Go to your inbox and set it to show messages from the beginning - it's in the upper left hand side just above where the messages would be listed. Also, on the right hand side go to the Sent items and the Message tracking folders. All of those add to the number of messages you have and can cause an Inbox full message. BTW, this information is in the Frequently Asked Questions at the top of the page, we're trying hard to put the most commonly asked things in there. Check it out if you haven't done so.
-Steve St. Laurent
Webmaster