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   

         QUIT 

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

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

                     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

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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