This is a problem I just encountered. Any time you delete a user from within Exim4u, their mailbox remains on the file system. Plus, if you later add a new user with the same username and domain, they inherit all the previous user's mailbox information, including messages and folders.
Is there a way that this could be remedied? It's a pain in the A** for the site admin to have to go in and remove old mailboxes manually after the domain admin deletes the account.