Recently, I had to delete and restore my Hotmail account from my PC and iPhone. Since that moment, the Mail App uses another account as my default when I send emails. The iPhone uses another account as default. It uses my ISP mail account. I went to the settings to try to change the default account. It is currently my Hotmail that is selected. I tried also to erase my ISp account int the mail Settings. Even with that change, the Mail app continues to use the ISp account as default.
I have an iPhone 5s with IOS 10.0.2 installed. Maybe there is a setting that I don't know. If you could help, it would be appreciated.