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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)
SMTP Replies and Reply Codes
(Page 1 of 4)
SMTP communication begins with the
establishment of an SMTP
session between an SMTP sender (the device
acting as a client) and an SMTP receiver (the device acting as a server).
All protocol operations then consist of plain ASCII text SMTP commands
issued by the sender to the receiver. The receiver analyzes each command,
carries out the instruction requested by the sender if possible, and
then responds back with a reply to the sender. The reply serves several
functions: confirming command receipt; indicating if the command was
accepted or not; and communicating the result of processing the command.
Just as SMTP commands are sent in
a manner reminiscent of how FTP
internal commands work, SMTP replies are
formatted and interpreted in a way almost identical to that of FTP
replies. As with FTP, the reply consists
not just of a string of reply text, but a combination of reply text
and a numerical reply code. And as with FTP, these reply codes
use three digits to encode various information about the reply, with
each digit having a particular significance. The reply code is really
the key part of the reply, with the reply text being merely descriptive.
Note: The topic on FTP reply codes contains a thorough explanation of the benefits of using these structure numeric reply codes. Since the SMTP system is so similar, I won't duplicate that discussion in this topic. I will, however, fully describe the syntax and semantics of SMTP reply codes here, in order both to keep this section on SMTP self-contained, and to indicate where the differences are between FTP and SMTP reply codes.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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