One of the main ways in which our clients use up their disk space is by storing email on the server. This is a problem, because if your disk space fills up completely, your email will start bouncing. If you are storing your email on a server indefinitely, we highly recommend that you start downloading it to a mail program. But you might be doing so without even knowing it. Your login account (the username and password with which you FTP and access your admin panel) is also you default email address. The address receives all of the catchall emails mentioned above, and fills up rapidly if not checked.
So, if you want to keep your disk space clear, be sure to check your main account regularly. You can do this through a mail program (POP settings info here) or through webmail (info here).