courier-mta (esmtpd) only delivers messages to /var/lib/courier/tmp

I swear that courier-mta was working before, but now it has stopped working. The problem first started out with getting "submit: Permission denied" errors and I was unable to send emails to the machine. I chmod'd /var/lib/courier/tmp (via chmod 771 /var/lib/courier/tmp) which fixed this. But, now all the messages are stuck in /var/lib/courier/tmp/NNNNNN and never get migrated to $HOME/Maildir . I cannot figure out what moves the files from /var/lib/courier/tmp to $HOME/Maildir so that I can use courier-pop. What does the moving? Note that my /etc/courier/courierd has DEFAULTDELIVERY=./Maildir in it. Thanks.