20] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
             For example, mail received at relay host A with arguments
                FROM:<USERX@HOSTY.ARPA>
                TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>
             will be relayed on to host B with arguments
                FROM:<@HOSTA.ARPA:USERX@HOSTY.ARPA>
                TO:<@HOSTB.ARPA:USERC@HOSTD.ARPA>.
          This command causes its forward-path argument to be appended
          to the forward-path buffer.
       DATA (DATA)
          The receiver treats the lines following the command as mail
          data from the sender.  This command causes the mail data
          from this command to be appended to the mail data buffer.
          The mail data may contain any of the 128 ASCII character
          codes.
          The mail data is terminated by a line containing only a
          period, that is the character sequence "<CRLF>.<CRLF>" (see
          Section 4.5.2 on Transparency).  This is the end of mail
          data indication.
          The end of mail data indication requires that the receiver
          must now 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 completely the
          receiver must send a failure reply.
          When the receiver-SMTP accepts a message either for relaying
          or for final delivery it inserts at the beginning of the
          mail data a time stamp line.  The time stamp line indicates
          the identity of the host that sent the message, and 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.
          When the receiver-SMTP makes the "final delivery" of a
          message it inserts at the beginning of the mail data a

Postel [Page 21]

August 1982 RFC 821 Simple Mail Transfer Protocol

          return path line.  The return path line preserves the
          information in the <reverse-path> from the MAIL command.
          Here, final delivery means the message leaves the SMTP
          world.  Normally, this would mean it has been delivered to
          the destination user, 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 be
             different from the actual sender's mailbox, for example,
             if error responses are to be delivered a special error
             handling mailbox rather than the message senders.
          The preceding two paragraphs imply 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 header and body [2].  See Example 8.
          Special mention is needed of the response and further action
          required when the processing following the end of mail data
          indication is partially successful.  This could arise if
          after accepting several recipients and the mail data, the
          receiver-SMTP finds that the mail data can be successfully
          delivered to some of the recipients, but it cannot be to
          others (for example, due to mailbox space allocation
          problems).  In such a situation, the response to the DATA
          command must be an OK reply.  But, the receiver-SMTP must
          compose and send an "undeliverable mail" notification
          message to the originator of the message.  Either a single
          notification which lists all of the recipients that failed
          to get the message, or separate notification messages must
          be sent for each failed recipient (see Example 7).  All
          undeliverable mail notification messages are sent using the
          MAIL command (even if they result from processing a SEND,
          SOML, or SAML command).

[Page 22] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
  1. ————————————————————
          Example of Return Path and Received Time Stamps
    Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>   
    Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST
    Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST
    Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST
    Date: 27 Oct 81 15:01:01 PST                                
    From: JOE@ABC.ARPA                                          
    Subject: Improved Mailing System Installed                  
    To: SAM@JKL.ARPA                                            
                                  
    This is to inform you that ...                              
                             Example 8
  1. ————————————————————
       SEND (SEND)
          This command is used to initiate a mail transaction in which
          the mail data is delivered to one or more terminals.  The
          argument field contains a reverse-path.  This command is
          successful if the message is delivered to a terminal.
          The reverse-path consists of an optional list of hosts and
          the sender mailbox.  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 IPCE to which it is
          relaying the mail rather than the IPCE from which the mail
          came (if they are different).
          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.
       SEND OR MAIL (SOML)
          This command is used to initiate a mail transaction in which
          the mail data is delivered to one or more terminals or

Postel [Page 23]

August 1982 RFC 821 Simple Mail Transfer Protocol

          mailboxes. For each recipient the mail data is delivered to
          the recipient's terminal if the recipient is active on the
          host (and accepting terminal messages), otherwise to the
          recipient's mailbox.  The argument field contains a
          reverse-path.  This command is successful if the message is
          delivered to a terminal or the mailbox.
          The reverse-path consists of an optional list of hosts and
          the sender mailbox.  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 IPCE to which it is
          relaying the mail rather than the IPCE from which the mail
          came (if they are different).
          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.
       SEND AND MAIL (SAML)
          This command is used to initiate a mail transaction in which
          the mail data is delivered to one or more terminals and
          mailboxes. For each recipient the mail data is delivered to
          the recipient's terminal if the recipient is active on the
          host (and accepting terminal messages), and for all
          recipients to the recipient's mailbox.  The argument field
          contains a reverse-path.  This command is successful if the
          message is delivered to the mailbox.
          The reverse-path consists of an optional list of hosts and
          the sender mailbox.  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 IPCE to which it is
          relaying the mail rather than the IPCE from which the mail
          came (if they are different).
          This command clears the reverse-path buffer, the

[Page 24] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
          forward-path buffer, and the mail data buffer; and inserts
          the reverse-path information from this command into the
          reverse-path buffer.
       RESET (RSET)
          This command specifies that the current mail transaction is
          to be aborted.  Any stored sender, recipients, and mail data
          must be discarded, and all buffers and state tables cleared.
          The receiver must send an OK reply.
       VERIFY (VRFY)
          This command asks the receiver to confirm that the argument
          identifies a user.  If it is a user name, the full name of
          the user (if known) and the fully specified mailbox are
          returned.
          This command has no effect on any of the reverse-path
          buffer, the forward-path buffer, or the mail data buffer.
       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.  The full name of the users (if
          known) and the fully specified mailboxes are returned in a
          multiline reply.
          This command has no effect on any of the reverse-path
          buffer, the forward-path buffer, or the mail data buffer.
       HELP (HELP)
          This command causes the receiver to send helpful information
          to the sender of the HELP command.  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 any of the reverse-path
          buffer, the forward-path buffer, or the mail data buffer.

Postel [Page 25]

August 1982 RFC 821 Simple Mail Transfer Protocol

       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.
          This command has no effect on any of the reverse-path
          buffer, the forward-path buffer, or the mail data buffer.
       QUIT (QUIT)
          This command specifies that the receiver must send an OK
          reply, and then close the transmission channel.
          The receiver should not close the transmission channel until
          it receives and replies to a QUIT command (even if there was
          an error).  The sender should not close the transmission
          channel until it send a QUIT command and receives the reply
          (even if there was an error response to a previous command).
          If the connection is closed prematurely the receiver should
          act as if a RSET command had been received (canceling any
          pending transaction, but not undoing any previously
          completed transaction), the sender should act as if the
          command or transaction in progress had received a temporary
          error (4xx).
       TURN (TURN)
          This command specifies that the receiver must either (1)
          send an OK reply and then take on the role of the
          sender-SMTP, or (2) send a refusal reply and retain the role
          of the receiver-SMTP.
          If program-A is currently the sender-SMTP and it sends the
          TURN command and receives an OK reply (250) then program-A
          becomes the receiver-SMTP.  Program-A is then in the initial
          state as if the transmission channel just opened, and it
          then sends the 220 service ready greeting.
          If program-B is currently the receiver-SMTP and it receives
          the TURN command and sends an OK reply (250) then program-B
          becomes the sender-SMTP.  Program-B is then in the initial
          state as if the transmission channel just opened, and it
          then expects to receive the 220 service ready greeting.
          To refuse to change roles the receiver sends the 502 reply.

[Page 26] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
       There are restrictions on the order in which these command may
       be used.
          The first command in a session must be the HELO command.
          The HELO command may be used later in a session as well.  If
          the HELO command argument is not acceptable a 501 failure
          reply must be returned and the receiver-SMTP must stay in
          the same state.
          The NOOP, HELP, EXPN, and VRFY commands can be used at any
          time during a session.
          The MAIL, SEND, SOML, or SAML commands begin a mail
          transaction.  Once started a mail transaction consists of
          one of the transaction beginning commands, one or more RCPT
          commands, and a DATA command, in that order.  A mail
          transaction may be aborted by the RSET command.  There may
          be zero or more transactions in a session.
          If the transaction beginning command argument is not
          acceptable a 501 failure reply must be returned and the
          receiver-SMTP must stay in the same state.  If the commands
          in a transaction are out of order a 503 failure reply must
          be returned and the receiver-SMTP must stay in the same
          state.
          The last command in a session must be the QUIT command.  The
          QUIT command can not be used at any other time in a session.
    4.1.2.  COMMAND SYNTAX
       The commands consist of a command code followed by an argument
       field.  Command codes are four alphabetic characters.  Upper
       and lower case alphabetic characters are to be treated
       identically.  Thus, any of the following may represent the mail
       command:
          MAIL    Mail    mail    MaIl    mAIl
       This also applies to any symbols representing parameter values,
       such as "TO" or "to" for the forward-path.  Command codes and
       the argument fields are separated by one or more spaces.
       However, within the reverse-path and forward-path arguments
       case is important.  In particular, in some hosts the user
       "smith" is different from the user "Smith".

Postel [Page 27]

August 1982 RFC 821 Simple Mail Transfer Protocol

       The argument field consists of a variable length character
       string ending with the character sequence <CRLF>.  The receiver
       is to take no action until this sequence is received.
       Square brackets denote an optional argument field.  If the
       option is not taken, the appropriate default is implied.

[Page 28] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
       The following are the SMTP commands:
          HELO <SP> <domain> <CRLF>
          MAIL <SP> FROM:<reverse-path> <CRLF>
          RCPT <SP> TO:<forward-path> <CRLF>
          DATA <CRLF>
          RSET <CRLF>
          SEND <SP> FROM:<reverse-path> <CRLF>
          SOML <SP> FROM:<reverse-path> <CRLF>
          SAML <SP> FROM:<reverse-path> <CRLF>
          VRFY <SP> <string> <CRLF>
          EXPN <SP> <string> <CRLF>
          HELP [<SP> <string>] <CRLF>
          NOOP <CRLF>
          QUIT <CRLF>
          TURN <CRLF>

Postel [Page 29]

August 1982 RFC 821 Simple Mail Transfer Protocol

       The syntax of the above argument fields (using BNF notation
       where applicable) is given below.  The "..." notation indicates
       that a field may be repeated one or more times.
          <reverse-path> ::= <path>
          <forward-path> ::= <path>
          <path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"
          <a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>
          <at-domain> ::= "@" <domain>
          <domain> ::=  <element> | <element> "." <domain>
          <element> ::= <name> | "#" <number> | "[" <dotnum> "]"
          <mailbox> ::= <local-part> "@" <domain>
          <local-part> ::= <dot-string> | <quoted-string>
          <name> ::= <a> <ldh-str> <let-dig>
          <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
          <let-dig> ::= <a> | <d>
          <let-dig-hyp> ::= <a> | <d> | "-"
          <dot-string> ::= <string> | <string> "." <dot-string>
          <string> ::= <char> | <char> <string>
          <quoted-string> ::=  """ <qtext> """
          <qtext> ::=  "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>
          <char> ::= <c> | "\" <x>
          <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
          <number> ::= <d> | <d> <number>
          <CRLF> ::= <CR> <LF>

[Page 30] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
          <CR> ::= the carriage return character (ASCII code 13)
          <LF> ::= the line feed character (ASCII code 10)
          <SP> ::= the space character (ASCII code 32)
          <snum> ::= one, two, or three digits representing a decimal
                    integer value in the range 0 through 255
          <a> ::= any one of the 52 alphabetic characters A through Z
                    in upper case and a through z in lower case
          <c> ::= any one of the 128 ASCII characters, but not any
                    <special> or <SP>
          <d> ::= any one of the ten digits 0 through 9
          <q> ::= any one of the 128 ASCII characters except <CR>,
                    <LF>, quote ("), or backslash (\)
          <x> ::= any one of the 128 ASCII characters (no exceptions)
          <special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
                    | "," | ";" | ":" | "@"  """ | the control
                    characters (ASCII codes 0 through 31 inclusive and
                    127)
       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" could be used to indicate a single nine character
       user field with comma being the fourth character of the field.
       Hosts are generally known by names which are translated to
       addresses in each host.  Note that the name elements of domains
       are the official names -- no use of nicknames or aliases is
       allowed.
       Sometimes a host is not known to the translation function and
       communication is blocked.  To bypass this barrier two numeric
       forms are also allowed for host "names".  One form is a decimal
       integer prefixed by a pound sign, "#", which indicates the
       number is the address of the host.  Another form is four small
       decimal integers separated by dots and enclosed by brackets,
       e.g., "[123.255.37.2]", which indicates a 32-bit ARPA Internet
       Address in four 8-bit fields.

