Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site sdcc3.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!genrad!decvax!harpo!whuxlm!akgua!sdcsvax!sdcc3!brian
From: brian@sdcc3.UUCP (Brian Kantor)
Newsgroups: net.sources
Subject: RFC821 (SMTP) [pt 2 of 2]
Message-ID: <2625@sdcc3.UUCP>
Date: Fri, 11-Jan-85 13:44:18 EST
Article-I.D.: sdcc3.2625
Posted: Fri Jan 11 13:44:18 1985
Date-Received: Mon, 14-Jan-85 01:47:22 EST
Organization: UCSD wombat breeding society
Lines: 2147



                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



         The time stamp line and the return path line are formally
         defined as follows:

         <return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>

         <time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>

            <stamp> ::= <from-domain> <by-domain> <opt-info> ";"
                      <daytime>

            <from-domain> ::= "FROM" <SP> <domain> <SP>

            <by-domain> ::= "BY" <SP> <domain> <SP>

            <opt-info> ::= [<via>] [<with>] [<id>] [<for>]

            <via> ::= "VIA" <SP> <link> <SP>

            <with> ::= "WITH" <SP> <protocol> <SP>

            <id> ::= "ID" <SP> <string> <SP>

            <for> ::= "FOR" <SP> <path> <SP>

            <link> ::= The standard names for links are registered with
                      the Network Information Center.

            <protocol> ::= The standard names for protocols are
                      registered with the Network Information Center.

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

            <date> ::= <dd> <SP> <mon> <SP> <yy>

            <time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>

            <dd> ::= the one or two decimal integer day of the month in
                      the range 1 to 31.

            <mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
                      "JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"

            <yy> ::= the two decimal integer year of the century in the
                      range 00 to 99.





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                                           Simple Mail Transfer Protocol



            <hh> ::= the two decimal integer hour of the day in the
                      range 00 to 24.

            <mm> ::= the two decimal integer minute of the hour in the
                      range 00 to 59.

            <ss> ::= the two decimal integer second of the minute in the
                      range 00 to 59.

            <zone> ::= "UT" for Universal Time (the default) or other
                      time zone designator (as in [2]).

            

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

                          Return Path Example

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

                               Example 9

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

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

                        Time Stamp Line Example

      Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT

         Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
                   id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT

                               Example 10

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













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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   4.2.  SMTP REPLIES

      Replies to SMTP commands are devised to ensure the synchronization
      of requests and actions in the process of mail transfer, and to
      guarantee that the sender-SMTP always knows the state of the
      receiver-SMTP.  Every command must generate exactly one reply.

         The details of the command-reply sequence are made explicit in
         Section 5.3 on Sequencing and Section 5.4 State Diagrams.

      An SMTP reply consists of a three digit number (transmitted as
      three alphanumeric characters) followed by some text.  The number
      is intended for use by automata to determine what state to enter
      next; the text is meant for the human user.  It is intended that
      the three digits contain enough encoded information that the
      sender-SMTP need not examine the text and may either discard it or
      pass it on to the user, as appropriate.  In particular, the text
      may be receiver-dependent and context dependent, so there are
      likely to be varying texts for each reply code.  A discussion of
      the theory of reply codes is given in Appendix E.  Formally, a
      reply is defined to be the sequence:  a three-digit code, <SP>,
      one line of text, and <CRLF>, or a multiline reply (as defined in
      Appendix E).  Only the EXPN and HELP commands are expected to
      result in multiline replies in normal circumstances, however
      multiline replies are allowed for any command.
























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                                           Simple Mail Transfer Protocol



      4.2.1.  REPLY CODES BY FUNCTION GROUPS

         500 Syntax error, command unrecognized
            [This may include errors such as command line too long]
         501 Syntax error in parameters or arguments
         502 Command not implemented
         503 Bad sequence of commands
         504 Command parameter not implemented
          
         211 System status, or system help reply
         214 Help message
            [Information on how to use the receiver or the meaning of a
            particular non-standard command; this reply is useful only
            to the human user]
          
         220 <domain> Service ready
         221 <domain> Service closing transmission channel
         421 <domain> Service not available,
             closing transmission channel
            [This may be a reply to any command if the service knows it
            must shut down]
          
