The default mail transfer agent (MTA) which Debian comes installed with is called Exim.

A MTA is a system which will receive, spool, store and deliver e-mail for MUAs (mail user agents). Examples of MUAs include pine, mutt and Netscape Communicator.

Exim was developed by Philip Hazel at the University of Cambridge for use on Unix systems connected to the Internet.

Exim is an incredibly powerful piece of software, and is able to handle complex mail routing requirements. It can be used as a drop-in replacement for the venerable Sendmail application, which is the standard Unix MTA.

This course will bring you up to speed with the basics of running an Exim based mail system, but you should consult Exim's excellent documentation for further information and help. http://www.exim.org/

You can also find for information in the exim(8) man page, as well as in the "/usr/share/doc/exim/" directory on your local Debian system. This documentation is installed as part of the Exim Debian package.