Wow! What a heavy-handed way to fix a problem which has a purely technological solution.
I can set up a closed e-mail system now. We all agree that's technically straightforward. The problem is that it is of great value to me that any of the 100 million legitimate users on the net can easily send me e-mail and I can respond to them equally easily, and a "solution" that cuts them out to get rid of the spammers is cutting off your nose, both ears, and about nine fingers, to spite your face.
Now, communities will have to perform some kind of authentication of its members to exclude abuse. Which means that USENET cannot be covered by this scheme; but isn't it already nearly dead?
Neither can the existing SMTP mail network, unless you want to overlay a crypto system on that. But you could do that with usenet as well if you wanted to. Besides, as soon as the communities got large enough to be interesting, you'd find spam leaking in via providers who value short term profit over long term interests, same as now. Spam is a social problem, not a technical one, which is why technical solutions will never be more than a stopgap. There's lots of other places to discuss spam, anyone who doesn't already know what they are is welcome to e-mail me for a list, or visit http://spam.abuse.net. Regards, John Levine, postmaster@abuse.net, http://www.abuse.net, Trumansburg NY abuse.net postmaster