         250 Requested mail action okay, completed
         251 User not local; will forward to <forward-path>
         450 Requested mail action not taken: mailbox unavailable
            [E.g., mailbox busy]
         550 Requested action not taken: mailbox unavailable
            [E.g., mailbox not found, no access]
         451 Requested action aborted: error in processing
         551 User not local; please try <forward-path>
         452 Requested action not taken: insufficient system storage
         552 Requested mail action aborted: exceeded storage allocation
         553 Requested action not taken: mailbox name not allowed
            [E.g., mailbox syntax incorrect]
         354 Start mail input; end with <CRLF>.<CRLF>
         554 Transaction failed
         












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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



      4.2.2.  NUMERIC ORDER LIST OF REPLY CODES

         211 System status, or system help reply
         214 Help message
            [Information on how to use the receiver or the meaning of a
            particular non-standard command; this reply is useful only
            to the human user]
         220 <domain> Service ready
         221 <domain> Service closing transmission channel
         250 Requested mail action okay, completed
         251 User not local; will forward to <forward-path>
          
         354 Start mail input; end with <CRLF>.<CRLF>
          
         421 <domain> Service not available,
             closing transmission channel
            [This may be a reply to any command if the service knows it
            must shut down]
         450 Requested mail action not taken: mailbox unavailable
            [E.g., mailbox busy]
         451 Requested action aborted: local error in processing
         452 Requested action not taken: insufficient system storage
          
         500 Syntax error, command unrecognized
            [This may include errors such as command line too long]
         501 Syntax error in parameters or arguments
         502 Command not implemented
         503 Bad sequence of commands
         504 Command parameter not implemented
         550 Requested action not taken: mailbox unavailable
            [E.g., mailbox not found, no access]
         551 User not local; please try <forward-path>
         552 Requested mail action aborted: exceeded storage allocation
         553 Requested action not taken: mailbox name not allowed
            [E.g., mailbox syntax incorrect]
         554 Transaction failed
         












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                                           Simple Mail Transfer Protocol



   4.3.  SEQUENCING OF COMMANDS AND REPLIES

      The communication between the sender and receiver is intended to
      be an alternating dialogue, controlled by the sender.  As such,
      the sender issues a command and the receiver responds with a
      reply.  The sender must wait for this response before sending
      further commands.

      One important reply is the connection greeting.  Normally, a
      receiver will send a 220 "Service ready" reply when the connection
      is completed.  The sender should wait for this greeting message
      before sending any commands.

         Note: all the greeting type replies have the official name of
         the server host as the first word following the reply code.

            For example,

               220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>

      The table below lists alternative success and failure replies for
      each command.  These must be strictly adhered to; a receiver may
      substitute text in the replies, but the meaning and action implied
      by the code numbers and by the specific command reply sequence
      cannot be altered.

      COMMAND-REPLY SEQUENCES

         Each command is listed with its possible replies.  The prefixes
         used before the possible replies are "P" for preliminary (not
         used in SMTP), "I" for intermediate, "S" for success, "F" for
         failure, and "E" for error.  The 421 reply (service not
         available, closing transmission channel) may be given to any
         command if the SMTP-receiver knows it must shut down.  This
         listing forms the basis for the State Diagrams in Section 4.4.

            CONNECTION ESTABLISHMENT
               S: 220
               F: 421
            HELO
               S: 250
               E: 500, 501, 504, 421
            MAIL
               S: 250
               F: 552, 451, 452
               E: 500, 501, 421



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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



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




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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   4.4.  STATE DIAGRAMS

      Following are state diagrams for a simple-minded SMTP
      implementation.  Only the first digit of the reply codes is used.
      There is one state diagram for each group of SMTP commands.  The
      command groupings were determined by constructing a model for each
      command and then collecting together the commands with
      structurally identical models.

      For each command there are three possible outcomes:  "success"
      (S), "failure" (F), and "error" (E). In the state diagrams below
      we use the symbol B for "begin", and the symbol W for "wait for
      reply".

      First, the diagram that represents most of the SMTP commands:

         
                                  1,3    +---+
                             ----------->| E |
                            |            +---+
                            |
         +---+    cmd    +---+    2      +---+
         | B |---------->| W |---------->| S |
         +---+           +---+           +---+
                            |
                            |     4,5    +---+
                             ----------->| F |
                                         +---+
         

         This diagram models the commands:

            HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
            NOOP, QUIT, TURN.















