Exim

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. Provided by Wikipedia