Which sounds awfully like a whitelist system to me. Implementable
today, no protocol enhancements, minimal training, and decent systems
can be pre--programmed to minimize the hassles to people you WANT
sending to you....
Im not sure that the test I proposed implies any particular system,
including whitelists. My own personal preference is a sender-pays
system, implemented at the ISP level, which would be equally effective.
Im sure others have proposals that would satisfy the test.
Sender pays requires a set of standards, cooperative implementation,
some non-trivial infrastructure to make it work, and a lot of time and
energy. Whitelisting is something that can be built on an individual,
site or organizational level without a need of new standards, new
protocols, financial transactions, certification, PKI infrastructures,
or anything else. It can be implemented today, in fact, layering on top
of what's there in most cases.