You will receive an e-mail each month showing how much storage you are using, and how much it costs. You can also use “Mail Manager” (go to your control panel to search for stored mails and the results will give you a grand total of storage used. (Note that the total storage used can be less than the sum of every individual e-mail, as some e-mails are received twice but only need to be stored once, e.g. they are sent to two separate addresses on the same domain, or forwarded locally within the domain).
Posted in: Questions about e-mail backup