The TCP/IP Guide - SMTP Replies and Reply Codes

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.

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	Meaning	Description
1yz	Positive Preliminary Reply	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.
2yz	Positive Completion Reply	The command has been successfully processed and completed.
3yz	Positive Intermediate Reply	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.
4yz	Transient Negative Completion Reply	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.
5yz	Permanent Negative Completion Reply	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.

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	Meaning	Description
x0z	Syntax	Syntax errors or miscellaneous messages.
x1z	Information	Replies to requests for information, such as status requests.
x2z	Connections	Replies related to the connection between the SMTP sender and SMTP receiver.
x3z	Unspecified	Not defined.
x4z	Unspecified	Not defined.
x5z	Mail System	Replies related to the SMTP mail service itself.

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.)

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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