Postfix through Fink. Use the "FinkCommander" and get info for the package, it will tell you how to move Sendmail away. It won't be removed, but it will not be active. Postfix will take completely over.
I've used Postfix for a long time, and I think it is great. On Mac OS X Hints, there is also a hint, and great comments about installing WU-IMAP (POP3 and IMAP server, can also do both with ssl), I've had no problems with none of it. It is really simple, believe me.