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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



      A more complex diagram models the DATA command:

         
         +---+   DATA    +---+ 1,2                 +---+
         | B |---------->| W |-------------------->| E |
         +---+           +---+        ------------>+---+
                         3| |4,5     |
                          | |        |
            --------------   -----   |
           |                      |  |             +---+
           |               ----------     -------->| S |
           |              |       |      |         +---+
           |              |  ------------
           |              | |     |
           V           1,3| |2    |
         +---+   data    +---+     --------------->+---+
         |   |---------->| W |                     | F |
         +---+           +---+-------------------->+---+
                              4,5


         Note that the "data" here is a series of lines sent from the
         sender to the receiver with no response expected until the last
         line is sent.

























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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   4.5.  DETAILS

      4.5.1.  MINIMUM IMPLEMENTATION

         In order to make SMTP workable, the following minimum
         implementation is required for all receivers:

            COMMANDS -- HELO
                        MAIL
                        RCPT
                        DATA
                        RSET
                        NOOP
                        QUIT

      4.5.2.  TRANSPARENCY

         Without some provision for data transparency the character
         sequence "<CRLF>.<CRLF>" ends the mail text and cannot be sent
         by the user.  In general, users are not aware of such
         "forbidden" sequences.  To allow all user composed text to be
         transmitted transparently the following procedures are used.

            1. Before sending a line of mail text the sender-SMTP checks
            the first character of the line.  If it is a period, one
            additional period is inserted at the beginning of the line.

            2. When a line of mail text is received by the receiver-SMTP
            it checks the line.  If the line is composed of a single
            period it is the end of mail.  If the first character is a
            period and there are other characters on the line, the first
            character is deleted.

         The mail data may contain any of the 128 ASCII characters.  All
         characters are to be delivered to the recipient's mailbox
         including format effectors and other control characters.  If
         the transmission channel provides an 8-bit byte (octets) data
         stream, the 7-bit ASCII codes are transmitted right justified
         in the octets with the high order bits cleared to zero.

            In some systems it may be necessary to transform the data as
            it is received and stored.  This may be necessary for hosts
            that use a different character set than ASCII as their local
            character set, or that store data in records rather than





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Simple Mail Transfer Protocol                                           



            strings.  If such transforms are necessary, they must be
            reversible -- especially if such transforms are applied to
            mail being relayed.

      4.5.3.  SIZES

         There are several objects that have required minimum maximum
         sizes.  That is, every implementation must be able to receive
         objects of at least these sizes, but must not send objects
         larger than these sizes.

                                    
          ****************************************************
          *                                                  *
          *  TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION  *
          *  TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
          *  OF THESE OBJECTS SHOULD BE USED.                *
          *                                                  *
          ****************************************************

            user

               The maximum total length of a user name is 64 characters.

            domain

               The maximum total length of a domain name or number is 64
               characters.

            path

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

            command line

               The maximum total length of a command line including the
               command word and the <CRLF> is 512 characters.

            reply line

               The maximum total length of a reply line including the
               reply code and the <CRLF> is 512 characters.





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            text line

               The maximum total length of a text line including the
               <CRLF> is 1000 characters (but not counting the leading
               dot duplicated for transparency).

            recipients buffer

               The maximum total number of recipients that must be
               buffered is 100 recipients.

                                    
          ****************************************************
          *                                                  *
          *  TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION  *
          *  TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
          *  OF THESE OBJECTS SHOULD BE USED.                *
          *                                                  *
          ****************************************************

         Errors due to exceeding these limits may be reported by using
         the reply codes, for example:

            500 Line too long.

            501 Path too long

            552 Too many recipients.

            552 Too much mail data.



















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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



APPENDIX A

   TCP Transport service

      The Transmission Control Protocol [3] is used in the ARPA
      Internet, and in any network following the US DoD standards for
      internetwork protocols.

      Connection Establishment

         The SMTP transmission channel is a TCP connection established
         between the sender process port U and the receiver process port
         L.  This single full duplex connection is used as the
         transmission channel.  This protocol is assigned the service
         port 25 (31 octal), that is L=25.

      Data Transfer

         The TCP connection supports the transmission of 8-bit bytes.
         The SMTP data is 7-bit ASCII characters.  Each character is
         transmitted as an 8-bit byte with the high-order bit cleared to
         zero.



























