RFC du protocole SMTP : Model

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
   two-way 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

   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].  When the transport service provides an 8-bit byte
   (octet) transmission channel, each 7-bit character is transmitted
   right justified in an 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 meta-linguistic variables.
   However, some arguments use the angle brackets literally.  For
   example, an actual reverse-path is enclosed in angle brackets, i.e.,
   "<John.Smith@USC-ISI.ARPA>" is an instance of <reverse-path> (the
   angle brackets are actually transmitted in the command or reply).