Chapter 1. Introduction

Exim is a mail transfer agent (MTA) that can be run as an alternative to Sendmail on Unix systems.^[1] Exim is open-source software that is distributed under the GNU General Public License (GPL), and it runs on all the most popular flavors of Unix and many more besides. A number of Unix distributions now include Exim as their default MTA.

^[1] The terms mail transfer agent and mail transport agent are basically synonymous, and are used interchangeably.

I wrote Exim for use on medium-sized servers with permanent Internet connections in a university environment, but it is now used in a wide variety of different situations, from single-user machines on dial-up connections to clusters of servers supporting millions of customers at some large ISP sites. The code is small (between 500 KB and 1.2 MB on most hardware, depending on the compiler and which optional modules are included), and its performance scales well.