Exim
Exim is a mail transfer agent (MTA) used in Unixllike operating systems. The first version was written in 1995 by Philip Hazel for use in the University of Cambridge Computing Service's e-mail systems. Exim is distributed under the GPL, and therefore is free to download, use and modify. Exim somewhat resembles Smail 3, but it has diverged and now surpasses it in user friendliness and flexibility. They both follow the Sendmail design model where a single main binary controls all the facilities of the MTA. This monolithic design is considered by some to be inherently less secure and slower, but despite this, Exim's security record is much better than Sendmail and comparable with Qmail and Postfix, as is its speed. In advanced areas such as queue handling, address routing and testing, it exhibits excellent performance. Exim doesn't have a Graphical User Interface (GUI) to help you configure it, but some Linux distributions add one.
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Ajenti
An admin's tool for a more civilized age, providing you with a fast and secure way to manage a remote Linux box at any time using everyday tools like a web terminal, text editor, file manager and others. Ajenti Core is a streamlined and reusable framework for building web interfaces of all kinds. Includes lots of plugins for system and software configuration, monitoring and management. Ajenti won't damage your existing configuration or tell you how to do your job. Preserves config structure and comments. Easily extensible using Python. Plugin development is fast and pleasant with rich APIs. Automatic configuration of Exim 4 and Courier IMAP, including virtual e-mails, DKIM, DMARC and SPF.
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Rspamd
Rspamd has been started to handle mail flows that has grown over the last decade by more than ten times. From the very beginning of the project, Rspamd was oriented on highly loaded mail systems with development focus on performance and scan speed. Rspamd is written in plain C language and it uses a number of techniques to run fast especially on the modern hardware. On the other hand, it is possible to run Rspamd even on an embedded device with a very constrained environment. You can also check the recent performance analyze article to have a better impression about how fast Rspamd could be. Rspamd can be treated as a faster replacement for SpamAssassin mail filter with the ability to scan ten times more messages using the same rules (by means of SpamAssassin plugin). In the next graph, you can see how switch to Rspamd from SA helped to reduce CPU load on scanner machines. Rspamd can work with the most popular mail transfer systems, such as Postfix, Exim, Sendmail or Haraka.
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Courier
The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. Individual components can be enabled or disabled at will. The Courier mail server now implements basic web-based calendaring and scheduling services integrated in the webmail module. Advanced groupware calendaring services will follow soon.
The Courier mail server's source code should compile on most POSIX-based operating systems based on Linux, and BSD-derived kernels. The Courier mail server should also compile on Solaris and AIX, with some help from Sun's or IBM's freeware add-on tools for their respective operating systems.
The Courier mail server evolved out of several related projects, that merged together (more on that later). The Courier mail server implements SMTP extensions for mailing list management.
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