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

                               Example 2

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





 

                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



   3.3.  VERIFYING AND EXPANDING

      SMTP provides as additional features, commands to verify a user
      name or expand a mailing list.  This is done with the VRFY and
      EXPN commands, which have character string arguments.  For the
      VRFY command, the string is a user name, and the response may
      include the full name of the user and must include the mailbox of
      the user.  For the EXPN command, the string identifies a mailing
      list, and the multiline response may include the full name of the
      users and must give the mailboxes on the mailing list.

      "User name" is a fuzzy term and used purposely.  If a host
      implements the VRFY or EXPN commands then at least local mailboxes
      must be recognized as "user names".  If a host chooses to
      recognize other strings as "user names" that is allowed.

      In some hosts the distinction between a mailing list and an alias
      for a single mailbox is a bit fuzzy, since a common data structure
      may hold both types of entries, and it is possible to have mailing
      lists of one mailbox.  If a request is made to verify a mailing
      list a positive response can be given if on receipt of a message
      so addressed it will be delivered to everyone on the list,
      otherwise an error should be reported (e.g., "550 That is a
      mailing list, not a user").  If a request is made to expand a user
      name a positive response can be formed by returning a list
      containing one name, or an error can be reported (e.g., "550 That
      is a user name, not a mailing list").

      In the case of a multiline reply (normal for EXPN) exactly one
      mailbox is to be specified on each line of the reply.  In the case
      of an ambiguous request, for example, "VRFY Smith", where there
      are two Smith's the response must be "553 User ambiguous".

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















 

                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



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

                    Example of Verifying a User Name

         Either

            S: VRFY Smith
            R: 250 Fred Smith <Smith@USC-ISIF.ARPA>

         Or

            S: VRFY Smith
            R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>

         Or

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

         Or

            S: VRFY Jones
            R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>