Mail is sent by a series of request/response transactions
between a client, the "sender-SMTP," and a server, the
"receiver-SMTP". These transactions pass (1) the message
proper, which is composed of header and body, and (2) SMTP
source and destination addresses, referred to as the
"envelope".
The SMTP programs are analogous to Message Transfer Agents
(MTAs) of X.400. There will be another level of protocol
software, closer to the end user, that is responsible for
composing and analyzing RFC-822 message headers; this
component is known as the "User Agent" in X.400, and we
use that term in this document. There is a clear logical
distinction between the User Agent and the SMTP
implementation, since they operate on different levels of
protocol. Note, however, that this distinction is may not
be exactly reflected the structure of typical
implementations of Internet mail. Often there is a
program known as the "mailer" that implements SMTP and
also some of the User Agent functions; the rest of the
User Agent functions are included in a user interface used
for entering and reading mail.
The SMTP envelope is constructed at the originating site,
typically by the User Agent when the message is first
queued for the Sender-SMTP program. The envelope
addresses may be derived from information in the message
header, supplied by the user interface (e.g., to implement
a bcc: request), or derived from local configuration
information (e.g., expansion of a mailing list). The SMTP
envelope cannot in general be re-derived from the header
at a later stage in message delivery, so the envelope is
transmitted separately from the message itself using the
MAIL and RCPT commands of SMTP.
The text of RFC-821 suggests that mail is to be delivered
to an individual user at a host. With the advent of the
domain system and of mail routing using mail-exchange (MX)
resource records, implementors should now think of
delivering mail to a user at a domain, which may or may
not be a particular host. This DOES NOT change the fact
that SMTP is a host-to-host mail exchange protocol.
