To view you disk space usage please see this kb article:
Once you have determined the user that needs to free up some disk space log in to that email account.
1) The first thing you can do is delete all mail marked as junk.
2) Delete all " Deleted Items" :
3) Set up an Auto-Clean to delete old emails:
Note: You can change the time you want to set before mail is deleted I set it at a year as an example.