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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)
SMTP Special Features, Capabilities and Extensions
(Page 1 of 2)
The primary job of the Simple Mail
Transfer Protocol is of course to implement the TCP/IP electronic mail
delivery system. Whenever the user of an SMTP server gives it an e-mail
message addressed to a non-local mailbox, the server will attempt to
transfer it to the appropriate destination server, using the SMTP
mail transaction process. Many billions
of such transfers are performed every day on the Internet, allowing
e-mail to quickly reach its destination anywhere around the world.
SMTP Special Features
In addition to the basic mail transfer
mechanism, SMTP includes a number of other features and capabilities.
These allow SMTP to support special requirements and auxiliary needs
of the mail system, and are described in detail in RFC 2821. It would
take many topics to describe them all in detail, so I will just provide
a quick summary of the more important ones here so you know a bit about
Other Capabilities and Functions of SMTP Servers
- Mail Relaying: As discussed in the SMTP
communication overview, the protocol was
once widely used in a relaying mode where e-mail was routed
from one SMTP server to another to reach its destination. Today, the
normal method of e-mail transfer on the Internet today is directly from
the sender's SMTP server to the recipient's, using DNS
MX records to determine the recipient
SMTP server address. SMTP still includes the ability to relay mail from
one server to another, provided certain conditions are met. Note that
in addition to the inefficiency of relaying, many servers won't relay
mail because this feature has been abused
for spamming and hacking.
- Mail Forwarding: Under certain conditions,
an SMTP server may agree to accept e-mail for a non-local mailbox and
forward it to the appropriate destination. This sounds similar to relaying
but is used in a different way. A common example is when a user changes
his or her e-mail address. If I have worked at XYZ Industries for years
and then retire, the company may no longer wish to let me receive e-mail
at the company's SMTP server. As a courtesy, however, they may forward
e-mail sent to me there so I still receive it at my new company.
- Mail Gatewaying: Certain SMTP servers
may be configured as e-mail gateways. These devices translate
TCP/IP e-mail into a form suitable for another e-mail system, and vice-versa.
Gatewaying is a complex topic because e-mail systems can be so different;
one of the more important problems is the inconsistency
of addressing methods of different e-mail systems.
- Address Debugging: SMTP includes a VRFY
(verify) command, which can be used to check the validity of an
e-mail address without actually sending mail to it.
- Mailing List Expansion: The SMTP command
EXPN (expand) can be used to determine the individual e-mail
addresses associated with a mailing
list. (Note however that this has nothing
directly to do with mailing list software like Majordomo.)
- Turning: The original SMTP
protocol included a command that allows the SMTP sender and SMTP receiver
to change roles. This could be used to allow SMTP server A to send e-mail
to server B, and then have B send e-mail it has queued for A in the
same session. In practice, this capability was not widely used for a
variety of reasons, including security considerations. It is now officially
not recommended, but may still be implemented in some SMTP
The list above represents just a
few of the features that are mentioned in the SMTP standards. In addition
to these, a given type of SMTP server software may be given other features
as well by its developers. The HELP command is one way to determine
what some of the commands that a given SMTP server supports.
SMTP servers also must perform a
great deal of background processing that doesn't get a great
deal of attention. This includes managing connections, checking for
errors in commands and e-mail messages, and reacting accordingly. They
must also be on the lookout for problem conditions, such as looping
that may result in an e-mail message being passed back and forth between
two SMTP servers, each thinking the other is the intended recipient.
In the event of an initial failure to deliver mail, SMTP servers are
also required to periodically retry communication with the destination
device, and return a failure message to the sender if it cannot deliver
the message after a certain period of time. Again, RFC 2821 contains
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Version 3.0 - Version Date: September 20, 2005
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