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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



APPENDIX B

   NCP Transport service

      The ARPANET Host-to-Host Protocol [4] (implemented by the Network
      Control Program) may be used in the ARPANET.

      Connection Establishment

         The SMTP transmission channel is established via NCP between
         the sender process socket U and receiver process socket L.  The
         Initial Connection Protocol [5] is followed resulting in a pair
         of simplex connections.  This pair of connections is used as
         the transmission channel.  This protocol is assigned the
         contact socket 25 (31 octal), that is L=25.

      Data Transfer

         The NCP data connections are established in 8-bit byte mode.
         The SMTP data is 7-bit ASCII characters.  Each character is
         transmitted as an 8-bit byte with the high-order bit cleared to
         zero.



























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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



APPENDIX C

   NITS

      The Network Independent Transport Service [6] may be used.

      Connection Establishment

         The SMTP transmission channel is established via NITS between
         the sender process and receiver process.  The sender process
         executes the CONNECT primitive, and the waiting receiver
         process executes the ACCEPT primitive.

      Data Transfer

         The NITS connection supports the transmission of 8-bit bytes.
         The SMTP data is 7-bit ASCII characters.  Each character is
         transmitted as an 8-bit byte with the high-order bit cleared to
         zero.






























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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



APPENDIX D

   X.25 Transport service

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









































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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



APPENDIX E

   Theory of Reply Codes

      The three digits of the reply each have a special significance.
      The first digit denotes whether the response is good, bad or
      incomplete.  An unsophisticated sender-SMTP will be able to
      determine its next action (proceed as planned, redo, retrench,
      etc.) by simply examining this first digit.  A sender-SMTP that
      wants to know approximately what kind of error occurred (e.g.,
      mail system error, command syntax error) may examine the second
      digit, reserving the third digit for the finest gradation of
      information.

         There are five values for the first digit of the reply code:

            1yz   Positive Preliminary reply

               The command has been accepted, but the requested action
               is being held in abeyance, pending confirmation of the
               information in this reply.  The sender-SMTP should send
               another command specifying whether to continue or abort
               the action.

                  [Note: SMTP does not have any commands that allow this
                  type of reply, and so does not have the continue or
                  abort commands.]

            2yz   Positive Completion reply

               The requested action has been successfully completed.  A
               new request may be initiated.

            3yz   Positive Intermediate reply

               The command has been accepted, but the requested action
               is being held in abeyance, pending receipt of further
               information.  The sender-SMTP should send another command
               specifying this information.  This reply is used in
               command sequence groups.

            4yz   Transient Negative Completion reply

               The command was not accepted and the requested action did
               not occur.  However, the error condition is temporary and
               the action may be requested again.  The sender should



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                                           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



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Simple Mail Transfer Protocol                                           



         illustrates this.  Each reply text is recommended rather than
         mandatory, and may even change according to the command with
         which it is associated.  On the other hand, the reply codes
         must strictly follow the specifications in this section.
         Receiver implementations should not invent new codes for
         slightly different situations from the ones described here, but
         rather adapt codes already defined.

         For example, a command such as NOOP whose successful execution
         does not offer the sender-SMTP any new information will return
         a 250 reply.  The response is 502 when the command requests an
         unimplemented non-site-specific action.  A refinement of that
         is the 504 reply for a command that is implemented, but that
         requests an unimplemented parameter.

      The reply text may be longer than a single line; in these cases
      the complete text must be marked so the sender-SMTP knows when it
      can stop reading the reply.  This requires a special format to
      indicate a multiple line reply.

         The format for multiline replies requires that every line,
         except the last, begin with the reply code, followed
         immediately by a hyphen, "-" (also known as minus), followed by
         text.  The last line will begin with the reply code, followed
         immediately by <SP>, optionally some text, and <CRLF>.

            For example:
                                123-First line
                                123-Second line
                                123-234 text beginning with numbers
                                123 The last line

         In many cases the sender-SMTP then simply needs to search for
         the reply code followed by <SP> at the beginning of a line, and
         ignore all preceding lines.  In a few cases, there is important
         data for the sender in the reply "text".  The sender will know
         these cases from the current context.












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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



APPENDIX F

   Scenarios

      This section presents complete scenarios of several types of SMTP
      sessions.