Postel [Page 31]

August 1982 RFC 821 Simple Mail Transfer Protocol

       The time stamp line and the return path line are formally
       defined as follows:
       <return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
       <time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>
          <stamp> ::= <from-domain> <by-domain> <opt-info> ";"
                    <daytime>
          <from-domain> ::= "FROM" <SP> <domain> <SP>
          <by-domain> ::= "BY" <SP> <domain> <SP>
          <opt-info> ::= [<via>] [<with>] [<id>] [<for>]
          <via> ::= "VIA" <SP> <link> <SP>
          <with> ::= "WITH" <SP> <protocol> <SP>
          <id> ::= "ID" <SP> <string> <SP>
          <for> ::= "FOR" <SP> <path> <SP>
          <link> ::= The standard names for links are registered with
                    the Network Information Center.
          <protocol> ::= The standard names for protocols are
                    registered with the Network Information Center.
          <daytime> ::= <SP> <date> <SP> <time>
          <date> ::= <dd> <SP> <mon> <SP> <yy>
          <time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>
          <dd> ::= the one or two decimal integer day of the month in
                    the range 1 to 31.
          <mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
                    "JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
          <yy> ::= the two decimal integer year of the century in the
                    range 00 to 99.

[Page 32] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
          <hh> ::= the two decimal integer hour of the day in the
                    range 00 to 24.
          <mm> ::= the two decimal integer minute of the hour in the
                    range 00 to 59.
          <ss> ::= the two decimal integer second of the minute in the
                    range 00 to 59.
          <zone> ::= "UT" for Universal Time (the default) or other
                    time zone designator (as in [2]).
  1. ————————————————————
                        Return Path Example
       Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>
                             Example 9
  1. ————————————————————
  1. ————————————————————
                      Time Stamp Line Example
    Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
       Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
                 id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT
                             Example 10
  1. ————————————————————

Postel [Page 33]

August 1982 RFC 821 Simple Mail Transfer Protocol

 4.2.  SMTP REPLIES
    Replies to SMTP commands are devised to ensure the synchronization
    of requests and actions in the process of mail transfer, and to
    guarantee that the sender-SMTP always knows the state of the
    receiver-SMTP.  Every command must generate exactly one reply.
       The details of the command-reply sequence are made explicit in
       Section 5.3 on Sequencing and Section 5.4 State Diagrams.
    An SMTP reply consists of a three digit number (transmitted as
    three alphanumeric characters) followed by some text.  The number
    is intended for use by automata to determine what state to enter
    next; the text is meant for the human user.  It is intended that
    the three digits contain enough encoded information that the
    sender-SMTP need not examine the text and may either discard it or
    pass it on to the user, as appropriate.  In particular, 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 Appendix E.  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
    Appendix E).  Only the EXPN and HELP commands are expected to
    result in multiline replies in normal circumstances, however
    multiline replies are allowed for any command.

[Page 34] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
    4.2.1.  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
       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]
        
       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>
       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]
       451 Requested action aborted: error in processing
       551 User not local; please try <forward-path>
       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
       

Postel [Page 35]

August 1982 RFC 821 Simple Mail Transfer Protocol

    4.2.2.  NUMERIC ORDER LIST OF REPLY CODES
       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>
        
       354 Start mail input; end with <CRLF>.<CRLF>
        
       421 <domain> Service not available,
           closing transmission channel
          [This may be a reply to any command if the service knows it
          must shut down]
       450 Requested mail action not taken: mailbox unavailable
          [E.g., mailbox busy]
       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
       503 Bad sequence of commands
       504 Command parameter not implemented
       550 Requested action not taken: mailbox unavailable
          [E.g., mailbox not found, no access]
       551 User not local; please try <forward-path>
       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
       

[Page 36] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 4.3.  SEQUENCING OF COMMANDS AND REPLIES
    The communication between the sender and receiver is intended to
    be an alternating dialogue, controlled by the sender.  As such,
    the sender issues a command and the receiver responds with a
    reply.  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 of
       the server host as the first word following the reply code.
          For example,
             220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>
    The table below lists alternative success and failure replies for
    each command.  These must 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.
    COMMAND-REPLY SEQUENCES
       Each command is listed with its possible replies.  The prefixes
       used before the possible replies are "P" for preliminary (not
       used in SMTP), "I" for intermediate, "S" for success, "F" for
       failure, and "E" for error.  The 421 reply (service not
       available, closing transmission channel) may be given to any
       command if the SMTP-receiver knows it must shut down.  This
       listing forms the basis for the State Diagrams in Section 4.4.
          CONNECTION ESTABLISHMENT
             S: 220
             F: 421
          HELO
             S: 250
             E: 500, 501, 504, 421
          MAIL
             S: 250
             F: 552, 451, 452
             E: 500, 501, 421

Postel [Page 37]

August 1982 RFC 821 Simple Mail Transfer Protocol

          RCPT
             S: 250, 251
             F: 550, 551, 552, 553, 450, 451, 452
             E: 500, 501, 503, 421
          DATA
             I: 354 -> data -> S: 250
                               F: 552, 554, 451, 452
             F: 451, 554
             E: 500, 501, 503, 421
          RSET
             S: 250
             E: 500, 501, 504, 421
          SEND
             S: 250
             F: 552, 451, 452
             E: 500, 501, 502, 421
          SOML
             S: 250
             F: 552, 451, 452
             E: 500, 501, 502, 421
          SAML
             S: 250
             F: 552, 451, 452
             E: 500, 501, 502, 421
          VRFY
             S: 250, 251
             F: 550, 551, 553
             E: 500, 501, 502, 504, 421
          EXPN
             S: 250
             F: 550
             E: 500, 501, 502, 504, 421
          HELP
             S: 211, 214
             E: 500, 501, 502, 504, 421
          NOOP
             S: 250
             E: 500, 421
          QUIT
             S: 221
             E: 500
          TURN
             S: 250
             F: 502
             E: 500, 503

[Page 38] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 4.4.  STATE DIAGRAMS
    Following are state diagrams for a simple-minded SMTP
    implementation.  Only the first digit of the reply codes is used.
    There is one state diagram for each group of SMTP commands.  The
    command groupings were determined by constructing a model for each
    command and then collecting together the commands with
    structurally identical models.
    For each command there are three possible outcomes:  "success"
    (S), "failure" (F), and "error" (E). In the state diagrams below
    we use the symbol B for "begin", and the symbol W for "wait for
    reply".
    First, the diagram that represents most of the SMTP commands:
       
                                1,3    +---+
                           ----------->| E |
                          |            +---+
                          |
       +---+    cmd    +---+    2      +---+
       | B |---------->| W |---------->| S |
       +---+           +---+           +---+
                          |
                          |     4,5    +---+
                           ----------->| F |
                                       +---+
       
       This diagram models the commands:
          HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
          NOOP, QUIT, TURN.

Postel [Page 39]

August 1982 RFC 821 Simple Mail Transfer Protocol

    A more complex diagram models the DATA command:
       
       +---+   DATA    +---+ 1,2                 +---+
       | B |---------->| W |-------------------->| E |
       +---+           +---+        ------------>+---+
                       3| |4,5     |
                        | |        |
          --------------   -----   |
         |                      |  |             +---+
         |               ----------     -------->| S |
         |              |       |      |         +---+
         |              |  ------------
         |              | |     |
         V           1,3| |2    |
       +---+   data    +---+     --------------->+---+
       |   |---------->| W |                     | F |
       +---+           +---+-------------------->+---+
                            4,5
       Note that the "data" here is a series of lines sent from the
       sender to the receiver with no response expected until the last
       line is sent.

[Page 40] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 4.5.  DETAILS
    4.5.1.  MINIMUM IMPLEMENTATION
       In order to make SMTP workable, the following minimum
       implementation is required for all receivers:
          COMMANDS -- HELO
                      MAIL
                      RCPT
                      DATA
                      RSET
                      NOOP
                      QUIT
    4.5.2.  TRANSPARENCY
       Without some provision for data transparency the character
       sequence "<CRLF>.<CRLF>" ends the mail text and cannot be sent
       by the user.  In general, users are not aware of such
       "forbidden" sequences.  To allow all user composed text to be
       transmitted transparently the following procedures are used.
          1. Before sending a line of mail text the sender-SMTP checks
          the first character of the line.  If it is a period, one
          additional period is inserted at the beginning of the line.
          2. When a line of mail text is received by the receiver-SMTP
          it checks the line.  If the line is composed of a single
          period it is the end of mail.  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 format effectors and other control characters.  If
       the transmission channel provides an 8-bit byte (octets) data
       stream, the 7-bit ASCII codes are transmitted right justified
       in the octets with the high order bits cleared to zero.
          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, or that store data in records rather than

Postel [Page 41]

August 1982 RFC 821 Simple Mail Transfer Protocol

          strings.  If such transforms are necessary, they must be
          reversible -- especially if such transforms are applied to
          mail being relayed.
    4.5.3.  SIZES
       There are several objects that have required minimum maximum
       sizes.  That is, every implementation must be able to receive
       objects of at least these sizes, but must not send objects
       larger than these sizes.
                                  
        ****************************************************
        *                                                  *
        *  TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION  *
        *  TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
        *  OF THESE OBJECTS SHOULD BE USED.                *
        *                                                  *
        ****************************************************
          user
             The maximum total length of a user name is 64 characters.
          domain
             The maximum total length of a domain name or number is 64
             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.
          reply line
             The maximum total length of a reply line including the
             reply code and the <CRLF> is 512 characters.

[Page 42] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
          text line
             The maximum total length of a text line including the
             <CRLF> is 1000 characters (but not counting the leading
             dot duplicated for transparency).
          recipients buffer
             The maximum total number of recipients that must be
             buffered is 100 recipients.
                                  
        ****************************************************
        *                                                  *
        *  TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION  *
        *  TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
        *  OF THESE OBJECTS SHOULD BE USED.                *
        *                                                  *
        ****************************************************
       Errors due to exceeding these limits may be reported by using
       the reply codes, for example:
          500 Line too long.
          501 Path too long
          552 Too many recipients.
          552 Too much mail data.

