If the user has no email in his inbox at all, then make sure to check the system account, however if email forwarded to another account on the same domain works, that shouldnt be the problem. What I have found in the past works is deleting the account and recreating it, because it is most likely a problem with the quota attatched to the account, this would reset that. If he has email they would like to save you can make a copy of the inbox the copy it back when the account has been re-created. You may want to go in and check to make sure they dont have an inbox_lock file, if they do try and remove it, however I've never tried that and just a thought.