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):

• SMTP over TCP/IP (Exim daemon or inetd);
• SMTP over the standard input and output (the -bs option);
• Batched SMTP on the standard input (the -bS option).

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):

• SMTP over TCP/IP (the smtp transport);
• LMTP over TCP/IP (the smtp transport with the protocol option set to “lmtp”);
• LMTP over a pipe to a process running in the local host (the lmtp transport);
• Batched SMTP to a file or pipe (the appendfile and pipetransports with the use_bsmtp option set).

Configuration

• Access Control Lists - flexible policy controls.
• Content scanning, including easy integration with and other spam and virus scanners like SpamAssassin and ClamAV.
• Encrypted SMTP connections using TLS/SSL.
• Authentication with a variety of front end and back end methods, including PLAIN, LOGIN, sasl, dovecot, spa, cram_md5.
• Rewrite - rewrite envelope and/or header addresses using regular expressions.
• Routing controls - use routers to redirect, quarantine, or deliver messages.
• Transports - use transports to deliver messages by smtp, lmtp, or to files, directories, or other programs.
• Flexible retry rules for temporary delivery problems.

Support

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.

Exim Utilities

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.

• SUBMIT. "Message Submission for Mail" (RFC 4409) - [section 7.1]
• PIPELINING. "SMTP Extension for Command Pipelining" (RFC 2197) - [pipelining_advertise_hosts]. Pipelining is advertised to all hosts by default.
• SIZE. "SMTP Service Extension for Message Size Declaration" (RFC 1870) - on by default.
• SMTP AUTH. "SMTP Service Extension for Authentication" (RFC 2554) - [auth_advertise_hosts]
• START-TLS. “SMTP Service Extension for Secure SMTP over TLS” (RFC 3207) - [tls_advertise_hosts]
• 8BITMIME. "SMTP Service Extension for 8bit MIME Transport" (RFC 1652) - [accept_8bitmime]. Setting accept_8bitmime allows Exim to accept 8 bit messages, and is on by default, since version 4.80. Exim doesn't do anything special with such messages. You might want to switch this off if you know that you're sending messages to systems that can't handle 8bitmime, but these appear to be rare.
• DSN. "Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications" (RFC 3461). Note that Exim will issue delivery delay and failure notifications normally; this extension allows clients to say when and how notifications should be issued.
• CHUNKING. "“SMTP Service Extensions for Transmission of Large and Binary Messages" (RFC 3030) - [chunking_advertise_hosts]

But does not support:

• BURL. "Message Submission BURL Extension" (RFC 4468)
• BINARYMIME. "SMTP Service Extensions for Transmission of Large and Binary Messages" (RFC 3030)
• ENHANCEDSTATUSCODES. "SMTP Extension for Returning Enhanced Error Codes" (RFC 2034) - [section 40.16] note that Exim allows you to construct smtp replies with rfc1893 enhanced error codes, but it doesn't have any way of automatically generating them, and it won't advertise ENHANCEDSTATUSCODES in the reply to EHLO.

• SMTPUTF8
• PRDR

Exim is available from many places - see ObtainingExim.