Exim

Exim delivers electronic mail, both local and remote. It's the default mail transport agent installed on some Linux systems; it 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's open source, scalable, and rich in features. These include compatibility with sendmail options, database lookups, support for regular expressions and many kinds of address parsing, sophisticated error handling, and parameters for improving performance. Best of all, 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. Philip Hazel, the creator of Exim, is the author of this official guide, designed for access to quick information when you're in a hurry as well as thorough coverage of more advanced material.