Exim is a mail transfer agent (MTA) used in Unix-like operating systems. It was originally written in 1995 by Philip Hazel for use in the University of Cambridge mail systems.

Exim somewhat resembles Smail 3, but it has surpassed its model in user friendliness and flexibility. They are both built on the Sendmail model where the single main binary controls all the facilities of the MTA, but it's security record is better than the original. In fact, even with a poorer basic design model than MTAs such as Qmail or Postfix, Exim is on par with them with regards to security issues and speed of mail delivery. In inherently advanced areas such as queue handling and address routing and testing it is regarded as one of the best in the field.

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 systems since version 3.x; the version 4.x will become the new default in the next release.

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