For the first time in ages, a customer tossed a problem at me based on EXIM (EXperimental Internet Mailer, it seems, one of the few Cambridge projects not named after a bird), so I learned a bit about EXIM.
It seems to be an interesting sort-of cross between SendMail & PostFix, if you can picture that.
It has long, wordy config files like PostFix, but many of the words are arcanely structured scribble, like SendMail. It's smaller & faster than SendMail, too. Finally, it seems to not have SendMail’s iconic target status, with very few security issues.
So... not my first choice — but not too bad, either.