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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)
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SMTP Replies and Reply Codes
(Page 2 of 4)
Reply Code Structure and Digit Interpretation
SMTP reply codes can be considered
to be of the form xyz, where x is the first
digit, y the second and z the third. Here is
how these digits are used.
First Reply Code Digit (x)
The leading reply code digit indicates
the success or failure of the command in general terms, whether a successful
command is complete or incomplete, and whether an unsuccessful one should
be tried again or not. This particular digit is interpreted in exactly
the same way as it is in FTP, as shown in Table 253.
Table 253: SMTP Reply Code Format: First Digit Interpretation
Reply
Code Format
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Meaning
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Description
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1yz
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Positive
Preliminary Reply
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An initial response indicating
that the command has been accepted and processing of it is still in
progress. The SMTP sender should expect another reply before a new command
may be sent.
Note that while this first digit type is formally defined in the SMTP
specification for completeness, it is not currently actually used by
any of the SMTP commands. That is to say, there are no reply codes between
100 and 199 in SMTP.
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2yz
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Positive
Completion Reply
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The command
has been successfully processed and completed.
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3yz
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Positive
Intermediate Reply
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The command was accepted, but
processing of it has been delayed, pending receipt of additional information.
One example of where this type of reply is often made is after receipt
of a DATA command, to prompt the SMTP sender to then send the
actual e-mail message to be transferred.
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4yz
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Transient
Negative Completion Reply
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The command
was not accepted and no action was taken, but the error is temporary
and the command may be tried again. This is used for errors that may
be a result of temporary glitches or conditions that may change, such
as a resource on the SMTP server being temporarily busy.
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5yz
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Permanent
Negative Completion Reply
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The command was not accepted
and no action was taken. Trying the same command again is likely to
result in another error. An example would be sending an invalid command.
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Second Reply Code Digit (y)
The middle digit categorizes messages
into functional groups. This digit is used in the same general way as
in FTP, but some of the functional groups are different in SMTP, as
you can see in Table 254.
Table 254: SMTP Reply Code Format: Second Digit Interpretation
Reply
Code Format
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Meaning
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Description
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x0z
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Syntax
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Syntax errors or miscellaneous
messages.
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x1z
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Information
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Replies to
requests for information, such as status requests.
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x2z
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Connections
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Replies related to the connection
between the SMTP sender and SMTP receiver.
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x3z
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Unspecified
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Not defined.
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x4z
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Unspecified
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Not defined.
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x5z
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Mail
System
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Replies related
to the SMTP mail service itself.
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Third Reply Code Digit (z)
The last digit indicates a specific
type of message within each of the functional groups described by the
second digit. The third digit allows each functional group to have 10
different reply codes for each reply type given by the first code digit
(preliminary success, transient failure and so on.)
Combining Digit Values to Make Specific Reply Codes
Again, as in FTP, these x,
y and z digit meanings are combined to make
specific reply codes. For example, the reply code 250 is
a positive reply indicating command completion, related to the mail
system. It usually is used to indicate that a requested mail command
was completed successfully.
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Version 3.0 - Version Date: September 20, 2005
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