MTA
Exim.
That is all.
Seriously, though, I jumped that way a few years ago when the only sensible options were Exim or Postfix. Far too many people were still using Sendmail. Or maybe it was so log ago (Exim 3, that's for sure) that Postfix wasn't as mature. If I started again, I think I might go the other way - simply because it's more popular.
Or is that not a good basis on which to make a decision ??
(I find Exim's teergrubing facility particularly satisfying)
(Oh, and this was on RedHat - before I discovered Debian and that Exim was the default MTA there)