Concept

Exim

Summary
Exim is a mail transfer agent (MTA) used on Unix-like operating systems. Exim is a free software distributed under the terms of the GNU General Public License, and it aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail. Exim 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 Linux systems. Many Exim installations exist, especially within Internet service providers and universities in the United Kingdom. Exim is also widely used with the GNU Mailman mailing list manager, and cPanel. In March 2023 a study performed by E-Soft, Inc., approximated that 59% of the publicly reachable mail-servers on the Internet ran Exim. 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. Exim has well-defined stages during which it gains or loses privileges. Exim's security has had a number of serious security problems diagnosed over the years. Since the redesigned version 4 was released there have been four remote code execution flaws and one conceptual flaw concerning how much trust it is appropriate to place in the run-time user; the latter was fixed in a security lockdown in revision 4.73, one of the very rare occasions when Exim has broken backwards compatibility with working configurations. 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.
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