All of our managed Linux servers here at Anchor use Postfix, written by Wietse Venema, as their mail server. Postfix is easy to configure, works out of the box, written with security in mind, actively maintained, and very fast. These are all very good reasons to stick with Postfix, but I’ve just found another one for all the programmers out there:

http://dotat.at/writing/exim-turing.conf

From Tony Finch’s homepage:

I realised recently that Exim is Turing-equivalent so I decided to write a little demo which includes an informal description of how to translate a Turing machine into an Exim configuration, and an example configuration that implements combinator reduction like my IOCCC winner mentioned above.