smtp-simple mail transfer protocol.txt

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[Page 12]                                                         Postel


                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   3.5.  OPENING AND CLOSING

      At the time the transmission channel is opened there is an
      exchange to ensure that the hosts are communicating with the hosts
      they think they are.

      The following two commands are used in transmission channel
      opening and closing:

         HELO <SP> <domain> <CRLF>

         QUIT <CRLF>

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

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

                     Example of Connection Opening

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

                               Example 5

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

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

                     Example of Connection Closing

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

                               Example 6

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










Postel                                                         [Page 13]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   3.6.  RELAYING

      The forward-path may be a source route of the form
      "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts.  This
      form is used to emphasize the distinction between an address and a
      route.  The mailbox is an absolute address, and the route is
      information about how to get there.  The two concepts should not
      be confused.

      Conceptually the elements of the forward-path are moved to the
      reverse-path as the message is relayed from one server-SMTP to
      another.  The reverse-path is a reverse source route, (i.e., a
      source route from the current location of the message to the
      originator of the message).  When a server-SMTP deletes its
      identifier from the forward-path and inserts it into the
      reverse-path, it must use the name it is known by in the
      environment it is sending into, not the environment the mail came
      from, in case the server-SMTP is known by different names in
      different environments.

      If when the message arrives at an SMTP the first element of the
      forward-path is not the identifier of that SMTP the element is not
      deleted from the forward-path and is used to determine the next
      SMTP to send the message to.  In any case, the SMTP adds its own
      identifier to the reverse-path.

      Using source routing the receiver-SMTP receives mail to be relayed
      to another server-SMTP  The receiver-SMTP may accept or reject the
      task of relaying the mail in the same way it accepts or rejects
      mail for a local user.  The receiver-SMTP transforms the command
      arguments by moving its own identifier from the forward-path to
      the beginning of the reverse-path.  The receiver-SMTP then becomes
      a sender-SMTP, establishes a transmission channel to the next SMTP
      in the forward-path, and sends it the mail.

      The first host in the reverse-path should be the host sending the
      SMTP commands, and the first host in the forward-path should be
      the host receiving the SMTP commands.

      Notice that the forward-path and reverse-path appear in the SMTP
      commands and replies, but not necessarily in the message.  That
      is, there is no need for these paths and especially this syntax to
      appear in the "To:" , "From:", "CC:", etc. fields of the message
      header.

      If a server-SMTP has accepted the task of relaying the mail and



[Page 14]                                                         Postel


                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



      later finds that the forward-path is incorrect or that the mail
      cannot be delivered for whatever reason, then it must construct an
      "undeliverable mail" notification message and send it to the
      originator of the undeliverable mail (as indicated by the
      reverse-path).

      This notification message must be from the server-SMTP at this
      host.  Of course, server-SMTPs should not send notification
      messages about problems with notification messages.  One way to
      prevent loops in error reporting is to specify a null reverse-path
      in the MAIL command of a notification message.  When such a
      message is relayed it is permissible to leave the reverse-path
      null.  A MAIL command with a null reverse-path appears as follows:

         MAIL FROM:<>

      An undeliverable mail notification message is shown in example 7.
      This notification is in response to a message originated by JOE at
      HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
      to HOSTZ.  What we see in the example is the transaction between
      HOSTY and HOSTX, which is the first step in the return of the
      notification message.



























Postel                                                         [Page 15]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



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

            Example Undeliverable Mail Notification Message

         S: MAIL FROM:<>
         R: 250 ok
         S: RCPT TO:<@HOSTX.ARPA:[e-mail]>
         R: 250 ok
         S: DATA
         R: 354 send the mail data, end with .
         S: Date: 23 Oct 81 11:22:33
         S: From: [e-mail]
         S: To: [e-mail]
         S: Subject: Mail System Problem
         S:
         S:   Sorry JOE, your message to [e-mail] lost.
         S:   HOSTZ.ARPA said this:
         S:    "550 No Such User"
         S: .
         R: 250 ok

                               Example 7

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

























[Page 16]                                                         Postel


                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   3.7.  DOMAINS

      Domains are a recently introduced concept in the ARPA Internet
      mail system.  The use of domains changes the address space from a
      flat global space of simple character string host names to a
      hierarchically structured rooted tree of global addresses.  The
      host name is replaced by a domain and host designator which is a
      sequence of domain element strings separated by periods with the
      understanding that the domain elements are ordered from the most
      specific to the most general.

      For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
      "PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.

      Whenever domain names are used in SMTP only the official names are
      used, the use of nicknames or aliases is not allowed.

































Postel                                                         [Page 17]


                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   3.8.  CHANGING ROLES

      The TURN command may be used to reverse the roles of the two
      programs communicating over the transmission channel.

      If program-A is currently the sender-SMTP and it sends the TURN
      command and receives an ok reply (250) then program-A becomes the
      receiver-SMTP.

      If program-B is currently the receiver-SMTP and it receives the
      TURN command and sends an ok reply (250) then program-B becomes
      the sender-SMTP.

      To refuse to change roles the receiver sends the 502 reply.

      Please note that this command is optional.  It would not normally
      be used in situations where the transmission channel is TCP.
      However, when the cost of establishing the transmission channel is
      high, this command may be quite useful.  For example, this command
      may be useful in supporting be mail exchange using the public
      switched telephone system as a transmission channel, especially if
      some hosts poll other hosts for mail exchanges.



























[Page 18]                                                         Postel


                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



4.  THE SMTP SPECIFICATIONS

   4.1.  SMTP COMMANDS

      4.1.1.  COMMAND SEMANTICS

         The SMTP commands define the mail transfer or the mail system
         function requested by the user.  SMTP commands are character
         strings terminated by <CRLF>.  The command codes themselves are
         alphabetic characters terminated by <SP> if parameters follow
         and <CRLF> otherwise.  The syntax of mailboxes must conform to
         receiver site conventions.  The SMTP commands are discussed
         below.  The SMTP replies are discussed in the Section 4.2.

         A mail transaction involves several data objects which are
         communicated as arguments to different commands.  The