    RFC788

                         SIMPLE MAIL TRANSFER PROTOCOL

                               Jonathan B. Postel

                                 November 1981

                         Information Sciences Institute
                       University of Southern California
                               4676 Admiralty Way
                       Marina del Rey, California  90291

                                 (213) 822-1511

    RFC 788                                                    November 1981
                                               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 ..................................... 10
          3.5.  Opening and Closing ..................................... 12
          3.6.  Relaying ................................................ 13
          3.7.  Domains ................................................. 15

       4.  THE SMTP SPECIFICATIONS ...................................... 16

          4.1.  SMTP Commands ........................................... 16
          4.1.1.  Command Semantics ..................................... 16
          4.1.2.  Command Syntax ........................................ 23
          4.2.  SMTP Replies ............................................ 28
          4.2.1.  Reply Codes by Function Group ......................... 29
          4.2.2.  Reply Codes in Numeric Order .......................... 30
          4.3.  Sequencing of Commands and Replies ...................... 31
          4.4.  State Diagrams .......................................... 33
          4.5.  Details ................................................. 35
          4.5.1.  Minimum Implementation ................................ 35
          4.5.2.  Transparency .......................................... 35
          4.5.3.  Sizes ................................................. 36

       APPENDIX A:  TCP ................................................. 38
       APPENDIX B:  NCP ................................................. 39
       APPENDIX C:  NITS ................................................ 40
       APPENDIX D:  X.25 ................................................ 41
       APPENDIX E:  Theory of Reply Codes ............................... 42
       APPENDIX F:  Scenarios ........................................... 45

       GLOSSARY ......................................................... 58

       REFERENCES ....................................................... 61

    Network Working Group                                          J. Postel
    Request for Comments: 788                                            ISI
    Replaces: RFC 780, 772                                     November 1981

                         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]

    November 1981                                                    RFC 788
    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
       full-duplex 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.

         -------------------------------------------------------------

                   +----------+                +----------+
       +------+    |          |                |          |
       | User |<-->|          |      SMTP      |          |
       +------+    |  Sender- |Commands/Replies| Receiver-|
       +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
       | File |<-->|          |    and Mail    |          |<-->| File |
       |System|    |          |                |          |    |System|
       +------+    +----------+                +----------+    +------+

                    Sender-SMTP                Receiver-SMTP

                               Model for SMTP Use

                                    Figure 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 788                                                    November 1981
                                               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].  Each 7-bit character is transmitted right
       justified in an 8-bit byte (or 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 a meta-linguistic variables.
       However, some arguments use the angle brackets literally.  For
       example, an actual reverse-path is enclosed in angle brackets, i.e.,
       "<Smith@ISIA>" is an instance of <reverse-path> (the angle brackets
       are actually transmitted in the command or reply).

    Postel                                                          [Page 3]

    November 1981                                                    RFC 788
    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, and a note on
       mail domains.  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 a SMTP mail transaction.  The transaction
          is started with a MAIL command which gives the sender
          identification.  A series of one or more RCPT commands follow
          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 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 788                                                    November 1981
                                               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
             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 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 SMTP 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.

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          -------------------------------------------------------------

                         Example of the SMTP Procedure

             This SMTP example shows mail sent by Smith at host Alpha, to
             Jones, Green, and Brown at host Beta.  Here we assume that host
             Alpha contacts host Beta directly.

                S: MAIL FROM:<Smith@Alpha>
                R: 250 OK

                S: RCPT TO:<Jones@Beta>
                R: 250 OK

                S: RCPT TO:<Green@Beta>
                R: 550 No such user here

                S: RCPT TO:<Brown@Beta>
                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

          -------------------------------------------------------------

    [Page 6]                                                          Postel

    RFC 788                                                    November 1981
                                               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 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.

          -------------------------------------------------------------

                             Example of Forwarding

             Either

                S: RCPT TO:<Postel@ISI>
                R: 251 User not local; will forward to <Postel@ISIF>

             Or

                S: RCPT TO:<Paul@ISIB>
                R: 551 User not local; please try <Mockapetris@ISIF>

                                   Example 2

          -------------------------------------------------------------

    Postel                                                          [Page 7]

    November 1981                                                    RFC 788
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       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 a character string arguments.  For the
          VRFY command, the string is a user name, and the 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.

