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 slower and less secure, not least because of a lack of separation between the individual components of the system; but despite this, Exim's actual security record is much better than Sendmail and comparable with Qmail and Postfix.

Exim exhibits performance comparable to other MTAs when most mail is immediately deliverable. However, its performance is comparatively much poorer when circumstances cause more than a tiny amount of mail to be queued. Unlike Qmail, Postfix, and Zmailer , Exim does not have a central queue manager (i.e. an equivalent of qmail-send, qmgr, or scheduler). There is thus no centralised load balancing either of queue processing (leading to disproportionate amounts of time being spent on processing the same queue entries repeatedly) or of system-wide remote transport concurrency (leading to a "thundering herd" problem when a lot of messages are submitted addressed to a single domain). In Philip Hazel's own words:

"The bottom line is that Exim does not perform particularly well in environments where the queue regularly gets very large. It was never designed for this; deliveries from the queue were always intended to be 'exceptions' rather than the norm."

In 1997, Philip 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.

External links

• Exim Internet Mailer
• PCRE - Perl Compatible Regular Expressions