Exim delivers electronic mail, both local and remote. It has all the virtues of a good postman: it's easy to talk to, reliable, efficient, and eager to accommodate even the most complex special requests. It serves as the default mail transport agent installed on some Linux systems, runs on many versions of Unix, and is suitable for any TCP/IP network with any combination of hosts and end-user mail software. Exim is growing in popularity because it is open source, scalable, and rich in features: compatibility with the calling interfaces and options of sendmail (for which Exim is usually a drop-in replacement); lookups in LDAP servers, MySQL and PostgreSQL databases, and NIS or NIS+ services; support for many kinds of address parsing, including regular expressions that are compatible with Perl 5; sophisticated error handling; and innumerable tuning parameters for improving performance and handling enormous volumes of mail; Exim is easy to configure. You never have to deal with ruleset 3 or worry that a misplaced asterisk will cause an inadvertent mail bomb.
While a basic configuration is easy to read and can be created quickly, Exim's syntax and behaviour does get more subtle as you enter complicated areas like virtual hosting, filtering, and automatic replies. With this comprehensive survey, you can find basic information in a hurry as well as thorough coverage of more advanced material. The author of this text, Philip Hazel, is the creator of Exim. He writes its online documentation, answers many questions on the Exim mailing list each week, and frequently adds new features in response to user requests.