Postel [Page 43]

August 1982 RFC 821 Simple Mail Transfer Protocol

APPENDIX A

 TCP Transport service
    The Transmission Control Protocol [3] is used in the ARPA
    Internet, and in any network following the US DoD standards for
    internetwork protocols.
    Connection Establishment
       The SMTP transmission channel is a TCP connection established
       between the sender process port U and the receiver process port
       L.  This single full duplex connection is used as the
       transmission channel.  This protocol is assigned the service
       port 25 (31 octal), that is L=25.
    Data Transfer
       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.

[Page 44] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol

APPENDIX B

 NCP Transport service
    The ARPANET Host-to-Host Protocol [4] (implemented by the Network
    Control Program) may be used in the ARPANET.
    Connection Establishment
       The SMTP transmission channel is established via NCP between
       the sender process socket U and receiver process socket L.  The
       Initial Connection Protocol [5] is followed resulting in a pair
       of simplex connections.  This pair of connections is used as
       the transmission channel.  This protocol is assigned the
       contact socket 25 (31 octal), that is L=25.
    Data Transfer
       The NCP data connections are established in 8-bit byte mode.
       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.

Postel [Page 45]

August 1982 RFC 821 Simple Mail Transfer Protocol

APPENDIX C

 NITS
    The Network Independent Transport Service [6] may be used.
    Connection Establishment
       The SMTP transmission channel is established via NITS between
       the sender process and receiver process.  The sender process
       executes the CONNECT primitive, and the waiting receiver
       process executes the ACCEPT primitive.
    Data Transfer
       The NITS 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.

[Page 46] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol

APPENDIX D

 X.25 Transport service
    It may be possible to use the X.25 service [7] as provided by the
    Public Data Networks directly, however, it is suggested that a
    reliable end-to-end protocol such as TCP be used on top of X.25
    connections.

Postel [Page 47]

August 1982 RFC 821 Simple Mail Transfer Protocol

APPENDIX E

 Theory of Reply Codes
    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 sender-SMTP will be able to
    determine its next action (proceed as planned, redo, retrench,
    etc.) by simply examining this first digit.  A sender-SMTP that
    wants to know approximately what kind of error occurred (e.g.,
    mail system error, command syntax error) may examine the second
    digit, reserving the third digit 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 sender-SMTP should send
             another command specifying whether to continue or abort
             the action.
                [Note: SMTP does not have any commands that allow this
                type of reply, and so does not have the 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 sender-SMTP should send another command
             specifying this information.  This reply is used in
             command sequence groups.
          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

[Page 48] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
             return to the beginning of the command sequence (if any).
             It is difficult to assign a meaning to "transient" when
             two different sites (receiver- and sender- SMTPs) must
             agree on the interpretation.  Each reply in this category
             might have a different time value, but the sender-SMTP is
             encouraged to try again.  A rule of thumb to determine if
             a reply fits into the 4yz or the 5yz category (see below)
             is that replies are 4yz if they can be repeated without
             any change in command form or in properties of the sender
             or receiver.  (E.g., 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 sender-SMTP 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 sender-SMTP 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 don't 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 as yet.
          x4z   Unspecified as yet.
          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

Postel [Page 49]

August 1982 RFC 821 Simple Mail Transfer Protocol

       illustrates this.  Each reply text is recommended rather than
       mandatory, and may even change according to the command with
       which it is associated.  On the other hand, the reply codes
       must strictly follow the specifications in this section.
       Receiver implementations should not invent new codes for
       slightly different situations from the ones described here, but
       rather adapt codes already defined.
       For example, a command such as NOOP whose successful execution
       does not offer the sender-SMTP any new information will return
       a 250 reply.  The response 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 sender-SMTP 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>.
          For example:
                              123-First line
                              123-Second line
                              123-234 text beginning with numbers
                              123 The last line
       In many cases the sender-SMTP then simply needs to search for
       the reply code followed by <SP> at the beginning of a line, and
       ignore all preceding lines.  In a few cases, there is important
       data for the sender in the reply "text".  The sender will know
       these cases from the current context.

[Page 50] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol

APPENDIX F

 Scenarios
    This section presents complete scenarios of several types of SMTP
    sessions.
 A Typical SMTP Transaction Scenario
    This SMTP example shows mail sent by Smith at host USC-ISIF, to
    Jones, Green, and Brown at host BBN-UNIX.  Here we assume that
    host USC-ISIF contacts host BBN-UNIX directly.  The mail is
    accepted for Jones and Brown.  Green does not have a mailbox at
    host BBN-UNIX.
  1. ————————————————————
       R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
       S: HELO USC-ISIF.ARPA
       R: 250 BBN-UNIX.ARPA
       S: MAIL FROM:<Smith@USC-ISIF.ARPA>
       R: 250 OK
       S: RCPT TO:<Jones@BBN-UNIX.ARPA>
       R: 250 OK
       S: RCPT TO:<Green@BBN-UNIX.ARPA>
       R: 550 No such user here
       S: RCPT TO:<Brown@BBN-UNIX.ARPA>
       R: 250 OK
       S: DATA
       R: 354 Start mail input; end with <CRLF>.<CRLF>
       S: Blah blah blah...
       S: ...etc. etc. etc.
       S: .
       R: 250 OK
       S: QUIT
       R: 221 BBN-UNIX.ARPA Service closing transmission channel
                             Scenario 1
  1. ————————————————————

Postel [Page 51]

August 1982 RFC 821 Simple Mail Transfer Protocol

 Aborted SMTP Transaction Scenario
  1. ————————————————————
       R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
       S: HELO ISI-VAXA.ARPA
       R: 250 MIT-Multics.ARPA
       S: MAIL FROM:<Smith@ISI-VAXA.ARPA>
       R: 250 OK
       S: RCPT TO:<Jones@MIT-Multics.ARPA>
       R: 250 OK
       S: RCPT TO:<Green@MIT-Multics.ARPA>
       R: 550 No such user here
       S: RSET
       R: 250 OK
       S: QUIT
       R: 221 MIT-Multics.ARPA Service closing transmission channel
                             Scenario 2
  1. ————————————————————

[Page 52] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 Relayed Mail Scenario
  1. ————————————————————
       Step 1  --  Source Host to Relay Host
          R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
          S: HELO MIT-AI.ARPA
          R: 250 USC-ISIE.ARPA
          S: MAIL FROM:<JQP@MIT-AI.ARPA>
          R: 250 OK
          S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
          R: 250 OK
          S: DATA
          R: 354 Start mail input; end with <CRLF>.<CRLF>
          S: Date: 2 Nov 81 22:33:44
          S: From: John Q. Public <JQP@MIT-AI.ARPA>
          S: Subject:  The Next Meeting of the Board
          S: To: Jones@BBN-Vax.ARPA
          S:
          S: Bill:
          S: The next meeting of the board of directors will be
          S: on Tuesday.
          S:                                              John.
          S: .
          R: 250 OK
          S: QUIT
          R: 221 USC-ISIE.ARPA Service closing transmission channel

Postel [Page 53]

August 1982 RFC 821 Simple Mail Transfer Protocol

       Step 2  --  Relay Host to Destination Host
          R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
          S: HELO USC-ISIE.ARPA
          R: 250 BBN-VAX.ARPA
          S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
          R: 250 OK
          S: RCPT TO:<Jones@BBN-VAX.ARPA>
          R: 250 OK
          S: DATA
          R: 354 Start mail input; end with <CRLF>.<CRLF>
          S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
             2 Nov 81 22:40:10 UT
          S: Date: 2 Nov 81 22:33:44
          S: From: John Q. Public <JQP@MIT-AI.ARPA>
          S: Subject:  The Next Meeting of the Board
          S: To: Jones@BBN-Vax.ARPA
          S:
          S: Bill:
          S: The next meeting of the board of directors will be
          S: on Tuesday.
          S:                                              John.
          S: .
          R: 250 OK
          S: QUIT
          R: 221 USC-ISIE.ARPA Service closing transmission channel
                             Scenario 3
  1. ————————————————————

[Page 54] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 Verifying and Sending Scenario
  1. ————————————————————
       R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
       S: HELO MIT-MC.ARPA
       R: 250 SU-SCORE.ARPA
       S: VRFY Crispin
       R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
       S: SEND FROM:<EAK@MIT-MC.ARPA>
       R: 250 OK
       S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
       R: 250 OK
       S: DATA
       R: 354 Start mail input; end with <CRLF>.<CRLF>
       S: Blah blah blah...
       S: ...etc. etc. etc.
       S: .
       R: 250 OK
       S: QUIT
       R: 221 SU-SCORE.ARPA Service closing transmission channel
                             Scenario 4
  1. ————————————————————

Postel [Page 55]

August 1982 RFC 821 Simple Mail Transfer Protocol

 Sending and Mailing Scenarios
    First the user's name is verified, then  an attempt is made to
    send to the user's terminal.  When that fails, the messages is
    mailed to the user's mailbox.
  1. ————————————————————
       R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
       S: HELO MIT-MC.ARPA
       R: 250 SU-SCORE.ARPA
       S: VRFY Crispin
       R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
       S: SEND FROM:<EAK@MIT-MC.ARPA>
       R: 250 OK
       S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
       R: 450 User not active now
       S: RSET
       R: 250 OK
       S: MAIL FROM:<EAK@MIT-MC.ARPA>
       R: 250 OK
       S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
       R: 250 OK
       S: DATA
       R: 354 Start mail input; end with <CRLF>.<CRLF>
       S: Blah blah blah...
       S: ...etc. etc. etc.
       S: .
       R: 250 OK
       S: QUIT
       R: 221 SU-SCORE.ARPA Service closing transmission channel
                             Scenario 5
  1. ————————————————————

[Page 56] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
    Doing the preceding scenario more efficiently.
  1. ————————————————————
       R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
       S: HELO MIT-MC.ARPA
       R: 250 SU-SCORE.ARPA
       S: VRFY Crispin
       R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
       S: SOML FROM:<EAK@MIT-MC.ARPA>
       R: 250 OK
       S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
       R: 250 User not active now, so will do mail.
       S: DATA
       R: 354 Start mail input; end with <CRLF>.<CRLF>
       S: Blah blah blah...
       S: ...etc. etc. etc.
       S: .
       R: 250 OK
       S: QUIT
       R: 221 SU-SCORE.ARPA Service closing transmission channel
                             Scenario 6
  1. ————————————————————

Postel [Page 57]

August 1982 RFC 821 Simple Mail Transfer Protocol

 Mailing List Scenario
    First each of two mailing lists are expanded in separate sessions
    with different hosts.  Then the message is sent to everyone that
    appeared on either list (but no duplicates) via a relay host.
  1. ————————————————————
       Step 1  --  Expanding the First List
          R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
          S: HELO SU-SCORE.ARPA
          R: 250 MIT-AI.ARPA
          S: EXPN Example-People
          R: 250-<ABC@MIT-MC.ARPA>
          R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
          R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
          R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
          R: 250-<joe@foo-unix.ARPA>
          R: 250 <xyz@bar-unix.ARPA>
          S: QUIT
          R: 221 MIT-AI.ARPA Service closing transmission channel

[Page 58] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
       Step 2  --  Expanding the Second List
          R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
          S: HELO SU-SCORE.ARPA
          R: 250 MIT-MC.ARPA
          S: EXPN Interested-Parties
          R: 250-Al Calico <ABC@MIT-MC.ARPA>
          R: 250-<XYZ@MIT-AI.ARPA>
          R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
          R: 250-<fred@BBN-UNIX.ARPA>
          R: 250 <xyz@bar-unix.ARPA>
          S: QUIT
          R: 221 MIT-MC.ARPA Service closing transmission channel

