John upgraded his iPhone 4S to iOS 8.4 and now his email won't work. Leo says that the iPhone 4S is pretty long in the tooth these days, but since Apple is still offering it, they have to support it. This is why it's wise to be slow in upgrading older devices. But it should work since it installed.
Leo recommends backing up to iTunes completely, and then reset it. Others in the chatroom say they've had no issues with the iOS 8.4 update. So wiping the phone and starting over is a good way to fix the issues. Once John does that, he can restore his backup.