The first version of Exim was written in 1995 by Philip Hazel for use in the University of Cambridge Computing Service’s e-mail systems. The name initially stood for EXperimental Internet Mailer. It was originally based on an older MTA, Smail-3, but it has since diverged from Smail-3 in its design and philosophy.
Exim, like Smail, still follows the Sendmail design model, where a single binary controls all the facilities of the MTA. This monolithic design is generally considered to be inherently less secure, largely due to the lack of separation between the individual components of the system. Nevertheless, Exim’s security record has been fairly clean, with only a handful of serious security problems diagnosed over the years. There have been no serious issues since the redesigned version 4 was released. This is probably due to having been written from scratch and with security in mind (the security issues with Sendmail have long been well known).
Exim is highly configurable, and therefore has features that are lacking in other MTAs. It has always had substantial facilities for mail policy controls, providing facilities for the administrator to control who may send or relay mail through the system. In version 4.x this has matured to an Access Control List based system allowing very detailed and flexible controls. The integration of a framework for content scanning, which allowed for easier integration of anti-virus and anti-spam measures, happened in the 4.x releases. This made Exim very suitable for enforcing diverse mail policies.
The configuration is done through one or more configuration files, which must include the main section with generic settings and variables, as well as the following optional sections:
Exim has been deployed in busy environments, often handling thousands of emails per hour efficiently. Exim is designed to deliver email immediately, without queueing. However, its queue processing performance is comparatively poor when queues are large (which happens rarely on typical low-traffic sites, but can happen regularly on high-traffic sites). 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 centralized 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 multiple messages addressed to a single domain are submitted at once). In Philip Hazel's own words:
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, and PCRE has become popular in applications other than Exim.
Exim is free software distributed under the terms of the GNU General Public Licence. It has been ported to most Unix-like systems, as well as to Microsoft Windows using the Cygwin emulation layer. Exim 4 is currently the default MTA on Debian GNU/Linux systems.
Exim uses a peculiar version numbering scheme where the first decimal digit is updated only whenever the documentation is fully up to date. For this reason, a 0.01 version change can signify important changes, not necessarily fully documented. In 2005, changes to Exim's version numbering were on the table of discussion.