Postel [Page 59]

August 1982 RFC 821 Simple Mail Transfer Protocol

       Step 3  --  Mailing to All via a Relay Host
          R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
          S: HELO SU-SCORE.ARPA
          R: 250 USC-ISIE.ARPA
          S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
          R: 250 OK
          S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
          R: 250 OK
          S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
          R: 250 OK
          S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
          R: 250 OK
          S: RCPT
              TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
          R: 250 OK
          S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
          R: 250 OK
          S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
          R: 250 OK
          S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
          R: 250 OK
          S: DATA
          R: 354 Start mail input; end with <CRLF>.<CRLF>
          S: Blah blah blah...
          S: ...etc. etc. etc.
          S: .
          R: 250 OK
          S: QUIT
          R: 221 USC-ISIE.ARPA Service closing transmission channel
                             Scenario 7
  1. ————————————————————

[Page 60] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 Forwarding Scenarios
  1. ————————————————————
       R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
       S: HELO LBL-UNIX.ARPA
       R: 250 USC-ISIF.ARPA
       S: MAIL FROM:<mo@LBL-UNIX.ARPA>
       R: 250 OK
       S: RCPT TO:<fred@USC-ISIF.ARPA>
       R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
       S: DATA
       R: 354 Start mail input; end with <CRLF>.<CRLF>
       S: Blah blah blah...
       S: ...etc. etc. etc.
       S: .
       R: 250 OK
       S: QUIT
       R: 221 USC-ISIF.ARPA Service closing transmission channel
                             Scenario 8
  1. ————————————————————

Postel [Page 61]

August 1982 RFC 821 Simple Mail Transfer Protocol

  1. ————————————————————
       Step 1  --  Trying the Mailbox at the First Host
          R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
          S: HELO LBL-UNIX.ARPA
          R: 250 USC-ISIF.ARPA
          S: MAIL FROM:<mo@LBL-UNIX.ARPA>
          R: 250 OK
          S: RCPT TO:<fred@USC-ISIF.ARPA>
          R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>
          S: RSET
          R: 250 OK
          S: QUIT
          R: 221 USC-ISIF.ARPA Service closing transmission channel
       Step 2  --  Delivering the Mail at the Second Host
          R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
          S: HELO LBL-UNIX.ARPA
          R: 250 USC-ISI.ARPA
          S: MAIL FROM:<mo@LBL-UNIX.ARPA>
          R: 250 OK
          S: RCPT TO:<Jones@USC-ISI.ARPA>
          R: OK
          S: DATA
          R: 354 Start mail input; end with <CRLF>.<CRLF>
          S: Blah blah blah...
          S: ...etc. etc. etc.
          S: .
          R: 250 OK
          S: QUIT
          R: 221 USC-ISI.ARPA Service closing transmission channel
                             Scenario 9
  1. ————————————————————

[Page 62] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 Too Many Recipients Scenario
  1. ————————————————————
       R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
       S: HELO USC-ISIF.ARPA
       R: 250 BERKELEY.ARPA
       S: MAIL FROM:<Postel@USC-ISIF.ARPA>
       R: 250 OK
       S: RCPT TO:<fabry@BERKELEY.ARPA>
       R: 250 OK
       S: RCPT TO:<eric@BERKELEY.ARPA>
       R: 552 Recipient storage full, try again in another transaction
       S: DATA
       R: 354 Start mail input; end with <CRLF>.<CRLF>
       S: Blah blah blah...
       S: ...etc. etc. etc.
       S: .
       R: 250 OK
       S: MAIL FROM:<Postel@USC-ISIF.ARPA>
       R: 250 OK
       S: RCPT TO:<eric@BERKELEY.ARPA>
       R: 250 OK
       S: DATA
       R: 354 Start mail input; end with <CRLF>.<CRLF>
       S: Blah blah blah...
       S: ...etc. etc. etc.
       S: .
       R: 250 OK
       S: QUIT
       R: 221 BERKELEY.ARPA Service closing transmission channel
                            Scenario 10
  1. ————————————————————
    Note that a real implementation must handle many recipients as
    specified in Section 4.5.3.

Postel [Page 63]

August 1982 RFC 821 Simple Mail Transfer Protocol

GLOSSARY

 ASCII
    American Standard Code for Information Interchange [1].
 command
    A request for a mail service action sent by the sender-SMTP to the
    receiver-SMTP.
 domain
    The hierarchially structured global character string address of a
    host computer in the mail system.
 end of mail data indication
    A special sequence of characters that indicates the end of the
    mail data.  In particular, the five characters carriage return,
    line feed, period, carriage return, line feed, in that order.
 host
    A computer in the internetwork environment on which mailboxes or
    SMTP processes reside.
 line
    A a sequence of ASCII characters ending with a <CRLF>.
 mail data
    A sequence of ASCII characters of arbitrary length, which conforms
    to the standard set in the Standard for the Format of ARPA
    Internet Text Messages (RFC 822 [2]).
 mailbox
    A character string (address) which identifies a user to whom mail
    is to be sent.  Mailbox normally consists of the host and user
    specifications.  The standard mailbox naming convention is defined
    to be "user@domain".  Additionally, the "container" in which mail
    is stored.

[Page 64] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol
 receiver-SMTP process
    A process which transfers mail in cooperation with a sender-SMTP
    process.  It waits for a connection to be established via the
    transport service.  It receives SMTP commands from the
    sender-SMTP, sends replies, and performs the specified operations.
 reply
    A 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 completion code
    (including error codes) followed by a text string.  The codes are
    for use by programs and the text is usually intended for human
    users.
 sender-SMTP process
    A process which transfers mail in cooperation with a receiver-SMTP
    process.  A local language may be used in the user interface
    command/reply dialogue.  The sender-SMTP initiates the transport
    service connection.  It initiates SMTP commands, receives replies,
    and governs the transfer of mail.
 session
    The set of exchanges that occur while the transmission channel is
    open.
 transaction
    The set of exchanges required for one message to be transmitted
    for one or more recipients.
 transmission channel
    A full-duplex communication path between a sender-SMTP and a
    receiver-SMTP for the exchange of commands, replies, and mail
    text.
 transport service
    Any reliable stream-oriented data communication services.  For
    example, NCP, TCP, NITS.

Postel [Page 65]

August 1982 RFC 821 Simple Mail Transfer Protocol

 user
    A human being (or a process on behalf of a human being) wishing to
    obtain mail transfer service.  In addition, a recipient of
    computer mail.
 word
    A sequence of printing characters.
 <CRLF>
    The characters carriage return and line feed (in that order).
 <SP>
    The space character.

[Page 66] Postel

RFC 821 August 1982

                                         Simple Mail Transfer Protocol

REFERENCES

 [1]  ASCII
    ASCII, "USA Code for Information Interchange", United States of
    America Standards Institute, X3.4, 1968.  Also in:  Feinler, E.
    and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
    the Defense Communications Agency by SRI International, Menlo
    Park, California, Revised January 1978.
 [2]  RFC 822
    Crocker, D., "Standard for the Format of ARPA Internet Text
    Messages," RFC 822, Department of Electrical Engineering,
    University of Delaware, August 1982.
 [3]  TCP
    Postel, J., ed., "Transmission Control Protocol - DARPA Internet
    Program Protocol Specification", RFC 793, USC/Information Sciences
    Institute, NTIS AD Number A111091, September 1981.  Also in:
    Feinler, E. and J. Postel, eds., "Internet Protocol Transition
    Workbook", SRI International, Menlo Park, California, March 1982.
 [4]  NCP
    McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
    January 1972.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
    Protocol Handbook", NIC 7104, for the Defense Communications
    Agency by SRI International, Menlo Park, California, Revised
    January 1978.
 [5]  Initial Connection Protocol
    Postel, J., "Official Initial Connection Protocol", NIC 7101,
    11 June 1971.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
    Protocol Handbook", NIC 7104, for the Defense Communications
    Agency by SRI International, Menlo Park, California, Revised
    January 1978.
 [6]  NITS
    PSS/SG3, "A Network Independent Transport Service", Study Group 3,
    The Post Office PSS Users Group, February 1980.  Available from
    the DCPU, National Physical Laboratory, Teddington, UK.

Postel [Page 67]

August 1982 RFC 821 Simple Mail Transfer Protocol

 [7]  X.25
    CCITT, "Recommendation X.25 - Interface Between Data Terminal
    Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
    Terminals Operating in the Packet Mode on Public Data Networks,"
    CCITT Orange Book, Vol. VIII.2, International Telephone and
    Telegraph Consultative Committee, Geneva, 1976.

[Page 68] Postel

Network Working Group Craig Partridge Request for Comments: 974 CSNET CIC BBN Laboratories Inc

                                                          January 1986
                 MAIL ROUTING AND THE DOMAIN SYSTEM

Status of this Memo

 This RFC presents a description of how mail systems on the Internet
 are expected to route messages based on information from the domain
 system described in RFCs 882, 883 and 973.  Distribution of this memo
 is unlimited.

Introduction

 The purpose of this memo is to explain how mailers are to decide how
 to route a message addressed to a given Internet domain name.  This
 involves a discussion of how mailers interpret MX RRs, which are used
 for message routing.  Note that this memo makes no statement about
 how mailers are to deal with MB and MG RRs, which are used for
 interpreting mailbox names.
 Under RFC-882 and RFC-883 certain assumptions about mail addresses
 have been changed.  Up to now, one could usually assume that if a
 message was addressed to a mailbox, for example, at LOKI.BBN.COM,
 that one could just open an SMTP connection to LOKI.BBN.COM and pass
 the message along.  This system broke down in certain situations,
 such as for certain UUCP and CSNET hosts which were not directly
 attached to the Internet, but these hosts could be handled as special
 cases in configuration files (for example, most mailers were set up
 to automatically forward mail addressed to a CSNET host to
 CSNET-RELAY.ARPA).
 Under domains, one cannot simply open a connection to LOKI.BBN.COM,
 but must instead ask the domain system where messages to LOKI.BBN.COM
 are to be delivered. And the domain system may direct a mailer to
 deliver messages to an entirely different host, such as SH.CS.NET.
 Or, in a more complicated case, the mailer may learn that it has a
 choice of routes to LOKI.BBN.COM.  This memo is essentially a set of
 guidelines on how mailers should behave in this more complex world.
 Readers are expected to be familiar with RFCs 882, 883, and the
 updates to them (e.g., RFC-973).

Partridge [Page 1]

RFC 974 January 1986 Mail Routing and the Domain System

What the Domain Servers Know

 The domain servers store information as a series of resource records
 (RRs), each of which contains a particular piece of information about
 a given domain name (which is usually, but not always, a host).  The
 simplest way to think of a RR is as a typed pair of datum, a domain
 name matched with relevant data, and stored with some additional type
 information to help systems determine when the RR is relevant.  For
 the purposes of message routing, the system stores RRs known as MX
 RRs. Each MX matches a domain name with two pieces of data, a
 preference value (an unsigned 16-bit integer), and the name of a
 host.  The preference number is used to indicate in what order the
 mailer should attempt deliver to the MX hosts, with the lowest
 numbered MX being the one to try first.  Multiple MXs with the same
 preference are permitted and have the same priority.
 In addition to mail information, the servers store certain other
 types of RR's which mailers may encounter or choose to use.  These
 are: the canonical name (CNAME) RR, which simply states that the
 domain name queried for is actually an alias for another domain name,
 which is the proper, or canonical, name; and the Well Known Service
 (WKS) RR, which stores information about network services (such as
 SMTP) a given domain name supports.

