I have posted instructions on how to install qmail for OS X.
Not for the faint of heart. You should have a good reason for needing qmail before proceeding. Also, because I'm unsure whether it is safe to have the qmail queue installed under HFS+, you might not wish to proceed unless you have a UFS filesystem on your machine.
That being said, the instructions should work. At least, they did for me with the minimal testing I did. As usual, your milage may very.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020709091327481