I have a virtual mailuser setup with MySQL backend. Courier and postfix are running correctly, courier-maildrop uses courier-authlib and gets the correct information. The user owning the mail is vmail (uid=2000).
Mail is delivered correctly when maildrop is called from any other user than vmail. But if it is called as vmail (either by sudo -u vmail -i or by postfix's user=vmail option in master.cf), it deliveres all mail to ~/Maildir, which is not the designated mailbox transmitted by authlib.
I don't know what the problem is, but on the debian-user-list I got the hint that maybe geteuid() was correct instead of getuid(). As I'm not good in C, I did not try it out.