General Routing Guidelines

 Before delving into a detailed discussion of how mailers are expected
 to do mail routing, it would seem to make sense to give a brief
 overview of how this memo is approaching the problems that routing
 poses.
 The first major principle is derived from the definition of the
 preference field in MX records, and is intended to prevent mail
 looping.  If the mailer is on a host which is listed as an MX for the
 destination host, the mailer may only deliver to an MX which has a
 lower preference count than its own host.
 It is also possible to cause mail looping because routing information
 is out of date or incomplete.  Out of date information is only a
 problem when domain tables are changed.  The changes will not be
 known to all affected hosts until their resolver caches time out.
 There is no way to ensure that this will not happen short of
 requiring mailers and their resolvers to always send their queries to
 an authoritative server, and never use data stored in a cache.  This
 is an impractical solution, since eliminating resolver caching would
 make mailing inordinately expensive.  What is more, the out-of-date
 RR problem should not happen if, when a domain table is changed,

Partridge [Page 2]

RFC 974 January 1986 Mail Routing and the Domain System

 affected hosts (those in the list of MXs) have their resolver caches
 flushed. In other words, given proper precautions, mail looping as a
 result of domain information should be avoidable, without requiring
 mailers to query authoritative servers.  (The appropriate precaution
 is to check with a host's administrator before adding that host to a
 list of MXs).
 The incomplete data problem also requires some care when handling
 domain queries.  If the answer section of a query is incomplete
 critical MX RRs may be left out.  This may result in mail looping, or
 in a message being mistakenly labelled undeliverable.  As a result,
 mailers may only accept responses from the domain system which have
 complete answer sections.  Note that this entire problem can be
 avoided by only using virtual circuits for queries, but since this
 situation is likely to be very rare and datagrams are the preferred
 way to interact with the domain system, implementors should probably
 just ensure that their mailer will repeat a query with virtual
 circuits should the truncation bit ever be set.

Determining Where to Send a Message

 The explanation of how mailers should decide how to route a message
 is discussed in terms of the problem of a mailer on a host with
 domain name LOCAL trying to deliver a message addressed to the domain
 name REMOTE. Both LOCAL and REMOTE are assumed to be syntactically
 correct domain names.  Furthermore, LOCAL is assumed to be the
 official name for the host on which the mailer resides (i.e., it is
 not a alias).

Issuing a Query

 The first step for the mailer at LOCAL is to issue a query for MX RRs
 for REMOTE.  It is strongly urged that this step be taken every time
 a mailer attempts to send the message.  The hope is that changes in
 the domain database will rapidly be used by mailers, and thus domain
 administrators will be able to re-route in-transit messages for
 defective hosts by simply changing their domain databases.
 Certain responses to the query are considered errors:
    Getting no response to the query.  The domain server the mailer
    queried never sends anything back.  (This is distinct from an
    answer which contains no answers to the query, which is not an
    error).
    Getting a response in which the truncation field of the header is

Partridge [Page 3]

RFC 974 January 1986 Mail Routing and the Domain System

    set.  (Recall discussion of incomplete queries above).  Mailers
    may not use responses of this type, and should repeat the query
    using virtual circuits instead of datagrams.
    Getting a response in which the response code is non-zero.
 Mailers are expected to do something reasonable in the face of an
 error.  The behaviour for each type of error is not specified here,
 but implementors should note that different types of errors should
 probably be treated differently.  For example, a response code of
 "non-existent domain" should probably cause the message to be
 returned to the sender as invalid, while a response code of "server
 failure" should probably cause the message to be retried later.
 There is one other special case.  If the response contains an answer
 which is a CNAME RR, it indicates that REMOTE is actually an alias
 for some other domain name. The query should be repeated with the
 canonical domain name.
 If the response does not contain an error response, and does not
 contain aliases, its answer section should be a (possibly zero
 length) list of MX RRs for domain name REMOTE (or REMOTE's true
 domain name if REMOTE was a alias).  The next section describes how
 this list is interpreted.

Interpreting the List of MX RRs

 NOTE: This section only discusses how mailers choose which names to
 try to deliver a message to, working from a list of RR's.  It does
 not discuss how the mailers actually make delivery.  Where ever
 delivering a message is mentioned, all that is meant is that the
 mailer should do whatever it needs to do to transfer a message to a
 remote site, given a domain name for that site.  (For example, an
 SMTP mailer will try to get an address for the domain name, which
 involves another query to the domain system, and then, if it gets an
 address, connect to the SMTP TCP port).  The mechanics of actually
 transferring the message over the network to the address associated
 with a given domain name is not within the scope of this memo.
 It is possible that the list of MXs in the response to the query will
 be empty.  This is a special case.  If the list is empty, mailers
 should treat it as if it contained one RR, an MX RR with a preference
 value of 0, and a host name of REMOTE.  (I.e., REMOTE is its only
 MX).  In addition, the mailer should do no further processing on the
 list, but should attempt to deliver the message to REMOTE.  The idea

Partridge [Page 4]

RFC 974 January 1986 Mail Routing and the Domain System

 here is that if a domain fails to advertise any information about a
 particular name we will give it the benefit of the doubt and attempt
 delivery.
 If the list is not empty, the mailer should remove irrelevant RR's
 from the list according to the following steps.  Note that the order
 is significant.
    For each MX, a WKS query should be issued to see if the domain
    name listed actually supports the mail service desired.  MX RRs
    which list domain names which do not support the service should be
    discarded.  This step is optional, but strongly encouraged.
    If the domain name LOCAL is listed as an MX RR, all MX RRs with a
    preference value greater than or equal to that of LOCAL's must be
    discarded.
 After removing irrelevant RRs, the list can again be empty.  This is
 now an error condition and can occur in several ways.  The simplest
 case is that the WKS queries have discovered that none of the hosts
 listed supports the mail service desired.  The message is thus deemed
 undeliverable, though extremely persistent mail systems might want to
 try a delivery to REMOTE's address (if it exists) before returning
 the message. Another, more dangerous, possibility is that the domain
 system believes that LOCAL is handling message for REMOTE, but the
 mailer on LOCAL is not set up to handle mail for REMOTE.  For
 example, if the domain system lists LOCAL as the only MX for REMOTE,
 LOCAL will delete all the entries in the list.  But LOCAL is
 presumably querying the domain system because it didn't know what to
 do with a message addressed to REMOTE. Clearly something is wrong.
 How a mailer chooses to handle these situations is to some extent
 implementation dependent, and is thus left to the implementor's
 discretion.
 If the list of MX RRs is not empty, the mailer should try to deliver
 the message to the MXs in order (lowest preference value tried
 first).  The mailer is required to attempt delivery to the lowest
 valued MX.  Implementors are encouraged to write mailers so that they
 try the MXs in order until one of the MXs accepts the message, or all
 the MXs have been tried.  A somewhat less demanding system, in which
 a fixed number of MXs is tried, is also reasonable.  Note that
 multiple MXs may have the same preference value.  In this case, all
 MXs at with a given value must be tried before any of a higher value
 are tried.  In addition, in the special case in which there are
 several MXs with the lowest preference value,  all of them should be
 tried before a message is deemed undeliverable.

Partridge [Page 5]

RFC 974 January 1986 Mail Routing and the Domain System

Minor Special Issues

 There are a couple of special issues left out of the preceding
 section because they complicated the discussion.  They are treated
 here in no particular order.
 Wildcard names, those containing the character '*' in them, may be
 used for mail routing.  There are likely to be servers on the network
 which simply state that any mail to a domain is to be routed through
 a relay. For example, at the time that this RFC is being written, all
 mail to hosts in the domain IL is routed through RELAY.CS.NET.  This
 is done by creating a wildcard RR, which states that *.IL has an MX
 of RELAY.CS.NET.  This should be transparent to the mailer since the
 domain servers will hide this wildcard match. (If it matches *.IL
 with HUJI.IL for example, a domain server will return an RR
 containing HUJI.IL, not *.IL). If by some accident a mailer receives
 an RR with a wildcard domain name in its name or data section it
 should discard the RR.
 Note that the algorithm to delete irrelevant RRs breaks if LOCAL has
 a alias and the alias is listed in the MX records for REMOTE.  (E.g.
 REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL).  This
 can be avoided if aliases are never used in the data section of MX
 RRs.
 Implementors should understand that the query and interpretation of
 the query is only performed for REMOTE.  It is not repeated for the
 MX RRs listed for REMOTE.  You cannot try to support more extravagant
 mail routing by building a chain of MXs.  (E.g. UNIX.BBN.COM is an MX
 for RELAY.CS.NET and RELAY.CS.NET is an MX for all the hosts in .IL,
 but this does not mean that UNIX.BBN.COM accepts any responsibility
 for mail for .IL).
 Finally, it should be noted that this is a standard for routing on
 the Internet.  Mailers serving hosts which lie on multiple networks
 will presumably have to make some decisions about which network to
 route through. This decision making is outside the scope of this
 memo, although mailers may well use the domain system to help them
 decide.  However, once a mailer decides to deliver a message via the
 Internet it must apply these rules to route the message.

Partridge [Page 6]

RFC 974 January 1986 Mail Routing and the Domain System

Examples

 To illustrate the discussion above, here are three examples of how
 mailers should route messages.  All examples work with the following
 database:
    A.EXAMPLE.ORG    IN    MX    10    A.EXAMPLE.ORG
    A.EXAMPLE.ORG    IN    MX    15    B.EXAMPLE.ORG
    A.EXAMPLE.ORG    IN    MX    20    C.EXAMPLE.ORG
    A.EXAMPLE.ORG    IN    WKS   10.0.0.1    TCP    SMTP
    B.EXAMPLE.ORG    IN    MX    0      B.EXAMPLE.ORG
    B.EXAMPLE.ORG    IN    MX    10     C.EXAMPLE.ORG
    B.EXAMPLE.ORG    IN    WKS   10.0.0.2    TCP    SMTP
    C.EXAMPLE.ORG    IN    MX    0     C.EXAMPLE.ORG
    C.EXAMPLE.ORG    IN    WKS   10.0.0.3    TCP    SMTP
    D.EXAMPLE.ORG    IN    MX    0     D.EXAMPLE.ORG
    D.EXAMPLE.ORG    IN    MX    0     C.EXAMPLE.ORG
    D.EXAMPLE.ORG    IN    WKS   10.0.0.4    TCP    SMTP
 In the first example, an SMTP mailer on D.EXAMPLE.ORG is trying to
 deliver a message addressed to A.EXAMPLE.ORG. From the answer to its
 query, it learns that A.EXAMPLE.ORG has three MX RRs.  D.EXAMPLE.ORG
 is not one of the MX RRs and all three MXs support SMTP mail
 (determined from the WKS entries), so none of the MXs are eliminated.
 The mailer is obliged to try to deliver to A.EXAMPLE.ORG as the
 lowest valued MX.  If it cannot reach A.EXAMPLE.ORG it can (but is
 not required to) try B.EXAMPLE.ORG. and if B.EXAMPLE.ORG is not
 responding, it can try C.EXAMPLE.ORG.
 In the second example, the mailer is on B.EXAMPLE.ORG, and is again
 trying to deliver a message addressed to A.EXAMPLE.ORG.  There are
 once again three MX RRs for A.EXAMPLE.ORG, but in this case the
 mailer must discard the RRs for itself and C.EXAMPLE.ORG (because the
 MX RR for C.EXAMPLE.ORG has a higher preference value than the RR for
 B.EXAMPLE.ORG).  It is left only with the RR for A.EXAMPLE.ORG, and
 can only try delivery to A.EXAMPLE.ORG.
 In the third example, consider a mailer on A.EXAMPLE.ORG trying to
 deliver a message to D.EXAMPLE.ORG.  In this case there are only two
 MX RRs, both with the same preference value.  Either MX will accept
 messages for D.EXAMPLE.ORG. The mailer should try one MX first (which
 one is up to the mailer, though D.EXAMPLE.ORG seems most reasonable),
 and if that delivery fails should try the other MX (e.g.
 C.EXAMPLE.ORG).

