Email storage limit

I keep getting the following email and it is frustrating.  I have deleted 100s of emails and a lot of them with large attachments of pictures so I should not be getting this email anymore.  Cox Chat could not help me and couldn't tell how much I had used and had left. What can I do to fix this issue.  

Dear Cox Customer,

You have exceeded your mailbox storage limit for your Cox Email account.
Effective immediately, incoming email messages will not be delivered and
will be rejected until you reduce your mailbox storage to below the limit.

To resume receiving new email messages, you must delete unwanted existing
messages and/or content in your mailbox to create necessary storage
space.

Some quick ways to reduce your mailbox size are to empty your junk and
trash folders, delete unwanted sent messages, delete messages with
unwanted large attachments, or save large attachments such as photos and
documents to an external storage location. Visit Cox.com for additional
information on how to manage your email messages.

Sincerely,
Cox Communications