          The case of verifying a user name is straightforward as shown in
          example 3.

          -------------------------------------------------------------

                        Example of Verifying a User Name

             Either

                S: VRFY Postel
                R: 250 Jon Postel <Postel@ISIF>

             Or

                S: VRFY Jones
                R: 550 String does not match anything.

                                   Example 3

          -------------------------------------------------------------

    [Page 8]                                                          Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

          The case of expanding a mailbox list requires a multiline reply as
          shown in example 4.

          -------------------------------------------------------------

                      Example of Expanding a Mailing List

             Either

                S: EXPN Example-People
                R: 250-Jon Postel <Postel@ISIF>
                R: 250-Fred Fonebone <Fonebone@ISIQ>
                R: 250-Sam Q. Smith <SQSmith@ISIQ>
                R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
                R: 250-<joe@foo-unix>
                R: 250 <xyz@bar-unix>

             Or

                S: EXPN Executive-Washroom-List
                R: 550 Access Denied to You.

                                   Example 4

          -------------------------------------------------------------

          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.

    Postel                                                          [Page 9]

    November 1981                                                    RFC 788
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       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).  User's should have the ability to control the
          writing of messages on their terminals.  Most hosts permit the
          user's 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.

    [Page 10]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

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

    Postel                                                         [Page 11]

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       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> <host> <CRLF>

             QUIT <CRLF>

          In the HELO command the host sending the command identifies
          itself; the command may be interpreted as saying "Hello, i am
          <host>".

          -------------------------------------------------------------

                         Example of Connection Opening

             R: 220 BBN-UNIX Simple Mail Transfer Service Ready
             S: HELO USC-ISIF
             R: 250 BBN-UNIX

                                   Example 5

          -------------------------------------------------------------

          -------------------------------------------------------------

                         Example of Connection Closing

             S: QUIT
             R: 221 BBN-UNIX Service closing transmission channel

                                   Example 6

          -------------------------------------------------------------

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    RFC 788                                                    November 1981
                                               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.

          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.

          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
          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).

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          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.

          -------------------------------------------------------------

                Example Undeliverable Mail Notification Message

             S: MAIL FROM:<>
             R: 250 ok
             S: RCPT TO:<@HOSTX,JOE@HOSTW>
             R: 250 ok
             S: DATA
             R: 354 send the mail data, end with .
             S: Date: 23 Oct 81
             S: Sender: SMTP@HOSTY
             S: Subject: Mail System Problem
             S:
             S:   Sorry JOE, your message to SAM@HOSTZ lost.
             S:   HOSTZ said this:
             S:    "550 No Such User"
             S: .
             R: 250 ok

                                   Example 7

          -------------------------------------------------------------

    [Page 14]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

       3.7.  DOMAINS

          At some not too distant future time it might be necessary to
          expand the mailbox format to include a region or name domain
          identifier.  There is quite a bit of discussion on this at
          present, and is likely that SMTP will be revised in the future to
          take into account naming domains.

          The examples in this document do not show mail domains.

    Postel                                                         [Page 15]

    November 1981                                                    RFC 788
    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.

             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

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    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

                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 is 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.

                   For example, mail received at relay host A with arguments

                      FROM:<X@Y>
                      TO:<@A,@B,C@D>

                   will be relayed on to host B with arguments

                      FROM:<@A,X@Y>
                      TO:<@B,C@D>.

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                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
                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.

                The preceding two paragraphs imply that the final mail data

    [Page 18]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

                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].  For example:

                   Return-Path: <@GHI,@DEF,@ABC,JOE@ABC>
                   Mail-From: GHI received by JKL at 27-Oct-81 15:27:39-PST
                   Mail-From: DEF received by GHI at 27-Oct-81 15:15:13-PST
                   Mail-From: ABC received by DEF at 27-Oct-81 15:01:59-PST
                   Date: 27-Oct-81 15:01:01-PST
                   From: JOE@ABC
                   Subject: Improved Mailing System Installed
                   To: SAM@JKL

                   This is to inform you that ...

                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).

             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 the 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.

