Generically speaking -- and I'm digesting several decades of
experience into a few paragraphs, so I'm going to make some
sweeping statements that, of course, have exceptions. Except
for the last one.
If you've never done this before, then stick with sendmail because it
minimizes the probability that you'll screw up.
Postfix is easier to configure than sendmail. It also benefits from
having been designed after many years of experience with sendmail, so
it incorporates some lessons learned. It is relatively straighforward
to switch between the two, once you've mastered some basic concepts.
In the contemporary environment, either is a good choice for
relatively secure, relatively high-performance environments. Both can
be configured/customized extensively and there is plenty of support for
both, from multiple sources.
Exim is newer and arguably still easier to configure. It might be a
good choice for someone with limited requirements and little experience.
Courier is well-integrated with the other components necessary to make
a fully-featured mail server, and is worth consideration if its feature
set overlaps well with your requirements.
Qmail is crap and is only used by people who don't know any better.
---Rsk
