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If what you'd like to do is to solve the problem at hand (if possible) using existing tools/without introducing a modification to an existing protocol, try DSPAM and SPF.
Between those two things, and some fairly aggressive DNS-based host rejection filters, I get between one and two spam messages per month in my email inbox -- and my email address was out there on the internet before spam was an issue. If DNS-based host rejection isn't feasible, DSPAM will just have to work harder, but work it will.
In general, I think it unwise to fiddle around with things that work for their intended purpose, especially when desired additional functionality can be had for minimal effort.
Specifically (intended constructively, and offered in a friendly tone), I see your proposed scheme suffering from the usual problems -- your ++RECEIVING MAIL++ section details a mechanism that suffers from the same old problems inherent in unintelligent filtering and/or challenge/response systems. I don't see that you've worked around any limitations in any existing system or provided any functionality not already available in a mature product.
However you go about it, I wish you the very best of luck with your project.