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rfc:rfc821
 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. ————————————————————

[Page 6] Postel

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.

[Page 12] Postel

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

[Page 14] Postel

RFC 821 August 1982

                                         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. ————————————————————

[Page 16] Postel

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.

Postel [Page 17]

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. ————————————————————

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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. ————————————————————

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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.

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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.

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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.

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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.

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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.

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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.

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