Haraka

Dovecot Pro is a full-service email platform that delivers dynamic scalability, high performance, efficient utilization of hardware, and outstanding support to the world’s largest Telcos, ISP's, and Hosters. With a marketshare of 76% of installed email backends, customers trust the performance of Dovecot Pro. Efficient hardware utilization and advanced monitoring tools for statistics and administration reduce ownership costs and save time and maintenance costs. In addition, Dovecot Pro scales up and out to hundreds of servers and millions of users and multiple physical sites. Its stateless native architecture offers maximum flexibility as each major component can be deployed on its own dedicated node. Dovecot Pro provides industry-leading support for all email standards necessary to provide both Mail Delivery Agent functionality (via LMTP and Sieve), and to facilitate mail retrieval by mail clients (via IMAP and POP3).

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.