Ok, I reviewed my current config and remenbered some things of my old debian setup.

I use postfixadmin to maintain my accounts, it stores passwords encrypted in the appropriate mysqltable. The only problem I've had was wrong file permission of the authdaemon file in /var/lib/courier/authdaemon - iirc.

The HowTos I used for this setup are
here and here.

If you're interested in my configs I can post them, too. I'd be glad to be able to help you

Cheers

tcs