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Mail software can be generally divided into two categories:
Mail Transfer Agents (MTAs) and
Mail User Agents (MUAs).
An MTA is a program that relays messages between machines.
An MUA is a client program responsible for interacting
with the user.
An exhaustive list of mail programs would be so large as to be
meaningless. For example, the Univerisity of Washington's
IMAP Connection
maintains a list of products that support the
IMAP Protocol.
At the time of this writing (March 1999), more than 50 MTAs
and 100 MUAs are listed - and this only for the IMAP protocol.
Another good source of information is Yahoo's
Electronic Mail page.
Mail Transfer Agents
Sendmail
is the probably the most widely-used UNIX Internet MTA.
Other UNIX Internet mail packages are often designed to operate much
like sendmail.
At its core, Sendmail is a SMTP server that accepts email messages
via network connections, then processes them according to
a bizarre series of rewrite rules,
which I won't dare to touch on here.
Based on each message's destination address, the configured
rewrite rules, and perhaps the phase of the moon (sendmail
is a notoriously complex program), the message can be
relayed to another machine via SMTP, queued for later delivery
if the remote machine is not responding, passed to another
program, or filed in a local mail file.
The program can also run standalone, accepting
email over standard input.
Sendmail operates
in push mode, connecting to remote sites when it has mail
to deliver to them.
Another major UNIX MTA is
fetchmail.
Fetchmail operates in pull mode, connecting to remote sites,
using POP or IMAP to retreive mail queued there, and
relaying it to another MTA, usually a local sendmail.
Fetchmail is a important component of mail configurations
that, for security, connectivity, or other reasons, can
not rely on inbound SMTP connections to deliver mail.
An important type of MTA is the anonymous remailer.
Remailers forward messages after stripping off various headers
that would identify the sender, creating an anonymous email
that (in theory) can't be traced back to its sender.
The use of public key cryptography allows email messages
to sent with an encrypted return address, which only
the remailer can decode. The recipient of such a message
can reply back to the sender (via the remailer),
but can't determine the sender's actual email address.
For more information, see the
Anonymous Remailers page
in the
WWW Virtual Library.
Another class of MTA is the automated mailing list manager, such as
Majordomo.
These MTAs, generally operating as program invoked from Sendmail,
take incoming mail messages, duplicate them, and relay them to
a list of mailing list subscribers. Majordomo is capable of
automating the process of subscribing and unsubscribing mailing
list members, and configuring a variety of mailing list styles.
Mail User Agents
MUAs retrieve, parse, and present incomming
mail, and format and transmit outgoing mail.
The original UNIX MUAs expected incoming email messages
to be placed in a special file by the MTA(s), and ran an MTA
program directly to send outgoing mail. Most newer MUAs attempt
to maintain backwards compatibility with this design, at some
cost in complexity. More modern MUA design use networking protocols
such as IMAP to access mailboxes, and SMTP to send messages.
Two common text-based UNIX MUAs are
Pine and
Elm. An
major disadvantage of text-based MUAs are their inability
to display MIME messages with embedded graphics.
Eudora is a major
commercial, Windows-based MUA.
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