couriercheckpassword anyone?

Does anyone have couriercheckpassword working? I'm trying to get it to 
work but I've been only partially successful.

Here is my authmodulelist parameter from the authdaemonrc file.

authmodulelist="authcustom authcram authuserdb authpam"

Using couriercheckpassword works when authenticating a system account. I 
  know couriercheckpassword is really being used in this case because if 
I remove authpam from the authmodulelist above, authentication for 
system accounts stops working.

When I try to authenticate a virtual mail user, however, 
couriercheckpassword fails. I have my virtual users in a userdb file as 
per the documentation on the courier IMAP web site. I know courier 
itself is able to authenticate users in the userdb file because that's 
where my mail user account is stored and I use it every day.

I've tried removing all authentication methods from authmodulelist 
except authuserdb but this didn't help.

Here is the command I've been using to test couriercheckpassword.

qmail-popup domain.com couriercheckpassword id

When I enter the user ID and password of a system account, I get the 
output of the id command. If I enter the user ID and password of a 
virtual mail account, qmail-popup returns this error.

-ERR authorization failed

Suggestions anyone?

Andrew