When a message arrives and is delivered by vdelivermail, the quotas are enforced. It also updates the maildirsize file (and uses the maildir++ naming conventions) thereby updating the maildir++ quota usage that is shared amongst all maildir++ compatible software.
From what I understand of maildir++, all the programs like the POP, IMAP, and any maildir++ compatable software should maintain the maildirsize file(s) for each folder, including INBOX, so the usage should remain the same when moving messages between folders. The usage should be restored when deleting a message.
It's all right when you receive messages because all the check is done in the receiving phase (vedelivermail). All right the same if you move messages between folders (sizes don't change).
But what happens when you create messages with sqwebmail or squirrelmail & courier, or copy them to a remote IMAP server from your local folder?
When you create a message or upload an attach, and keep them inside your "Sent" folder, vdelivermail is not used, so single users may create and store messages until their personal ".maildirsize" let them do it. So they may overflow domain quota.