   A Typical SMTP Transaction Scenario

      This SMTP example shows mail sent by Smith at host USC-ISIF, to
      Jones, Green, and Brown at host BBN-UNIX.  Here we assume that
      host USC-ISIF contacts host BBN-UNIX directly.  The mail is
      accepted for Jones and Brown.  Green does not have a mailbox at
      host BBN-UNIX.

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

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

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

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

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

         S: RCPT TO:<Brown@BBN-UNIX.ARPA>
         R: 250 OK

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

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

                               Scenario 1

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



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August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   Aborted SMTP Transaction Scenario

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

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

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

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

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

         S: RSET
         R: 250 OK

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

                               Scenario 2

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























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                                           Simple Mail Transfer Protocol



   Relayed Mail Scenario

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

         Step 1  --  Source Host to Relay Host

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

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

            S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
            R: 250 OK

            S: DATA
            R: 354 Start mail input; end with <CRLF>.<CRLF>
            S: Date: 2 Nov 81 22:33:44
            S: From: John Q. Public <JQP@MIT-AI.ARPA>
            S: Subject:  The Next Meeting of the Board
            S: To: Jones@BBN-Vax.ARPA
            S:
            S: Bill:
            S: The next meeting of the board of directors will be
            S: on Tuesday.
            S:                                              John.
            S: .
            R: 250 OK

            S: QUIT
            R: 221 USC-ISIE.ARPA Service closing transmission channel

















Postel                                                         [Page 53]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



         Step 2  --  Relay Host to Destination Host

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

            S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
            R: 250 OK

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

            S: DATA
            R: 354 Start mail input; end with <CRLF>.<CRLF>
            S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
               2 Nov 81 22:40:10 UT
            S: Date: 2 Nov 81 22:33:44
            S: From: John Q. Public <JQP@MIT-AI.ARPA>
            S: Subject:  The Next Meeting of the Board
            S: To: Jones@BBN-Vax.ARPA
            S:
            S: Bill:
            S: The next meeting of the board of directors will be
            S: on Tuesday.
            S:                                              John.
            S: .
            R: 250 OK

            S: QUIT
            R: 221 USC-ISIE.ARPA Service closing transmission channel

                               Scenario 3

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















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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   Verifying and Sending Scenario

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

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

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

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

         S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
         R: 250 OK

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

         S: QUIT
         R: 221 SU-SCORE.ARPA Service closing transmission channel

                               Scenario 4

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



















Postel                                                         [Page 55]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   Sending and Mailing Scenarios

      First the user's name is verified, then  an attempt is made to
      send to the user's terminal.  When that fails, the messages is
      mailed to the user's mailbox.

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

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

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

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

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

         S: RSET
         R: 250 OK

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

         S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
         R: 250 OK

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

         S: QUIT
         R: 221 SU-SCORE.ARPA Service closing transmission channel

                               Scenario 5

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






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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



      Doing the preceding scenario more efficiently.

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

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

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

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

         S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
         R: 250 User not active now, so will do mail.

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

         S: QUIT
         R: 221 SU-SCORE.ARPA Service closing transmission channel

                               Scenario 6

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



















Postel                                                         [Page 57]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   Mailing List Scenario

      First each of two mailing lists are expanded in separate sessions
      with different hosts.  Then the message is sent to everyone that
      appeared on either list (but no duplicates) via a relay host.

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

         Step 1  --  Expanding the First List

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

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

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

























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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



         Step 2  --  Expanding the Second List

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

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

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


































Postel                                                         [Page 59]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



         Step 3  --  Mailing to All via a Relay Host

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

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

            S: DATA
            R: 354 Start mail input; end with <CRLF>.<CRLF>
            S: Blah blah blah...
            S: ...etc. etc. etc.
            S: .
            R: 250 OK

            S: QUIT
            R: 221 USC-ISIE.ARPA Service closing transmission channel

                               Scenario 7

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












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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   Forwarding Scenarios

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

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

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

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

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

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

                               Scenario 8

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






















Postel                                                         [Page 61]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



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

         Step 1  --  Trying the Mailbox at the First Host

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

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

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

            S: RSET
            R: 250 OK

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

         Step 2  --  Delivering the Mail at the Second Host

            R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
            S: HELO LBL-UNIX.ARPA
            R: 250 USC-ISI.ARPA

