The only way I have found is through the administrator. It is not automatic.
We have many students that never look at their email accounts. Other will just decide to stop looking in the middle of the semester. To keep them from getting too big, we periodiaclly (about once a month) highlight all mailboxes in the student container, choose tools | clean. Then delete all messages older than 60 days and all unread messages older than 30 days. There is a check box to delete them rather than moving them to the trashcan.
Hope this helps.
Dan