When a mailbox get corrupt when changes the quota, here is the steps to fix it.
1) Login to your cPanel.
2) Select File Manager.
3) Select 'Home Directory' from File Manager Directory Selection popup.
4) Double click mail line from Size column.
5) Double click domain name (e.g. josh.iprobot.net) from Size Column.
6) Double click your email account username (e.g. [email protected]).
7) Select 'maildirsize' checkbox and click on the "Delete" button at the top menu as shown below:
NOTE: Once you delete maildirsize, cPanel will reform for the email account automatically and it should correct the email quota size.