The TCP/IP Guide - SMTP Special Features, Capabilities and 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	Extension	Defining Document	Description
*8BITMIME*	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.
*AUTH*	Authorization	RFC 2554	Used to implement an authorization mechanism for servers requiring enhanced security.
*DSN*	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.
*ENHANCEDSTATUSCODES*	Enhanced status codes. (They should have abbreviated the keyword. J)	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.
*PIPELINING*	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.
*SIZE*	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.