If you get notification that your webhosting account is overquota, and you get your email through your webhosting account, check to see if you have an email account that isn't getting regularly checked.
I only mention this because yesterday one of our clients exceeded their disk quota, and it was due to over 55,000 emails sitting in one account, unchecked (or left there) for the past year. Even deleting 25,000 of them yesterday (starting with the oldest—I figure that if the user in question didn't bother to delete them off the server, or even check that account, then that user isn't going to miss them) didn't bring them under quota. I had to delete about another 5,000 or so today (unlike yesterday where I deleted the oldest 25,000 I instead started with the largest messages, then obvious spam).
In fact, of the few customers that regularly go over their quota, it's because they fail to delete email off the server (or even check those accounts).