I've had VERY good luck with qmail over the years. I've been using for 300+ users for 4 years with no qmail related issues at all.
I've been using
courier at home for 2 years and have had very good luck as well.
I can't really speak about exim or postfix 'cause I have never used either.
If you haven't checked out
courier yet, give it a look.
congrats on choosing open source!!