Hi, welcome to iPF!
Usage for the Mail app stands for emails and attachments in your Inbox, Sent, Trash, Archive, Draft mailboxes as well as folders/mailboxes you added yourself.
When you tell us that you have no mail in both accounts, did you also mean not only the Inbox, but all the other mailboxes are empty as well?
See if restarting and/or resetting will reduce the used space:
Restart - turn off your device, turn it on again.
Reset: press and hold Home and Power button at the same time until the Apple logo appears, then release both and let your device start on it's own.