adding courier-imap to existing qmail install?

I have a production mail server, running qmail 1.03. I am used to
vpopmail, and this server does not have vpopmail installed.

I am updating the webmail application on this server, the new
installation needs to support things like a sent mail folder. it does
this using Imap, the server does not currently have imap running.

i added courier-imap (which is what ive always used on my qmail installs
that use vpopmail). It installed, and runs, but i cant seem to figure
out how to make it authenticate to qmail.

The server hosts virtualdomains, so users login using user@domain.com.
If i try to use a known working account through courier-imap, it rejects
the login.

I suspect that the problem is that courier-imap is oblivious to the
qmail install, and is trying to authenicate to itself, or /etc/passwd.

Qmail seems to have an authentication package called "checkpoppassswd"
installed. I didnt setup the server, so i do not know for certain.

So, how can i tell courier-imap where to authenticate to?

Thanks,
Nathan