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?