Exim is a Mail Transfer Agent, used to route and deliver email, for Unix-like operating systems. It is used as the default MTA of some Linux distributions and for some control panel software. Its documentation is extensive and exhaustive, in the form of The Exim Specification. An undocumented feature is a bug. More details can be found in the wikipedia article on Exim.
Questions about Exim configuration and debugging should have this tag. Optionally, questions specific to the Debianized configuration system should have the Exim4 tag also.
Exim uses an ordered list of Routers to decide how to deliver a given message to a given recipient, accomplishing the delivery with a Transport. It uses a powerful string expansion system to provide configuration flexibility, including lookups of various forms and a wide variety of primitives. Authenticators are used to manage both the client and server side of authentication.
Useful guides for debugging Exim are http://wiki.exim.org/TestingExim, http://wiki.exim.org/FAQ/General_Debugging, http://wiki.debian.org/PkgExim4UserFAQ and swaks.
Information for people new to Exim can be found at: http://www.exim-new-users.co.uk/
The exim-users mailing-list is a useful alternative source of assistance to ServerFault.
