Use postfix. It's (one of the, it not the) most common, and it's got a security secord to rival dovecot (the author is competent and cares). It integrates with everything under the sun for filtering. There's good documentation in the form of bundled text files.

Don't use qmail unless you really want to, as you need to patch it up a lot in any modern setting. And don't use sendmail if postfix will suffice. Exif will also do the job but there's really nothing it does better than the others.