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Unformatted text preview: r agent.
The minimal SMTP implementation supports eight commands. We saw five of them in
the previous example: HELO, MAIL, RCPT, DATA, and QUIT.
The RSET command aborts the current mail transaction and causes both ends to reset.
Any stored information about sender, recipients, or mail data is discarded.
The VRFY command lets the client ask the sender to verify a recipient address, without
sending mail to the recipient. It's often used by a system administrator, by hand, for
debugging mail delivery problems. We'll show an example of this in the next section.
The NOOP command does nothing besides force the server to respond with an OK reply
There are additional, optional commands. EXPN expands a mailing list, and is often used
by the system administrator, similar to VRFY. Indeed, most versions of Sendmail handle
the two identically.
Version 8 of Sendmail in 4.4BSD no longer handles the two identically. VRFY does not expand aliases
and doesn't follow .forward files. The TURN command lets the client and server switch roles, to send mail in the reverse
direction, without having to take down the TCP connection and create a new one.
(Sendmail does not support this command.) There are three other commands (SEND,
SOML, and SAML), which are rarely implemented, that replace the MAIL command.
These three allow combinations of the mail being delivered directly to the user's terminal
(if logged in), or sent to the recipient's mailbox.
Envelopes, Headers, and Body
Electronic mail is composed of three pieces.
1. The envelope is used by the MTAs for delivery. In our example the envelope was
specified by the two SMTP commands: file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (5 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol MAIL From: <firstname.lastname@example.org>
RFC 821 specifies the contents and interpretation of the envelope, and the protocol
used to exchange mail across a TCP co...
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- Spring '12