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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 4 of 4)
SMTP Multiple-Line Text Replies
Again, as in FTP, it is possible
for a reply to contain more than one line of text. In this case, each
line starts with the reply code, and all lines but the last have a hyphen
between the reply code and the reply text, to indicate that the reply
continues. The last line has a space between the reply code and text,
just like a single-line reply. An example of this was given in the
topic on SMTP connections where I showed
a multi-line response to a EHLO command.
Enhanced Status Code Replies
When the ENHANCEDSTATUSCODES
SMTP extension is enabled, this causes supplemental reply codes to be
issued by the SMTP receiver in response to each command. These codes
are similar in some respects to the standard reply codes; they also
use three digits, but the digits are separated by periods. These enhanced
codes provide more information about the results of operations, especially
For example, if you try to issue
a RCPT command specifying a non-local mailbox on a server not
supporting this feature, it will send back a 550 reply,
which is a generic error (requested action not taken: mailbox
unavailable). When enhanced status codes are active, the response
will be 550 5.7.1, the more specific message delivery
not authorized, request refused. A full description of these enhanced
codes can be found in RFC 1893.
Key Concept: Each time the SMTP sender issues a command, it receives a reply from the SMTP receiver. SMTP replies are similar to FTP replies, using both a three-digit reply code and a descriptive text line. A special enhanced status codes SMTP extension is also defined; when enabled, this causes the SMTP receiver to return more detailed result information after processing a command.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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