Partridge [Page 7]

Network Working Group J. Klensin, WG Chair Request For Comments: 1869 MCI STD: 10 N. Freed, Editor Obsoletes: 1651 Innosoft International, Inc. Category: Standards Track M. Rose

                                          Dover Beach Consulting, Inc.
                                                          E. Stefferud
                                   Network Management Associates, Inc.
                                                            D. Crocker
                                                Brandenburg Consulting
                                                         November 1995
                      SMTP Service Extensions

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.

1. Abstract

 This memo defines a framework for extending the SMTP service by
 defining a means whereby a server SMTP can inform a client SMTP as to
 the service extensions it supports.  Extensions to the SMTP service
 are registered with the IANA. This framework does not require
 modification of existing SMTP clients or servers unless the features
 of the service extensions are to be requested or provided.

2. Introduction

 The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable,
 effective basis for the relay function of message transfer agents.
 Although a decade old, SMTP has proven remarkably resilient.
 Nevertheless, the need for a number of protocol extensions has become
 evident. Rather than describing these extensions as separate and
 haphazard entities, this document enhances SMTP in a straightforward
 fashion that provides a framework in which all future extensions can
 be built in a single consistent way.

3. Framework for SMTP Extensions

 For the purpose of service extensions to SMTP, SMTP relays a mail
 object containing an envelope and a content.

Klensin, et al Standards Track [Page 1] RFC 1869 SMTP Service Extensions November 1995

(1) The SMTP envelope is straightforward, and is sent as a

     series of SMTP protocol units: it consists of an
     originator address (to which error reports should be
     directed); a delivery mode (e.g., deliver to recipient
     mailboxes); and, one or more recipient addresses.

(2) The SMTP content is sent in the SMTP DATA protocol unit

     and has two parts: the headers and the body. The
     headers form a collection of field/value pairs
     structured according to RFC 822 [2], whilst the body,
     if structured, is defined according to MIME [3]. The
     content is textual in nature, expressed using the US
     ASCII repertoire (ANSI X3.4-1986). Although extensions
     (such as MIME) may relax this restriction for the
     content body, the content headers are always encoded
     using the US ASCII repertoire. The algorithm defined in
     [4] is used to represent header values outside the US
     ASCII repertoire, whilst still encoding them using the
     US ASCII repertoire.
 Although SMTP is widely and robustly deployed, some parts of the
 Internet community might wish to extend the SMTP service.  This memo
 defines a means whereby both an extended SMTP client and server may
 recognize each other as such and the server can inform the client as
 to the service extensions that it supports.
 It must be emphasized that any extension to the SMTP service should
 not be considered lightly. SMTP's strength comes primarily from its
 simplicity.  Experience with many protocols has shown that:
   protocols with few options tend towards ubiquity, whilst
   protocols with many options tend towards obscurity.
 This means that 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.
 Given this environment, the framework for the extensions described in
 this memo consists of:

(1) a new SMTP command (section 4)

(2) a registry of SMTP service extensions (section 5)

(3) additional parameters to the SMTP MAIL FROM and RCPT TO

     commands (section 6).

Klensin, et al Standards Track [Page 2] RFC 1869 SMTP Service Extensions November 1995

4. The EHLO command

 A client SMTP supporting SMTP service extensions should start an SMTP
 session by issuing the EHLO command instead of the HELO command. If
 the SMTP server supports the SMTP service extensions it will give a
 successful response (see section 4.3), a failure response (see 4.4),
 or an error response (4.5). If the SMTP server does not support any
 SMTP service extensions it will generate an error response (see
 section 4.5).

4.1. Changes to STD 10, RFC 821

 This specification is intended to extend STD 10, RFC 821 without
 impacting existing services in any way.  The minor changes needed are
 enumerated below.

4.1.1. First command

 RFC 821 states that the first command in an SMTP session must be the
 HELO command. This requirement is hereby amended to allow a session
 to start with either EHLO or HELO.

4.1.2. Maximum command line length

 This specification extends the SMTP MAIL FROM and RCPT TO to allow
 additional parameters and parameter values.  It is possible that the
 MAIL FROM and RCPT TO lines that result will exceed the 512 character
 limit on command line length imposed by RFC 821.  This limit is
 hereby amended to only apply to command lines without any parameters.
 Each specification that defines new MAIL FROM or RCPT TO parameters
 must also specify maximum parameter value lengths for each parameter
 so that implementors of some set of extensions know how much buffer
 space must be allocated. The maximum command length that must be
 supported by an SMTP implementation with extensions is 512 plus the
 sum of all the maximum parameter lengths for all the extensions
 supported.

4.2. Command syntax

 The syntax for this command, using the ABNF notation of [2], is:
   ehlo-cmd ::= "EHLO" SP domain CR LF
 If successful, the server SMTP responds with code 250. On failure,
 the server SMTP responds with code 550. On error, the server SMTP
 responds with one of codes 500, 501, 502, 504, or 421.

Klensin, et al Standards Track [Page 3] RFC 1869 SMTP Service Extensions November 1995

 This command is issued instead of the HELO command, and may be issued
 at any time that a HELO command would be appropriate.  That is, if
 the EHLO command is issued, and a successful response is returned,
 then a subsequent HELO or EHLO command will result in the server SMTP
 replying with code 503.  A client SMTP must not cache any information
 returned if the EHLO command succeeds. That is, a client SMTP must
 issue the EHLO command at the start of each SMTP session if
 information about extended facilities is needed.

4.3. Successful response

 If the server SMTP implements and is able to perform the EHLO
 command, it will return code 250.  This indicates that both the
 server and client SMTP are in the initial state, that is, there is no
 transaction in progress and all state tables and buffers are cleared.
 Normally, this response will be a multiline reply. Each line of the
 response contains a keyword and, optionally, one or more parameters.
 The syntax for a positive response, using the ABNF notation of [2],
 is:
   ehlo-ok-rsp  ::=      "250"    domain [ SP greeting ] CR LF
                  / (    "250-"   domain [ SP greeting ] CR LF
                      *( "250-"      ehlo-line           CR LF )
                         "250"    SP ehlo-line           CR LF   )
                ; the usual HELO chit-chat
   greeting     ::= 1*<any character other than CR or LF>
   ehlo-line    ::= ehlo-keyword *( SP ehlo-param )
   ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                ; syntax and values depend on ehlo-keyword
   ehlo-param   ::= 1*<any CHAR excluding SP and all
                       control characters (US ASCII 0-31
                       inclusive)>
   ALPHA        ::= <any one of the 52 alphabetic characters
                     (A through Z in upper case, and,
                      a through z in lower case)>
   DIGIT        ::= <any one of the 10 numeric characters
                     (0 through 9)>
   CR           ::= <the carriage-return character
                     (ASCII decimal code 13)>
   LF           ::= <the line-feed character
                     (ASCII decimal code 10)>

Klensin, et al Standards Track [Page 4] RFC 1869 SMTP Service Extensions November 1995

   SP           ::= <the space character
                     (ASCII decimal code 32)>
 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 begun in
 RFC 821.
 The IANA maintains a registry of SMTP service extensions.  Associated
 with each such extension is a corresponding EHLO keyword value. Each
 service extension registered with the IANA must be defined in an RFC.
 Such RFCs must either be on the standards-track or must define an
 IESG-approved experimental protocol.  The definition must include:

(1) the textual name of the SMTP service extension;

(2) the EHLO keyword value associated with the extension;

(3) the syntax and possible values of parameters associated

     with the EHLO keyword value;

(4) 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);

(5) any new parameters the extension associates with the

     MAIL FROM or RCPT TO verbs;

(6) how support for the extension affects the behavior of a

     server and client SMTP; and,

(7) the increment by which the extension is increasing the

     maximum length of the commands MAIL FROM, RCPT TO, or
     both, over that specified in RFC 821.
 In addition, any EHLO keyword value that starts with an upper or
 lower case "X" refers to a local SMTP service extension, which is
 used through bilateral, rather than standardized, agreement. Keywords
 beginning with "X" may not be used in a registered service extension.
 Any 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.

Klensin, et al Standards Track [Page 5] RFC 1869 SMTP Service Extensions November 1995

 Additional verbs are bound by the same rules as EHLO keywords;
 specifically, verbs begining with "X" are local extensions that may
 not be registered or standardized and verbs not beginning with "X"
 must always be registered.

4.4. Failure response

 If for some reason the server SMTP is unable to list the service
 extensions it supports, it will return code 554.
 In the case of a failure response, the client SMTP should issue
 either the HELO or QUIT command.

4.5. Error responses from extended servers

 If the server SMTP recognizes the EHLO command, but the command
 argument is unacceptable, it will return code 501.
 If the server SMTP recognizes, but does not implement, the EHLO
 command, it will return code 502.
 If the server SMTP determines that the SMTP service is no longer
 available (e.g., due to imminent system shutdown), it will return
 code 421.
 In the case of any error response, the client SMTP should issue
 either the HELO or QUIT command.

4.6. Responses from servers without extensions

 A server SMTP that conforms to RFC 821 but does not support the
 extensions specified here will not recognize the EHLO command and
 will consequently return code 500, as specified in RFC 821.  The
 server SMTP should stay in the same state after returning this code
 (see section 4.1.1 of RFC 821).  The client SMTP may then issue
 either a HELO or a QUIT command.

4.7. Responses from improperly implemented servers

 Some SMTP servers are known to disconnect the SMTP transmission
 channel upon receipt of the EHLO command. The disconnect can occur
 immediately or after sending a response.  Such behavior violates
 section 4.1.1 of RFC 821, which explicitly states that disconnection
 should only occur after a QUIT command is issued.
 Nevertheless, in order to achieve maxmimum interoperablity it is
 suggested that extended SMTP clients using EHLO be coded to check for
 server connection closure after EHLO is sent, either before or after

Klensin, et al Standards Track [Page 6] RFC 1869 SMTP Service Extensions November 1995

 returning a reply.  If this happens the client must decide if the
 operation can be successfully completed without using any SMTP
 extensions. If it can a new connection can be opened and the HELO
 command can be used.
 Other improperly-implemented servers will not accept a HELO command
 after EHLO has been sent and rejected.  In some cases, this problem
 can be worked around by sending a RSET after the failure response to
 EHLO, then sending the HELO.  Clients that do this should be aware
 that many implementations will return a failure code (e.g., 503 Bad
 sequence of commands) in response to the RSET.  This code can be
 safely ignored.