    Postel                                                         [Page 19]

    November 1981                                                    RFC 788
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                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
                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 the 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

    [Page 20]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

                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
                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.

    Postel                                                         [Page 21]

    November 1981                                                    RFC 788
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                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.

             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).

             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.

    [Page 22]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

                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.

                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".

             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.

    Postel                                                         [Page 23]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

             The following are the SMTP commands:

                HELO <SP> <host> <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>

    [Page 24]                                                         Postel

    RFC 788                                                    November 1981
                                               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> ::= "<" ["@" <host> "," ...] <mailbox> ">"

                <host> ::= <a> <string> | "#" <number> | "[" <dotnum> "]"

                <mailbox> ::= <user> "@" <host>

                <user> ::= <string>

                <string> ::= <char> | <char> <string>

                <char> ::= <c> | '\' <c> | '\' <s>

                <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>

                <number> ::= <d> | <d> <number>

                <snum> ::= 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 except
                          <specials>

                <d> ::= any one of the ten digits 0 through 9

                <s> ::= any one of <specials>

                <specials> ::= '<', '>', '(', ')', '\', ',', ';', ':', '@',
                '"', and the control characters (ASCII codes 0 through 37
                octal inclusive and 177 octal)

             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.

    Postel                                                         [Page 25]

    November 1981                                                    RFC 788
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             Hosts are generally known by names which are translated to
             addresses in each host.  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.

             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> ::= "Mail-From:" <SP> <stamp> <CRLF>

                <stamp> ::= [<ptcl>] <from-host> <this-host> <daytime>

                <ptcl> ::= <protocol> <SP> "host" <SP>

                <from-host> ::= <host> <SP>

                <this-host> ::= "received by" <SP> <host> <SP>

                <protocol> ::= "TCP" | "NCP" | "NITS" | "X25" | "INTERNET" |
                          "ARPANET"

                   Note: INTERNET = TCP, ARPANET = NCP, and if the <ptcl> is
                             not present INTERNET is assumed.

                <daytime> ::= "at" <SP> <date> <SP> <time>

                <date> ::= <dd> "-" <mon> "-" <yy>

                <time> ::= <hh> ":" <mm> ":" <ss> "-" <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 01 to 99.

    [Page 26]                                                         Postel

    RFC 788                                                    November 1981
                                               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> ::= a time zone designator (as in [2]) or "UT" for
                          Universal Time (the default).

             Return Path Example:

                Return-Path: <@CHARLIE,@BAKER,JOE@ABLE>

             Mail From Example:

                Mail-From: ABC received by XYZ at 22-OCT-81 09:23:59-PDT

    Postel                                                         [Page 27]

    November 1981                                                    RFC 788
    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, so there are likely to be varying texts
          for each reply code.  A discussion of the theory of reply codes is
          given in the 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 command are expected to result in multiline replies in
          normal circumstances, however multiline replies are allowed for
          any command.

    [Page 28]                                                         Postel

    RFC 788                                                    November 1981
                                               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 <host> Service ready
             221 <host> Service closing transmission channel
             421 <host> 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 29]

    November 1981                                                    RFC 788
    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 <host> Service ready
             221 <host> 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 <host> 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 30]                                                         Postel

    RFC 788                                                    November 1981
                                               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 "Awaiting input" 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 <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 31]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

                RCPT
                   S: 250, 251
                   F: 550, 551, 552, 553, 450, 451, 452
                   E: 500, 501, 421
                DATA
                   I: 354 -> data -> S: 250
                                     F: 552, 554, 451, 452
                   F: 451, 554
                   E: 500, 501, 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
                   F: 550
                   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

    [Page 32]                                                         Postel

    RFC 788                                                    November 1981
                                               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.

    Postel                                                         [Page 33]

    November 1981                                                    RFC 788
    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 34]                                                         Postel

    RFC 788                                                    November 1981
                                               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 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 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 recipients mailbox
             including format effectors and other control characters.  The
             7-bit ASCII codes are transmitted right justified in 8-bit
             bytes (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
                strings.  If such transforms are necessary, they must be
                reversible -- especially if such transforms are applied to
                mail being relayed.

    Postel                                                         [Page 35]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

          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.

                host

                   The maximum total length of a host name or number is 40
                   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.

