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. -- Wikipedia Exim
Some people come here looking to get a complete email server package. Exim isn't that, it's a Mail Transport Agent and a Mail Submission Agent. Here are some of the things that you won't get with Exim:
No POP, No IMAP. Exim is not a mailstore. It does not support IMAP or POP protocols, though it can deliver messages to mailstores that do, either using SMTP or LMTP message delivery, or in some cases by saving messages directly into mailboxes.
No GUI. Exim doesn't have a Graphical User Interface (GUI) to help you configure it, but some Linux distributions add one. Various tools are available to give you graphical views of Exim's mail queues, traffic, and logs.
RFC 2821 SMTP and RFC 2033 LMTP email message transport.
Incoming (as SMTP server):
Exim also supports RFC 5068 Message Submission, as an SMTP server with (for example, encrypted and authenticated connections on port 587).
Outgoing email (as SMTP or LMTP client):
Exim includes excellent documentation, including a comprehensive online manual available in several formats including a 476 page pdf, this wiki, and a book. There's an email support list populated with helpful (if sometimes excitable) users and even with Exim developers who actually understand email. Exim - probably the best documented mail server in the world!
Some Linux distributions include Exim, and add on GUI configuration tools. Documentation for those tools is best obtained from the Linux distributors.
A number of utilities are also included in the distribution, to aid with log file inspection, queue management, reporting, configuration testing, and so on. See chapter 50 of the documentation.
Lemonade is a collection of extensions to IMAP and SMTP that support mobile messaging. Of the SMTP extensions, Exim supports the following. References are to sections of the manual, or to configuration options.
But does not support:
Exim is available from many places - see ObtainingExim.
The exim wiki, hosted on github. You need a github login to edit. Please add to and improve the content of this wiki