Productize your approach, and drive GoodMail out of business -- if you've nailed it so well.

I'm not sure the net social cost of your filters plus Turing tests would be less than that of tiny e-postage micropayments. My two levels of client-side content-based spam filters already render my laptop nearly unusable for about a 15-minute period when catching up on a day or two's worth of spam. (That's in addition to some server-side prelabelling.) And all the oh-so-poor newsletters inconvenienced by e-postage would be similarly inconvenienced by all the Turing-challenge-bounces they'd get.

Only a competitive economic process can really weigh alternatives against each other as to what approach, or hybrid approach, offers the most benefits at least cost. Perhaps it's even like your approach, but with a postage-due-challenge rather than a Turing-challenge. (I'd rather pay a penny than spend even 30 seconds answering a challenge -- and anyone who can't afford a penny, could probably earn it, Mechanical Turk-style, if they they were willing to solve some other Turing challenge.)

I also suspect your solution only works well for you because so few people are using exactly it. Scale it up, and spammers will figure out ways to pass your Turing test. Or poison others' spam filters against your challenges, so that they never get through. Or a thousand other things to gum up the system.

A nice thing about e-postage is that it can neatly sidestep the red queen race of esclating countermeasures: it honestly declares "this is how much it costs a stranger to arrive in my inbox". Then the whole execrable rathole industries developing ever-more-sophisticated-filters and ever-more-sophisticated-trickery can just go away, redeploying their resources to more productive endeavors. Wanted mail will get through and unwanted mail dropped, due simply to a consensual economic transaction between sender and recipient. No byzantine layers of technological weaponry need be designed and deployed between them, sucking up communication-created value that rightly belongs to correspondents.