I just showed how Apple could implement exactly its method on Android. I'm not sure why you're looking for other methods. As far as private antispam, you can i

I just showed how Apple could implement exactly its method on Android. I'm not sure why you're looking for other methods. As far as private antispam, you can imagine a hashcash-like system that takes into account how many messages you've accepted from the sender, but this is a completely different discussion.