Exim

Exim is a mail transfer agent (MTA) used in Unix-like operating systems. The first version was written in 1995 by Philip Hazel for use in the University of Cambridge Computing Service's e-mail systems.

Exim somewhat resembles Smail 3, but it has diverged and now surpasses it in user friendliness and flexibility. They both follow the Sendmail design model where a single main binary controls all the facilities of the MTA. This monolithic design is considered by some to be inherently less secure and slower, but despite this, Exim's security record is much better than Sendmail and comparable with Qmail and Postfix, as is its speed. In advanced areas such as queue handling, address routing and testing, it exhibits excellent performance.

In 1997, Phil Hazel replaced Exim's POSIX regular expression library written by Henry Spencer with a new library he developed called PCRE (Perl Compatible Regular Expressions). Perl regular expressions are much more powerful than POSIX and other common regular expressions, so PCRE has become popular in many applications other than Exim.

Exim is free software distributed under the terms of the GNU General Public Licence, and it has been ported to most Unix-like systems.

Exim has been the default MTA in Debian GNU/Linux systems since version 3.x; the version 4.x will become the new default in the next release.