Unfortunately it's rather hard to open an account on Proton Mail, probably for reasons of fighting spam, but it still doesn't help. They e.g. require already existing email, which defeats part of the purpose.
That's optional.
purpose of what? being fully anonymous? If all you want is a mail provider that doesn't scan your emails for marketing purposes, it shouldn't make a difference. If you really want to be anonymous, you can always use a burner email (no, not the disposable kind) to sign up.