+------+    |  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




 

                                                                        
RFC 821                                                      August 1982
                                           Simple Mail Transfer Protocol



   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, "" 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).






 

                                                                        
August 1982                                                      RFC 821
Simple Mail Transfer Protocol                                           



3.  THE SMTP PROCEDURES

   This section presents the procedures used in SMTP in several parts.
   First comes the basic mail procedure defined as a mail transaction.
   Following this are descriptions of forwarding mail, verifying mailbox
   names and expanding mailing lists, sending to terminals instead of or
   in combination with mailboxes, and the opening and closing exchanges.
   At the end of this section are comments on relaying, a note on mail
   domains, and a discussion of changing roles.  Throughout this section
   are examples of partial command and reply sequences, several complete
   scenarios are presented in Appendix F.

   3.1.  MAIL