Most of those providers already enforce DMARC between themselves, which provides all the anti-spam benefit you could hope for from a new protocol. (In terms of protocol beauty, it&#x27;s awful, but in terms of functionality it does what you want.)<p>If Gmail, Yahoo, Microsoft, and Comcast announced that they are making it more likely that outside email (from online stores, from individual Exchange installations, etc.) will be marked as spam <i>when it&#x27;s not actually spam</i>, people will find new email hosts. Somehow.<p>If they just want to mark traffic as spam when it is, they&#x27;re already doing that.