Exim
From SuperbHosting.net Support Wiki
Exim is a general and flexible mail transfer agent (MTA) used on Unix-like operating systems and offers extensive facilities for checking incoming e-mail.
Exim is a largely configurable MTA and as a result, offers features that might not be available in other MTA's. In addition to this, Exim has been successfully deployed in busy environments, often handling thousands of emails per hour efficiently. Exim is designed to efficiently deliver email immediately, without queueing.
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. It is also widely used with the GNU Mailman mailing list manager, and cPanel.
