I'd say that if you really want to understand what's going on, then sendmail's the only way to go. It's also the safest ( not my words, but those of the top internet security techie at British Telecom ( sorry Syntegra ) ).
I'd be interested in the results of a proper code review to see which is 'safest'. However, OpenBSD ship a sendmail, and they have pretty high standards too. DJB's "I'm safest because I've never had to give anyone any money for finding a security problem" argument just doesn't cut it for me ...
It depends on just what it is you feel you need to understand. Exim (and probably postfix) will help you understand the grokking of an SMTP session/message, but not much about queuing. Sendmail might get you to spend too much time thinking about non-SMTP things, and queues. qmail makes you think far too much about inadequate documentation, inadequate functionality, odd signals and obscure environment variables. Unfortunately it also makes you think about the speed of (local) delivery that other MTAs produce.
I'm playing at getting courier up at the moment, just to play with IMAP again. However, it's still in Beta, and has been for years.
I've happily set up BincIMAP a few times. BINC Is Not Courier. http://bincimap.org/
-jim
