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Table Of Contents  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)

Previous Topic/Section
SMTP Commands
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Pages in Current Topic/Section
1
2
34
Next Page
TCP/IP Electronic Mail Access and Retrieval Protocols and Methods
Next Topic/Section

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

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.


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

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.


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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SMTP Commands
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TCP/IP Electronic Mail Access and Retrieval Protocols and Methods
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