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What is a MTA?
MTA stands for Mail Transfer Agent. MTAs implement the control
mechanisms to send to and receive mail message from the internet
using SMTP (Simple Mail Transfer Protocol). MTAs are ususally
not responsible for delivering mail to the user (MDA - mail
delivery agent) or providing front ends to read and write mail
(MUA - mail user agent).
- Sendmail
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First, there was sendmail, and it was fine.
Sendmail
was the first mail transfer agent that has been used on a large
scale at many sites in the internet. Sendmail provided a
variety of interesting features at very early stage of its
development. The costs system administrators had to pay to use
these features were sendmails cryptic configuration file.
Nowadays, some of the configuration has been detached to unitize
the setup, but the central monilithic configuration file still
remains, although few people edit it directly, but use powerful
pre-defined macros. Apart from all the pros and cons, sendmail
remains to be one of the most wide spread MTAs being used all over
the world.
- Smail
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Being free software, too, smail has been created to provide an
alternative to the quite complex sendmail. Smail uses a straight
forward configuration file, which is much easier to understand
and learn than sendmail's. However, the price for simplicity is
a lack of certains features and the allover flexible design.
- Exim
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Exim
is based on smail, the latter being regarded its predessor. Exim
combines simple configuration interface with state-of-the-art
powerful features, like regular expression lists, SMTP AUTH aso.
- Qmail
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Qmail
was the first MTA to wield a complete different approach in regard
of modular design. Not only did it implement the abstract
layers and mechanisms of the delivery of a mail message into
one process, but used several processes communicating and
interacting with each other. Some of the processes even use
different UIDs to make their respective tasks more transparent
and to grant them exactly the rights they need. Qmail also
introduced a new format to store mail messages -
Maildir.
Until now, MDAs used a
single file to store the users' mail
(usually /var/spool/mail/user). Qmail
created one file for each new message, the access of which
is much more efficiently than mbox files. Many other MTAs/MDAs
adopted this format or, at least, support it optionally.
Qmails user aliasing mechanism is also based on separate
files for each alias and offers unequalled opportunities in
regard of usability and extendability.
- ZMailer
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ZMailer
implements an extremely flexible and configurable MTA in regard
of customization. As most new generation mailers ZMailer, too,
relies on several co-existing processes to provide best
throughput when deciding what as to be done with mail messages
(in ZMailer terms this decision is called "routing"). To
perform its duty it supports a number of different database
formats and network protocols. Among the latter are SMTP, UUCP,
bitnet and mail-to-news-gateways.
- Postfix
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Postfix
is another attempt to thwart sendmail's domination of MTA-field.
It has been developed with speed, simplicity of configuration
and security in mind.
Postfix works great with Courier
and Cyrus, both, with and without
the SASL-extensions of the latter.
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