| I personally think:
* sendmail: Not designed with security in mind. Bloated. Hard to configure. Only around because it’s the standard (not unlike wu-ftpd).
* exim: Not a bad mailer, I don’t have anything against it, really.
* qmail: Makefiles are abused in ways they were never meant to be, it just has a bizzare architecture, and you need to install everything of djb’s just to get it running. Ugh. Also non-free.
* postfix: A great, full-featured mailer written from the ground up to be secure.
So, my recommendations are, in order:
postfix, exim, qmail, sendmail.