                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).

    [Page 36]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

                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 37]

    November 1981                                                    RFC 788
    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 a 8-bit byte with the high-order bit cleared to
             zero.

    [Page 38]                                                         Postel

    RFC 788                                                    November 1981
                                               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 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 a 8-bit byte with the high-order bit cleared to
             zero.

    Postel                                                         [Page 39]

    November 1981                                                    RFC 788
    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 a 8-bit byte with the high-order bit cleared to
             zero.

    [Page 40]                                                         Postel

    RFC 788                                                    November 1981
                                               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, but there are indications that it
          is too error prone to qualify as a reliable channel.  It is
          suggested that a reliable end-to-end protocol such as TCP be used
          on top of X.25 connections.

    Postel                                                         [Page 41]

    November 1981                                                    RFC 788
    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 42]                                                         Postel

    RFC 788                                                    November 1981
                                               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 43]

    November 1981                                                    RFC 788
    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 multi-line 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

             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.

    [Page 44]                                                         Postel

    RFC 788                                                    November 1981
                                               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.

          -------------------------------------------------------------

             R: 220 BBN-UNIX Simple Mail Transfer Service Ready
             S: HELO USC-ISIF
             R: 250 BBN-UNIX

             S: MAIL FROM:<Smith@USC-ISIF>
             R: 250 OK

             S: RCPT TO:<Jones@BBN-UNIX>
             R: 250 OK

             S: RCPT TO:<Green@BBN-UNIX>
             R: 550 No such user here

             S: RCPT TO:<Brown@BBN-UNIX>
             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 Service closing transmission channel

                                   Scenario 1

          -------------------------------------------------------------

    Postel                                                         [Page 45]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

       Aborted SMTP Transaction Scenario

          -------------------------------------------------------------

             R: 220 MIT-Multics Simple Mail Transfer Service Ready
             S: HELO ISI-VAXA
             R: 250 MIT-Multics

             S: MAIL FROM:<Smith@ISI-VAXA>
             R: 250 OK

             S: RCPT TO:<Jones@MIT-Multics>
             R: 250 OK

             S: RCPT TO:<Green@MIT-Multics>
             R: 550 No such user here

             S: RSET
             R: 250 OK

             S: QUIT
             R: 221 MIT-Multics Service closing transmission channel

                                   Scenario 2

          -------------------------------------------------------------

    [Page 46]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

       Relayed Mail Scenario

          -------------------------------------------------------------

             Step 1  --  Source Host to Relay Host

                R: 220 USC-ISIE Simple Mail Transfer Service Ready
                S: HELO MIT-AI
                R: 250 USC-ISIE

                S: MAIL FROM:<JQP@MIT-AI>
                R: 250 OK

                S: RCPT TO:<@ISIE,Jones@BBN-VAX>
                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 at MIT-AI>
                S: Subject:  The Next Meeting of the Board
                S: To: Jones at BBN-Vax
                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 Service closing transmission channel

    Postel                                                         [Page 47]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

             Step 2  --  Relay Host to Destination Host

                R: 220 BBN-VAX Simple Mail Transfer Service Ready
                S: HELO USC-ISIE
                R: 250 BBN-VAX

                S: MAIL FROM:<@ISIE,JQP@MIT-AI>
                R: 250 OK

                S: RCPT TO:<Jones@BBN-VAX>
                R: 250 OK

                S: DATA
                R: 354 Start mail input; end with <CRLF>.<CRLF>
                S: Mail-From: NCP host MIT-AI received by USC-ISIE at
                   2-Nov-81 22:40:10
                S: Date: 2-Nov-81 22:33:44
                S: From: John Q. Public <JQP at MIT-AI>
                S: Subject:  The Next Meeting of the Board
                S: To: Jones at BBN-Vax
                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 Service closing transmission channel

                                   Scenario 3

          -------------------------------------------------------------

    [Page 48]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

       Verifying and Sending Scenario

          -------------------------------------------------------------

             R: 220 SU-SCORE Simple Mail Transfer Service Ready
             S: HELO MIT-MC
             R: 250 SU-SCORE

             S: VRFY Crispin
             R: 250 Mark Crispin <Admin.MRC@SU-SCORE>

             S: SEND FROM:<EAK@MIT-MC>
             R: 250 OK

             S: RCPT TO:<Admin.MRC@SU-SCORE>
             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 Service closing transmission channel

                                   Scenario 4

          -------------------------------------------------------------

    Postel                                                         [Page 49]

    November 1981                                                    RFC 788
    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.

