Like Postfix, we have Exim, which is another MTA.
Where Postfix is powerful and has its focus on security, Exim is known for being extremely customizable, and with a pedigree that goes all the way back to 1995 (inching out Postfix by three years). It's been the favorite of the Debian project for years, and, as a result, a lot of the documentation on setting up a mail server references Exim.
Also, like Postfix, this won't be a long section on fully configuring Exim, as that would require something much longer than a section, chapter, or realistically a book when done right. Instead, we're going to look at some configuration, the Exim log file, and what happens when you try to send a message from your local system.
