Exim. The mail transfer agent

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's 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 such as the following: - 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 - Innumerable tuning parameters for improving performance and handling enormous volumes of mail 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. While a basic configuration is easy to read and can be created quickly, Exim's syntax and behavior do get more subtle as you enter complicated areas like virtual hosting, filtering, and automatic replies. This book is a comprehensive survey that provides quick information for people in a hurry as well as thorough coverage of more advanced material.