If you still need info...
I'm still working with qmail, but the passwd checking is usually done by an external program. The one I'm using (that I've seen used the most on the net) is chkpasswd. I believe you can get it from
http://www.qmail.org .As far as configuring it, I'm still learning. You will need to add the chkpasswd program to the qmail-pop3 script, but I'm not sure how, exactly, to accomplish that.
If you'd rather start over with something a little more user friendly, you may want to consider some of the scripted installers. Two that I know of are:
http://www.qmailtheeasyway.com
and
http://obua.org
Best of luck!