          -------------------------------------------------------------

             R: 220 SU-SCORE Simple Mail Transfer Service Ready
             S: HELO MIT-MC
             R: 250 SU-SCORE

             S: VRFY Crispin
             R: 250 Mark Crispin <Admin.MRC@SU-SCORE>

             S: SEND FROM:<EAK@MIT-MC>
             R: 250 OK

             S: RCPT TO:<Admin.MRC@SU-SCORE>
             R: 450 User not active now

             S: RSET
             R: 250 OK

             S: MAIL FROM:<EAK@MIT-MC>
             R: 250 OK

             S: RCPT TO:<Admin.MRC@SU-SCORE>
             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 Service closing transmission channel

                                   Scenario 5

          -------------------------------------------------------------

    [Page 50]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

          Doing the preceding scenario more efficiently.

          -------------------------------------------------------------

             R: 220 SU-SCORE Simple Mail Transfer Service Ready
             S: HELO MIT-MC
             R: 250 SU-SCORE

             S: VRFY Crispin
             R: 250 Mark Crispin <Admin.MRC@SU-SCORE>

             S: SOML FROM:<EAK@MIT-MC>
             R: 250 OK

             S: RCPT TO:<Admin.MRC@SU-SCORE>
             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 Service closing transmission channel

                                   Scenario 6

          -------------------------------------------------------------

    Postel                                                         [Page 51]

    November 1981                                                    RFC 788
    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.

          -------------------------------------------------------------

             Step 1  --  Expanding the First List

                R: 220 MIT-AI Simple Mail Transfer Service Ready
                S: HELO SU-SCORE
                R: 250 MIT-AI

                S: EXPN Example-People
                R: 250-<ABC@MIT-MC>
                R: 250-Fred Fonebone <Fonebone@ISIQ>
                R: 250-Xenon Y. Zither <XYZ@MIT-AI>
                R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
                R: 250-<joe@foo-unix>
                R: 250 <xyz@bar-unix>

                S: QUIT
                R: 221 MIT-AI Service closing transmission channel

    [Page 52]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

             Step 2  --  Expanding the Second List

                R: 220 MIT-MC Simple Mail Transfer Service Ready
                S: HELO SU-SCORE
                R: 250 MIT-MC

                S: EXPN Interested-Parties
                R: 250-Al Calico <ABC@MIT-MC>
                R: 250-<XYZ@MIT-AI>
                R: 250-Quincy Smith <@ISIF,Q-Smith@ISI-VAXA>
                R: 250-<fred@BBN-UNIX>
                R: 250 <xyz@bar-unix>

                S: QUIT
                R: 221 MIT-MC Service closing transmission channel

    Postel                                                         [Page 53]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

             Step 3  --  Mailing to All via a Relay Host

                R: 220 USC-ISIE Simple Mail Transfer Service Ready
                S: HELO SU-SCORE
                R: 250 USC-ISIE

                S: MAIL FROM:<Account.Person@SU-SCORE>
                R: 250 OK
                S: RCPT TO:<@ISIE,ABC@MIT-MC>
                R: 250 OK
                S: RCPT TO:<@ISIE,Fonebone@ISIQ>
                R: 250 OK
                S: RCPT TO:<@ISIE,XYZ@MIT-AI>
                R: 250 OK
                S: RCPT TO:<@ISIE,@ISIF,Q-Smith@ISI-VAXA>
                R: 250 OK
                S: RCPT TO:<@ISIE,joe@FOO-UNIX>
                R: 250 OK
                S: RCPT TO:<@ISIE,xyz@BAR-UNIX>
                R: 250 OK
                S: RCPT TO:<@ISIE,fred@BBN-UNIX>
                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 Service closing transmission channel

