>That's not a fact that the recipients are users that signed up for it. It is if you use an email verification step as part of your sign-up flow, which we do.

>That's not a fact that the recipients are users that signed up for it. It is if you use an email verification step as part of your sign-up flow, which we do. At most, someone could sign up with someone else's address and generate a confirmation email, but it'd only be once per address and that's the price of an open Web. Anyway, I'm aware that not everyone does this, but at a certain point there's not much an ESP can do, beyond which there's essentially trust and monitoring.