I needed to figure out how I was using so much disk space and was looking for a quick way to tell me which domain was the culprit to the usage and since I never really did any domain or user quota settings which I should of and still will eventually I was needing a quick way to tell so this is the fastest way I could find and the best output.

# du -ksh /home/vpopmail/domains/*

If you wanted you could pretty much add this to your qmail stats if you wanted to monitor the disk usage for your virtual domains, you can easily get down into the mailboxes disk usage as well just change the command to something like..

# du -ksh /home/vpopmail/domains/myhosteddomain.com/*

Just some helpful things I've found to help myself and hopefully you as well.