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

            S: RCPT TO:<Jones@USC-ISI.ARPA>
            R: OK

            S: DATA
            R: 354 Start mail input; end with <CRLF>.<CRLF>
            S: Blah blah blah...
            S: ...etc. etc. etc.
            S: .
            R: 250 OK

            S: QUIT
            R: 221 USC-ISI.ARPA Service closing transmission channel

                               Scenario 9

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




[Page 62]                                                         Postel


                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   Too Many Recipients Scenario

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

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

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

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

         S: RCPT TO:<eric@BERKELEY.ARPA>
         R: 552 Recipient storage full, try again in another transaction

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

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

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

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

         S: QUIT
         R: 221 BERKELEY.ARPA Service closing transmission channel

                              Scenario 10

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

      Note that a real implementation must handle many recipients as
      specified in Section 4.5.3.



Postel                                                         [Page 63]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



GLOSSARY

   ASCII

      American Standard Code for Information Interchange [1].

   command

      A request for a mail service action sent by the sender-SMTP to the
      receiver-SMTP.

   domain

      The hierarchially structured global character string address of a
      host computer in the mail system.

   end of mail data indication

      A special sequence of characters that indicates the end of the
      mail data.  In particular, the five characters carriage return,
      line feed, period, carriage return, line feed, in that order.

   host

      A computer in the internetwork environment on which mailboxes or
      SMTP processes reside.

   line

      A a sequence of ASCII characters ending with a <CRLF>.

   mail data

      A sequence of ASCII characters of arbitrary length, which conforms
      to the standard set in the Standard for the Format of ARPA
      Internet Text Messages (RFC 822 [2]).

   mailbox

      A character string (address) which identifies a user to whom mail
      is to be sent.  Mailbox normally consists of the host and user
      specifications.  The standard mailbox naming convention is defined
      to be "user@domain".  Additionally, the "container" in which mail
      is stored.





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RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   receiver-SMTP process

      A process which transfers mail in cooperation with a sender-SMTP
      process.  It waits for a connection to be established via the
      transport service.  It receives SMTP commands from the
      sender-SMTP, sends replies, and performs the specified operations.

   reply

      A reply is an acknowledgment (positive or negative) sent from
      receiver to sender via the transmission channel in response to a
      command.  The general form of a reply is a completion code
      (including error codes) followed by a text string.  The codes are
      for use by programs and the text is usually intended for human
      users.

   sender-SMTP process

      A process which transfers mail in cooperation with a receiver-SMTP
      process.  A local language may be used in the user interface
      command/reply dialogue.  The sender-SMTP initiates the transport
      service connection.  It initiates SMTP commands, receives replies,
      and governs the transfer of mail.

   session

      The set of exchanges that occur while the transmission channel is
      open.

   transaction

      The set of exchanges required for one message to be transmitted
      for one or more recipients.

   transmission channel

      A full-duplex communication path between a sender-SMTP and a
      receiver-SMTP for the exchange of commands, replies, and mail
      text.

   transport service

      Any reliable stream-oriented data communication services.  For
      example, NCP, TCP, NITS.





Postel                                                         [Page 65]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   user

      A human being (or a process on behalf of a human being) wishing to
      obtain mail transfer service.  In addition, a recipient of
      computer mail.

   word

      A sequence of printing characters.

   <CRLF>

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

   <SP>

      The space character.
































[Page 66]                                                         Postel


                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



REFERENCES

   [1]  ASCII

      ASCII, "USA Code for Information Interchange", United States of
      America Standards Institute, X3.4, 1968.  Also in:  Feinler, E.
      and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
      the Defense Communications Agency by SRI International, Menlo
      Park, California, Revised January 1978.

   [2]  RFC 822

      Crocker, D., "Standard for the Format of ARPA Internet Text
      Messages," RFC 822, Department of Electrical Engineering,
      University of Delaware, August 1982.

   [3]  TCP

      Postel, J., ed., "Transmission Control Protocol - DARPA Internet
      Program Protocol Specification", RFC 793, USC/Information Sciences
      Institute, NTIS AD Number A111091, September 1981.  Also in:
      Feinler, E. and J. Postel, eds., "Internet Protocol Transition
      Workbook", SRI International, Menlo Park, California, March 1982.

   [4]  NCP

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

   [5]  Initial Connection Protocol

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

   [6]  NITS

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




Postel                                                         [Page 67]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   [7]  X.25

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

         








































[Page 68]                                                         Postel