5. Initial IANA Registry

 The IANA's initial registry of SMTP service extensions consists of
 these entries:
 Service Ext   EHLO Keyword Parameters Verb       Added Behavior
 ------------- ------------ ---------- ---------- ------------------
 Send             SEND         none       SEND    defined in RFC 821
 Send or Mail     SOML         none       SOML    defined in RFC 821
 Send and Mail    SAML         none       SAML    defined in RFC 821
 Expand           EXPN         none       EXPN    defined in RFC 821
 Help             HELP         none       HELP    defined in RFC 821
 Turn             TURN         none       TURN    defined in RFC 821
 which correspond to those SMTP commands which are defined as optional
 in [5].  (The mandatory SMTP commands, according to [5], are HELO,
 MAIL, RCPT, DATA, RSET, VRFY, NOOP, and QUIT.)

6. MAIL FROM and RCPT TO Parameters

 It is recognized that several of the extensions planned for SMTP will
 make use of additional parameters associated with the MAIL FROM and
 RCPT TO command. The syntax for these commands, again using the ABNF
 notation of [2] as well as underlying definitions from [1], is:
   esmtp-cmd        ::= inner-esmtp-cmd [SP esmtp-parameters] CR LF
   esmtp-parameters ::= esmtp-parameter *(SP esmtp-parameter)
   esmtp-parameter  ::= esmtp-keyword ["=" esmtp-value]
   esmtp-keyword    ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                        ; syntax and values depend on esmtp-keyword
   esmtp-value      ::= 1*<any CHAR excluding "=", SP, and all
                           control characters (US ASCII 0-31
                           inclusive)>

Klensin, et al Standards Track [Page 7] RFC 1869 SMTP Service Extensions November 1995

                        ; The following commands are extended to
                        ; accept extended parameters.
   inner-esmtp-cmd  ::= ("MAIL FROM:" reverse-path)   /
                        ("RCPT TO:" forward-path)
 All esmtp-keyword values must be registered as part of the IANA
 registration process described above. This definition only provides
 the framework for future extension; no extended MAIL FROM or RCPT TO
 parameters are defined by this RFC.

6.1. Error responses

 If the server SMTP does not recognize or cannot implement one or more
 of the parameters associated with a particular MAIL FROM or RCPT TO
 command, it will return code 555.
 If for some reason the server is temporarily unable to accomodate one
 or more of the parameters associated with a MAIL FROM or RCPT TO
 command, and if the definition of the specific parameter does not
 mandate the use of another code, it should return code 455.
 Errors specific to particular parameters and their values will be
 specified in the parameter's defining RFC.

7. Received: Header Field Annotation

 SMTP servers are required to add an appropriate Received: field to
 the headers of all messages they receive. A "with ESMTP" clause
 should be added to this field when any SMTP service extensions are
 used. "ESMTP" is hereby added to the list of standard protocol names
 registered with IANA.

8. Usage Examples

(1) An interaction of the form:

     S: <wait for connection on TCP port 25>
     C: <open connection to server>
     S: 220 dbc.mtview.ca.us SMTP service ready
     C: EHLO ymir.claremont.edu
     S: 250 dbc.mtview.ca.us says hello
      ...
     indicates that the server SMTP implements only those
     SMTP commands which are defined as mandatory in [5].

Klensin, et al Standards Track [Page 8] RFC 1869 SMTP Service Extensions November 1995

(2) In contrast, an interaction of the form:

     S: <wait for connection on TCP port 25>
     C: <open connection to server>
     S: 220 dbc.mtview.ca.us SMTP service ready
     C: EHLO ymir.claremont.edu
     S: 250-dbc.mtview.ca.us says hello
     S: 250-EXPN
     S: 250-HELP
     S: 250-8BITMIME
     S: 250-XONE
     S: 250 XVRB
      ...
     indicates that the server SMTP also implements the SMTP
     EXPN and HELP commands, one standard service extension
     (8BITMIME), and two nonstandard and unregistered
     service extensions (XONE and XVRB).

(3) Finally, a server that does not support SMTP service

     extensions would act as follows:
     S: <wait for connection on TCP port 25>
     C: <open connection to server>
     S: 220 dbc.mtview.ca.us SMTP service ready
     C: EHLO ymir.claremont.edu
     S: 500 Command not recognized: EHLO
      ...
     The 500 response indicates that the server SMTP does
     not implement the extensions specified here.  The
     client would normally send a HELO command and proceed
     as specified in RFC 821.   See section 4.7 for
     additional discussion.

9. Security Considerations

 This RFC does not discuss security issues and is not believed to
 raise any security issues not already endemic in electronic mail and
 present in fully conforming implementations of RFC-821.  It does
 provide an announcement of server mail capabilities via the response
 to the EHLO verb. However, all information provided by announcement
 of any of the initial set of service extensions defined by this RFC
 can be readily deduced by selective probing of the verbs required to
 transport and deliver mail. The security implications of service
 extensions described in other RFCs should be dealt with in those
 RFCs.

Klensin, et al Standards Track [Page 9] RFC 1869 SMTP Service Extensions November 1995

10. Acknowledgements

 This document represents a synthesis of the ideas of many people and
 reactions to the ideas and proposals of others.  Randall Atkinson,
 Craig Everhart, Risto Kankkunen, and Greg Vaudreuil contributed ideas
 and text sufficient to be considered co-authors.  Other important
 suggestions, text, or encouragement came from Harald Alvestrand, Jim
 Conklin, Mark Crispin, Frank da Cruz, 'Olafur Gudmundsson, Per
 Hedeland, Christian Huitma, Neil Katin, Eliot Lear, Harold A.
 Miller, Keith Moore, John Myers, Dan Oscarsson, Julian Onions, Rayan
 Zachariassen, and the contributions of the entire IETF SMTP Working
 Group. Of course, none of the individuals are necessarily responsible
 for the combination of ideas represented here. Indeed, in some cases,
 the response to a particular criticism was to accept the problem
 identification but to include an entirely different solution from the
 one originally proposed.

11. References

 [1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
     USC/Information Sciences Institute, August 1982.
 [2] Crocker, D., "Standard for the Format of ARPA Internet Text
     Messages", STD 11, RFC 822, UDEL, August 1982.
 [3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
     Extensions", RFC 1521, Bellcore, Innosoft, September 1993.
 [4] Moore, K., "Representation of Non-ASCII Text in Internet Message
     Headers", RFC 1522, University of Tennessee, September 1993.
 [5] Braden, R., "Requirements for Internet Hosts - Application and
     Support", STD 3, RFC 1123, USC/Information Sciences Institute,
     October 1989.

12. Chair, Editor, and Author Addresses

 John Klensin, WG Chair
 MCI
 2100 Reston Parkway
 Reston, VA 22091
 Phone: +1 703 715-7361
 Fax: +1 703 715-7436
 EMail: klensin@mci.net

Klensin, et al Standards Track [Page 10] RFC 1869 SMTP Service Extensions November 1995

 Ned Freed, Editor
 Innosoft International, Inc.
 1050 East Garvey Avenue South
 West Covina, CA 91790
 USA
 Phone: +1 818 919 3600
 Fax: +1 818 919 3614
 EMail: ned@innosoft.com
 Marshall T. Rose
 Dover Beach Consulting, Inc.
 420 Whisman Court
 Moutain View, CA  94043-2186
 USA
 Phone: +1 415 968 1052
 Fax: +1 415 968 2510
 EMail: mrose@dbc.mtview.ca.us
 Einar A. Stefferud
 Network Management Associates, Inc.
 17301 Drey Lane
 Huntington Beach, CA, 92647-5615
 USA
 Phone: +1 714 842 3711
 Fax: +1 714 848 2091
 EMail: stef@nma.com
 Dave Crocker
 Brandenburg Consulting
 675 Spruce Dr.
 Sunnyvale, CA 94086 USA
 USA
 Phone: +1 408 246 8253
 Fax: +1 408 249 6205
 EMail: dcrocker@mordor.stanford.edu

Klensin, et al Standards Track [Page 11]

Network Working Group J. Klensin, WG Chair Request For Comments: 1870 MCI STD: 10 N. Freed, Editor Obsoletes: 1653 Innosoft International, Inc. Category: Standards Track K. Moore

                                               University of Tennessee
                                                         November 1995
                       SMTP Service Extension
                    for Message Size Declaration

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.

1. Abstract

 This memo defines an extension to the SMTP service whereby an SMTP
 client and server may interact to give the server an opportunity to
 decline to accept a message (perhaps temporarily) based on the
 client's estimate of the message size.

2. Introduction

 The MIME extensions to the Internet message protocol provide for the
 transmission of many kinds of data which were previously unsupported
 in Internet mail.  One expected result of the use of MIME is that
 SMTP will be expected to carry a much wider range of message sizes
 than was previously the case.  This has an impact on the amount of
 resources (e.g. disk space) required by a system acting as a server.
 This memo uses the mechanism defined in [5] to define extensions to
 the SMTP service whereby a client ("sender-SMTP") may declare the
 size of a particular message to a server ("receiver-SMTP"), after
 which the server may indicate to the client that it is or is not
 willing to accept the message based on the declared message size and
 whereby a server ("receiver-SMTP") may declare the maximum message
 size it is willing to accept to a client ("sender-SMTP").

Klensin, et al Standards Track [Page 1] RFC 1870 SMTP Size Declaration November 1995

3. Framework for the Size Declaration Extension

 The following service extension is therefore defined:
 (1) the name of the SMTP service extension is "Message Size
     Declaration";
 (2) the EHLO keyword value associated with this extension is "SIZE";
 (3) one optional parameter is allowed with this EHLO keyword value, a
     decimal number indicating the fixed maximum message size in bytes
     that the server will accept.  The syntax of the parameter is as
     follows, using the augmented BNF notation of [2]:
         size-param ::= [1*DIGIT]
     A parameter value of 0 (zero) indicates that no fixed maximum
     message size is in force.  If the parameter is omitted no
     information is conveyed about the server's fixed maximum message
     size;
 (4) one optional parameter using the keyword "SIZE" is added to the
     MAIL FROM command.  The value associated with this parameter is a
     decimal number indicating the size of the message that is to be
     transmitted.  The syntax of the value is as follows, using the
     augmented BNF notation of [2]:
         size-value ::= 1*20DIGIT
 (5) the maximum length of a MAIL FROM command line is increased by 26
     characters by the possible addition of the SIZE keyword and
     value;
 (6) no additional SMTP verbs are defined by this extension.
 The remainder of this memo specifies how support for the extension
 affects the behavior of an SMTP client and server.

4. The Message Size Declaration service extension

 An SMTP server may have a fixed upper limit on message size.  Any
 attempt by a client to transfer a message which is larger than this
 fixed upper limit will fail.  In addition, a server normally has
 limited space with which to store incoming messages.  Transfer of a
 message may therefore also fail due to a lack of storage space, but
 might succeed at a later time.

