Jan Steinman writes:
Perhaps I know just enough about the underpinnings of the Internet to
see the possibilities, rather than the impossibilities. Sorry if my
examples were impractical, but I continue to dis-believe that it would
be impossible to implement a system whereby spam could be made
uneconomical.
On the contrary, it's very easy to make spam uneconomical. Just make all email uneconomical. The trick is to get a clean bath without losing the baby, which may be possible, but not by any of the schemes proposed in this thread, nor a host of related ones.
At the risk of incurring Brad's wrath, let me point out (I think for the second time in this thread) that policies that make it hard for unregistered parties to have mail delivered have a decidedly negative effect on GNU Mailman. Long before the spammers feel pain, many lists like this one will be long past feeling any pain.
Even worse, Mailman-Users might survive, but can you really see an ISP client going to Customer Service with "Hi, I'm a member of the PASS (Physically Abused Spouses and Siblings) mailing list. Will you please let it through?" Survival of the fittest is all very well, but I don't think that the winner of the <I-Don't-Feel-Like-Looking-It-Up> Prize for greatest contribution to society in open source (or something like that, congratulations again, Barry, Mark, Tokio, et cie.!) will be very happy about advocating measures that discriminate against use of Mailman by non-technical volunteers for charitable purposes. No clearly acceptable proposal has been made to deal with these important uses of email (to me and many others, YMMV).
By the way, my day job is Practitioner of the Dismal Science, and as a professional economist I can assure you that this particular idea, while not "dumb" in my opinion, rather the reverse, has been thoroughly debunked (see other references). It is what we call an example of the "Theorem" of the Second Best: while removing one commons (or other market failure) to get a perfect market economy always makes things "better for everybody", fixing one market failure to get an economy that still has market failures "usually" makes things worse.
(The phrases in quotation marks are trolls: as stated, it's not a theorem, "better for everybody" is defined in a very peculiar fashion, and "usually" is deliberately ambiguous. So don't call me on them, as stated it's just a sort of proverb that the pros need to keep in mind to avoid embarrassing themselves in front of their peers.)