                                   Scenario 7

          -------------------------------------------------------------

    [Page 54]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

       Forwarding Scenarios

          -------------------------------------------------------------

             R: 220 USC-ISIF Simple Mail Transfer Service Ready
             S: HELO LBL-UNIX
             R: 250 USC-ISIF

             S: MAIL FROM:<mo@LBL-UNIX>
             R: 250 OK

             S: RCPT TO:<fred@USC-ISIF>
             R: 251 User not local; will forward to <Jones@USC-ISIA>

             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 Service closing transmission channel

                                   Scenario 8

          -------------------------------------------------------------

    Postel                                                         [Page 55]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

          -------------------------------------------------------------

             Step 1  --  Trying the Mailbox at the First Host

                R: 220 USC-ISIF Simple Mail Transfer Service Ready
                S: HELO LBL-UNIX
                R: 250 USC-ISIF

                S: MAIL FROM:<mo@LBL-UNIX>
                R: 250 OK

                S: RCPT TO:<fred@USC-ISIF>
                R: 251 User not local; will forward to <Jones@USC-ISIA>

                S: RSET
                R: 250 OK

                S: QUIT
                R: 221 USC-ISIF Service closing transmission channel

             Step 2  --  Delivering the Mail at the Second Host

                R: 220 USC-ISIA Simple Mail Transfer Service Ready
                S: HELO LBL-UNIX
                R: 250 USC-ISIA

                S: MAIL FROM:<mo@LBL-UNIX>
                R: 250 OK

                S: RCPT TO:<Jones@USC-ISIA>
                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-ISIA Service closing transmission channel

                                   Scenario 9

          -------------------------------------------------------------

    [Page 56]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

       Too Many Recipients Scenario

          -------------------------------------------------------------

             R: 220 BERKELEY Simple Mail Transfer Service Ready
             S: HELO USC-ISIF
             R: 250 BERKELEY

             S: MAIL FROM:<Postel@USC-ISIF>
             R: 250 OK

             S: RCPT TO:<fabry@BERKELEY>
             R: 250 OK

             S: RCPT TO:<eric@BERKELEY>
             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>
             R: 250 OK

             S: RCPT TO:<eric@BERKELEY>
             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 Service closing transmission channel

                                  Scenario 10

          -------------------------------------------------------------

    Postel                                                         [Page 57]

    November 1981                                                    RFC 788
    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.

       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 line of text 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 Network
          Text Messages (RFC 733 [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@host".  Additionally, the "container" in which mail is
          stored.

       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.

    [Page 58]                                                         Postel

    RFC 788                                                    November 1981
                                               Simple Mail Transfer Protocol

       reply

          A reply is an acknowledgment (positive or negative) sent from
          receiver to sender via the transmission channel in response to a
          SMTP 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.

       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.

    Postel                                                         [Page 59]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

       word

          A sequence of printing characters.

       <CRLF>

          The characters carriage return and line feed (in that order).

       <SP>

          The space character.

    [Page 60]                                                         Postel

    RFC 788                                                    November 1981
                                               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 733

          Crocker, D., J. Vittal, K. Pogran, and D. Henderson, "Standard for
          the Format of ARPA Network Text Messages," RFC 733, NIC 41952,
          November 1977.  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.

       [3]  TCP

          Postel, J., ed., "Transmission Control Protocol - DARPA Internet
          Program Protocol Specification", RFC 793, USC/Information Sciences
          Institute, September 1981.

       [4]  NCP

          McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
          January 1972.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
          Protocol Handbook", NIC 7104, for the Defense Communications
          Agency by SRI International, Menlo Park, California, Revised
          January 1978.

       [5]  Initial Connection Protocol

          Postel, J., "Official Initial Connection Protocol", NIC 7101,
          11 June 1971.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
          Protocol Handbook", NIC 7104, for the Defense Communications
          Agency by SRI International, Menlo Park, California, Revised
          January 1978.

       [6]  NITS

          PSS/SG3, "A Network Independent Transport Service", Study Group 3,
          The Post Office PSS Users Group, February 1980.  Available from
          the DCPU, National Physical Laboratory, Teddington, UK.

    Postel                                                         [Page 61]

    November 1981                                                    RFC 788
    Simple Mail Transfer Protocol

       [7]  X.25

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

    [Page 62]                                                         Postel