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The SMTP Protocol
The Simple Mail Transfer Protocol defined in
RFC821 specifies how mail is delivered from one system to another.
It is a relatively straightfoward protocol.
Initially, an email client (usually the delivery agent software on
the originating machine) establishes a TCP connection to the SMTP
server (at port 25) on the destination machine.
The server responds with an informative message beginning with the
3-digit code 220 The client then sends a
HELO command identifying the domain name of the system
it is running on.
The client software then transmits one (or more) mail messages to
the server. Each message is preceded by a MAIL-FROM
and one or more RCPT-TO messages. The responses to
these messages always begin with 3-digit numbers followed by a
human readable message. Then the text of the message itself
(including its headers) is transmitted using a DATA
message.
Finally, a QUIT message from the client tells the
server to close the TCP connection. An example of this is given on
the next slide.
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