Exim is an open source mail transfer agent (MTA), which is software responsible for receiving, routing, and delivering e-mail messages (this type of program is sometimes referred to as an Internet mailer, or a mail server program).
Exim is highly configurable, and has features that are lacking in other MTAs. It has always had substantial facilities for mail policy controls, providing facilities for the administrator to control who may send or relay mail through the system. In version 4.x this has matured to an Access Control List based system allowing very detailed and flexible controls.
Exim somewhat resembles Smail 3, but it has diverged and now surpasses it in user friendliness and flexibility.
- 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).
- 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).
- 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.
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