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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 Mail Transaction Process
Previous Page
Pages in Current Topic/Section
1
2
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

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.


On The Web: The full current set of SMTP extensions can be found here: http://www.iana.org/assignments/mail-parameters


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
1
2
Next Page
SMTP Security Issues
Next Topic/Section

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