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!

Enjoy The TCP/IP Guide? Get the complete PDF!
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 Mail Transaction Process
Previous Page
Pages in Current Topic/Section
Next Page
SMTP Security Issues
Next Topic/Section

SMTP Special Features, Capabilities and Extensions
(Page 2 of 2)

SMTP Extensions

As discussed earlier in this section, during the 1990s many extensions to the basic operation of SMTP were defined. These are enabled when two SMTP servers supporting the extension set up a session using the EHLO command and appropriate extension response codes. Table 251 summarizes some of the more interesting SMTP extensions that have been defined, and gives the RFC number where each is described.

Table 251: SMTP Extensions

Extension Keyword


Defining Document



8-bit MIME support.

RFC 1652

Theoretically defines support for the 8-bit content transfer encoding type in MIME, but there are complications associated with this. See the discussion of content encoding in the MIME section.



RFC 2554

Used to implement an authorization mechanism for servers requiring enhanced security.


Delivery status notification

RFC 1891

Allows an SMTP sender to request that the SMTP receiver notify it a problem occurs in delivering a message the sender gives to it.


Enhanced status codes.
(They should have abbreviated the keyword.

RFC 2034, RFC 1893

Extends the traditional 3-digit SMTP reply code format with extra codes that provide more information. See the end of the topic on SMTP replies for more.


Command pipelining

RFC 2920

Allows multiple commands to be transmitted in batches from the SMTP sender to the receiver, rather than sending one command at a time and waiting for a response code.


Message size declaration

RFC 1870

Allows information about the size of a message to be declared by an SMTP sender prior to transmitting it, so the SMTP receiver can decide if it wants the message or not.

On The Web: The full current set of SMTP extensions can be found here:

Note: Certain commands in the basic SMTP protocol description that are considered optional are also sometimes considered extensions, such as the EXPN and HELP commands; I have not listed these here since they are not true SMTP extensions.

Previous Topic/Section
SMTP Mail Transaction Process
Previous Page
Pages in Current Topic/Section
Next Page
SMTP Security Issues
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.