exim

A Mail Transfer Agent (MTA) which was written by Philip Hazel at the University of Cambridge. Exim is similar to smail but has more features, including the ability to defend against junk mail by blocking messages from particular senders or networks. Exim comes with Debian Linux, and is freely available under the GNU GPL.

Exim's biggest advantage over sendmail is its human readable config file format, which it inherited from smail. Current versions of exim are much more powerful than smail ever was. Fairly simple changes to the config file can do some amazing things without a lot of pain. (Mostly, I am concerned with easy maintaince of configuration information such as mailing lists, filters, virtual user and domain addresses, spam blocking, etc., rather than elaborate routing and mangling of headers.)

The penalty, of course, is a huge set of documentation (which smail also lacked IMHO). But this is pretty much balanced out by config files that are still human readable. (And I've yet to read it all, as it wasn't necessary to get what I needed done.)

I'm sure that sendmail is still more flexible than exim. But then, assembly language is still much more flexible than C, not that this is a very good comparison. (But is sendmail's macro based configuration system as easy to use and as powerful as exim? idunno, I haven't tried it. I found exim first. You try both and tell us all. 8-P)