Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Searchable, convenient, complete TCP/IP information.
The TCP/IP Guide

Custom Search

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
Previous Page
Pages in Current Topic/Section
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




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.


Positive Completion Reply

The command has been successfully processed and completed.


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.


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.


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





Syntax errors or miscellaneous messages.



Replies to requests for information, such as status requests.



Replies related to the connection between the SMTP sender and SMTP receiver.



Not defined.



Not defined.


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.

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

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $

Home - Table Of Contents - Contact Us

The TCP/IP Guide (
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.