R1:4997b6bd028d

Never send normal mail to unverified addresses

Summary:
Ref T12237. This tightens our delivery rules, which previously sent normal mail to unverified addresses:

- We sent general mail to unverified addresses so that you wouldn't miss anything between the time you sign up (or have an account created) and the time you verify your address. This was imagined as a slight convenience for users.
- We sent automatic reply mail to unverified addresses if they sent mail to us first, saying "we don't recognize…