> Google requires you to register an application and get it approved to log in with OAuth, you can't just use it with arbitrary callback URLs. Why should I need to ask Google permission to use my own data, or ask for Google's permission to allow other apps to use my data?

I specifically mentioned the other approach; if your email client doesn't implement the OAuth style approach, you can generate an app password. https://support.google.com/accounts/answer/185833?hl=en