Two other suggestions.....
The first would be to go to your email accounts on the server. Log into your webmail using a browser on a computer. Look for an email with a large attachment or something that might be corrupt and try to delete that email. That approach could take some time if you have multiple email accounts and if you need to sort through a bunch of emails.
The other thing that I would try is to Reset all settings. You will not lose any data, but it will take some time to set all of the device settings back to where they were. Settings>General>Reset>Reset all settings.
I Just want to make sure that you did force close the mail app before you force restarted. Can you confirm that you did so?