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