Klensin, et al Standards Track [Page 2] RFC 1870 SMTP Size Declaration November 1995

 A client using the unextended SMTP protocol defined in [1], can only
 be informed of such failures after transmitting the entire message to
 the server (which discards the transferred message).  If, however,
 both client and server support the Message Size Declaration service
 extension, such conditions may be detected before any transfer is
 attempted.
 An SMTP client wishing to relay a large content may issue the EHLO
 command to start an SMTP session, to determine if the server supports
 any of several service extensions.  If the server responds with code
 250 to the EHLO command, and the response includes the EHLO keyword
 value SIZE, then the Message Size Declaration extension is supported.
 If a numeric parameter follows the SIZE keyword value of the EHLO
 response, it indicates the size of the largest message that the
 server is willing to accept.  Any attempt by a client to transfer a
 message which is larger than this limit will be rejected with a
 permanent failure (552) reply code.
 A server that supports the Message Size Declaration extension will
 accept the extended version of the MAIL command described below.
 When supported by the server, a client may use the extended MAIL
 command (instead of the MAIL command as defined in [1]) to declare an
 estimate of the size of a message it wishes to transfer.  The server
 may then return an appropriate error code if it determines that an
 attempt to transfer a message of that size would fail.

5. Definitions

 The message size is defined as the number of octets, including CR-LF
 pairs, but not the SMTP DATA command's terminating dot or doubled
 quoting dots, to be transmitted by the SMTP client after receiving
 reply code 354 to the DATA command.
 The fixed maximum message size is defined as the message size of the
 largest message that a server is ever willing to accept.  An attempt
 to transfer any message larger than the fixed maximum message size
 will always fail.  The fixed maximum message size may be an
 implementation artifact of the SMTP server, or it may be chosen by
 the administrator of the server.
 The declared message size is defined as a client's estimate of the
 message size for a particular message.

Klensin, et al Standards Track [Page 3] RFC 1870 SMTP Size Declaration November 1995

6. The extended MAIL command

 The extended MAIL command is issued by a client when it wishes to
 inform a server of the size of the message to be sent.  The extended
 MAIL command is identical to the MAIL command as defined in [1],
 except that a SIZE parameter appears after the address.
 The complete syntax of this extended command is defined in [5]. The
 esmtp-keyword is "SIZE" and the syntax for esmtp-value is given by
 the syntax for size-value shown above.
 The value associated with the SIZE parameter is a decimal
 representation of the declared message size in octets.  This number
 should include the message header, body, and the CR-LF sequences
 between lines, but not the SMTP DATA command's terminating dot or
 doubled quoting dots. Only one SIZE parameter may be specified in a
 single MAIL command.
 Ideally, the declared message size is equal to the true message size.
 However, since exact computation of the message size may be
 infeasable, the client may use a heuristically-derived estimate.
 Such heuristics should be chosen so that the declared message size is
 usually larger than the actual message size. (This has the effect of
 making the counting or non-counting of SMTP DATA dots largely an
 academic point.)
 NOTE: Servers MUST NOT use the SIZE parameter to determine end of
 content in the DATA command.

6.1 Server action on receipt of the extended MAIL command

 Upon receipt of an extended MAIL command containing a SIZE parameter,
 a server should determine whether the declared message size exceeds
 its fixed maximum message size.  If the declared message size is
 smaller than the fixed maximum message size, the server may also wish
 to determine whether sufficient resources are available to buffer a
 message of the declared message size and to maintain it in stable
 storage, until the message can be delivered or relayed to each of its
 recipients.
 A server may respond to the extended MAIL command with any of the
 error codes defined in [1] for the MAIL command.  In addition, one of
 the following error codes may be returned:
 (1) If the server currently lacks sufficient resources to accept a
     message of the indicated size, but may be able to accept the
     message at a later time, it responds with code "452 insufficient
     system storage".

Klensin, et al Standards Track [Page 4] RFC 1870 SMTP Size Declaration November 1995

 (2) If the indicated size is larger than the server's fixed maximum
     message size, the server responds with code "552 message size
     exceeds fixed maximium message size".
 A server is permitted, but not required, to accept a message which
 is, in fact, larger than declared in the extended MAIL command, such
 as might occur if the client employed a size-estimation heuristic
 which was inaccurate.

6.2 Client action on receiving response to extended MAIL command

 The client, upon receiving the server's response to the extended MAIL
 command, acts as follows:
 (1) If the code "452 insufficient system storage" is returned, the
     client should next send either a RSET command (if it wishes to
     attempt to send other messages) or a QUIT command. The client
     should then repeat the attempt to send the message to the server
     at a later time.
 (2) If the code "552 message exceeds fixed maximum message size" is
     received, the client should immediately send either a RSET command
     (if it wishes to attempt to send additional messages), or a QUIT
     command.  The client should then declare the message undeliverable
     and return appropriate notification to the sender (if a sender
     address was present in the MAIL command).
 A successful (250) reply code in response to the extended MAIL
 command does not constitute an absolute guarantee that the message
 transfer will succeed.  SMTP clients using the extended MAIL command
 must still be prepared to handle both temporary and permanent error
 reply codes (including codes 452 and 552), either immediately after
 issuing the DATA command, or after transfer of the message.

6.3 Messages larger than the declared size.

 Once a server has agreed (via the extended MAIL command) to accept a
 message of a particular size, it should not return a 552 reply code
 after the transfer phase of the DATA command, unless the actual size
 of the message transferred is greater than the declared message size.
 A server may also choose to accept a message which is somewhat larger
 than the declared message size.
 A client is permitted to declare a message to be smaller than its
 actual size.  However, in this case, a successful (250) reply code is
 no assurance that the server will accept the message or has
 sufficient resources to do so.  The server may reject such a message
 after its DATA transfer.

Klensin, et al Standards Track [Page 5] RFC 1870 SMTP Size Declaration November 1995

6.4 Per-recipient rejection based on message size.

 A server that implements this extension may return a 452 or 552 reply
 code in response to a RCPT command, based on its unwillingness to
 accept a message of the declared size for a particular recipient.
 (1) If a 452 code is returned, the client may requeue the message for
     later delivery to the same recipient.
 (2) If a 552 code is returned, the client may not requeue the message
     for later delivery to the same recipient.

7. Minimal usage

 A "minimal" client may use this extension to simply compare its
 (perhaps estimated) size of the message that it wishes to relay, with
 the server's fixed maximum message size (from the parameter to the
 SIZE keyword in the EHLO response), to determine whether the server
 will ever accept the message.  Such an implementation need not
 declare message sizes via the extended MAIL command.  However,
 neither will it be able to discover temporary limits on message size
 due to server resource limitations, nor per-recipient limitations on
 message size.
 A minimal server that employs this service extension may simply use
 the SIZE keyword value to inform the client of the size of the
 largest message it will accept, or to inform the client that there is
 no fixed limit on message size.  Such a server must accept the
 extended MAIL command and return a 552 reply code if the client's
 declared size exceeds its fixed size limit (if any), but it need not
 detect "temporary" limitations on message size.
 The numeric parameter to the EHLO SIZE keyword is optional.  If the
 parameter is omitted entirely it indicates that the server does not
 advertise a fixed maximum message size.  A server that returns the
 SIZE keyword with no parameter in response to the EHLO command may
 not issue a positive (250) response to an extended MAIL command
 containing a SIZE specification without first checking to see if
 sufficient resources are available to transfer a message of the
 declared size, and to retain it in stable storage until it can be
 relayed or delivered to its recipients.  If possible, the server
 should actually reserve sufficient storage space to transfer the
 message.

Klensin, et al Standards Track [Page 6] RFC 1870 SMTP Size Declaration November 1995

8. Example

 The following example illustrates the use of size declaration with
 some permanent and temporary failures.
 S: <wait for connection on TCP port 25>
 C: <open connection to server>
 S: 220 sigurd.innosoft.com -- Server SMTP (PMDF V4.2-6 #1992)
 C: EHLO ymir.claremont.edu
 S: 250-sigurd.innosoft.com
 S: 250-EXPN
 S: 250-HELP
 S: 250 SIZE 1000000
 C: MAIL FROM:<ned@thor.innosoft.com> SIZE=500000
 S: 250 Address Ok.
 C: RCPT TO:<ned@innosoft.com>
 S: 250 ned@innosoft.com OK; can accomodate 500000 byte message
 C: RCPT TO:<ned@ymir.claremont.edu>
 S: 552 Channel size limit exceeded: ned@YMIR.CLAREMONT.EDU
 C: RCPT TO:<ned@hmcvax.claremont.edu>
 S: 452 Insufficient channel storage: ned@hmcvax.CLAREMONT.EDU
 C: DATA
 S: 354 Send message, ending in CRLF.CRLF.
  ...
 C: .
 S: 250 Some recipients OK
 C: QUIT
 S: 221 Goodbye

9. Security Considerations

 The size declaration extensions described in this memo can
 conceivably be used to facilitate crude service denial attacks.
 Specifically, both the information contained in the SIZE parameter
 and use of the extended MAIL command make it somewhat quicker and
 easier to devise an efficacious service denial attack.  However,
 unless implementations are very weak, these extensions do not create
 any vulnerability that has not always existed with SMTP. In addition,
 no issues are addressed involving trusted systems and possible
 release of information via the mechanisms described in this RFC.

10. Acknowledgements

 This document was derived from an earlier Working Group work in
 progess contribution.  Jim Conklin, Dave Crocker, Neil Katin, Eliot
 Lear, Marshall T. Rose, and Einar Stefferud provided extensive
 comments in response to earlier works in progress of both this and
 the previous memo.

Klensin, et al Standards Track [Page 7] RFC 1870 SMTP Size Declaration November 1995

11. References

 [1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
     USC/Information Sciences Institute, August 1982.
 [2] Crocker, D., "Standard for the Format of ARPA Internet Text
     Messages", STD 11, RFC 822, UDEL, August 1982.
 [3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
     Extensions", RFC 1521, Bellcore, Innosoft, September 1993.
 [4] Moore, K., "Representation of Non-ASCII Text in Internet Message
     Headers", RFC 1522, University of Tennessee, September 1993.
 [5] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker,
     "SMTP Service Extensions", STD 11, RFC 1869, MCI, Innosoft
     International, Inc., Dover Beach Consulting, Inc., Network
     Management Associates, Inc., Brandenburg Consulting, November
     1995.
 [6] Partridge, C., "Mail Routing and the Domain System", STD 14, RFC
     974, BBN, January 1986.

Klensin, et al Standards Track [Page 8] RFC 1870 SMTP Size Declaration November 1995

12. Chair, Editor, and Author Addresses

 John Klensin, WG Chair
 MCI
 2100 Reston Parkway
 Reston, VA 22091
 Phone: +1 703 715-7361
 Fax: +1 703 715-7436
 EMail: klensin@mci.net
 Ned Freed, Editor
 Innosoft International, Inc.
 1050 East Garvey Avenue South
 West Covina, CA 91790
 USA
 Phone: +1 818 919 3600
 Fax: +1 818 919 3614
 EMail: ned@innosoft.com
 Keith Moore
 Computer Science Dept.
 University of Tennessee
 107 Ayres Hall
 Knoxville, TN 37996-1301
 USA
 EMail: moore@cs.utk.edu

Klensin, et al Standards Track [Page 9]

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