Exim is a mail transfer agent (MTA) used on Unix-like operating systems. Exim is 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 GNU/Linux systems.

A large number of Exim installations exist, especially within Internet service providers[2] and universities in the UK. Exim is also widely used with the GNU Mailman mailing list manager, and cPanel.

In March 2017 in a study performed by E-Soft, Inc.,[3] approximately 57% of the publicly reachable mail-servers on the Internet ran Exim.