Added mailbox but can't use it

Added mailbox but can't use it

Hi

My account (deslea.com) has seven mailboxes (standard 10GB plus one additional). It currently has six users.

When I attempt to add another user I get the error "Max user license limit reached. Please purchase/ugrade license to add more user(s) with mailboxes." It doesn't seem to recognise that I have an additional mailbox.

How do I add the extra user? Do I have to do it in some other part of the control panel because it's an extra?