1. Inbox is filled. This happens when you don't delete old mail OR you deleted old mail online AND didn't purge the trash folder.
2. You have spam filtering enabled and your spam goes to a folder (usually called SPAM) that has accumulated. If this mail is not deleted, it can have the same impact as a filled inbox, even though you have no mail waiting for you.
3. You (or someone) uploaded too many files to your website. This happens most often when an image gallery is installed and you allow others to upload to your site.
4. Your log files are configured to never delete. (Log files are usually configured to "roll-over" every month so this doesn't happen.)
First, make sure you have this configured!
Second, you should NOT put your normal domain-based email address** in this contact setting as that will do you little good if your email is down ... and it sends the alert to your dead email. Best advice: Use a secondary email address that you monitor or even better, the address to your cell phone. e.g., 7345551212@vtext.com (this is what it might look like for a Verizon phone) That way when something goes wrong, you'll know about the moment the server knows.
** If your domain is acme.com, do not put admin@acme.com here!