doesn't work for things like email, especially for people who get lots of previously-unknown senders.
> doesn't work for things like email
Which is why I was writing about Usenet☺
Controlling spam in general is a much tougher problem, but in the directed cyclic graph which is Usenet, it could have been done.
Nope. Each site would be its own verifying authority. All I care about as a site is that the sites which feed me are responsible for what they send me; all each of them cares about is the same. If any one site gets bad enough, I'll start to reject their articles.