Cannot delete mail from storage in Usage section of IPhone
Try rebooting the phone. Press and hold Home+Sleep until the Apple logo appears.

If that doesn't work, and the email account you are using with Mail is not connected using POP3, then try removing the email account and re-adding it. Go to Settings app > Mail, Contacts, Calendars > tap on the specific account > Delete Account. Confirm that you want to delete it, and reboot the phone - press and hold Home + Sleep until the Apple logo appears. Re-add the account by going to Settings app > Mail, Contacts, Calendars > Add Account.