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|| The TCP/IP Guide|
9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)
(Page 1 of 3)
I mentioned in the
overview and history of SMTP that early
TCP/IP e-mail mechanisms were developed by borrowing techniques and
elements from existing application protocols, especially Telnet
SMTP is an independent protocol, but its heritage can still be seen
clearly in a few areas. One of the more obvious of these is in the method
by which commands are issued by an SMTP sender and replies returned
by an SMTP receiver.
Like FTP, all SMTP commands are sent
as plain ASCII text over the TCP connection established between the
client and server in an SMTP connection. These commands must end with
the two-character CRLF sequence that normally terminates
ASCII text as required for the Telnet Network
Virtual Terminal (NVT). In fact, you can
check the function of an SMTP server and even issue commands to it yourself,
simply by using Telnet to connect to it on port 25.
SMTP Command Syntax
All SMTP commands are specified using
a four-letter command code. Some commands also either allow or
require parameters to be specified. The basic syntax of a command
When parameters are used, they follow
the command code and are separated from it by one or more space characters.
For example, the HELO and EHLO commands are specified
with the command code, a space character, and then the domain name of
the SMTP sender, as we saw in our look at SMTP
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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