I am guessing that you are downloading with SMTP but haven't set it to delete the emails after download. You should be able to use webmail to log in and clear out the old messages. See the posts above regarding possible time delay before the space is released.
If you have problems using webmail then raise a support ticket. Make a note of the ticket number and check online for the response.
Personally I never rely on a single email address. I have duplicate accounts for both business and personal use.