How to empty all emails data when account deleted? My situation is quite simple and I searched the Internet without any solution to my problem. So I decided to give a try here!

I have removed all email accounts (Gmail and Outlook) from my iPhone 5S accounts.

I supposed that it will automatically delete all local emails data. But still, when I check the Mail usage (in Settings > General > Storage usage), it indicates 1GB used, so obviously data are still there and I found no way to erase it.

So my question is simple, considering my situation, how can I free this email storage?
Assuming you have deleted the Gmail and Outlook accounts, if you preform a restore using iTunes and backups, all the email data will be free and so return available. 

EDIT: If you want you can also try with some specific application (e.g., PhoneClean) that help to clean the phone. Anyway, before download third part app I think the restore procedure (that is free) can work as well.