Which others? Give some examples. Gmail won't. Hotmail won't. Yahoo won't. In fact I can't think of a single authenticating mail service that doesn't also vali

Which others? Give some examples. Gmail won't. Hotmail won't. Yahoo won't. In fact I can't think of a single authenticating mail service that doesn't also validate authorization (e.g. you can only send as you). Now maybe you're going to point out micro-mail servers, but that's kind of beside the point because very few other people share it with you. e.g. you used gmail as your example, but services like gmail aren't vulnerable to that.