I can't follow your implication that Gmail, Hotmail, Evite, and other services couldn't survive e-postage. The free mail sites are, I suspect, net receivers of bulk and spam mail, and handling such mail is a major expense for them. So anything which lowers or offsets those costs makes those businesses easier to run. Evite, for all its faults, has not to my knowledge sunk to sending spam unrelated to event invites & reminders -- so it would be on my personal whitelist, if not the overall whitelist of any inbox provider I use. As would some (but not all) social networking sites be on most whitelists.

Getting mail from new and marginal services can already be dicey with current filtering. Postage ensures the really valuable mail (like say a utilities-disconnect notice) can get through -- it can be a filter-error safety valve much like your preferred Turing challenge. And even without any sender-side infrastructure upgrade, or 'money accounts for users', the 'postage due' bounce could offer the sender an URL where they can pay out-of-band. That's no more cumbersome than your preferred Turing challenge for suspect emails. (Indeed, I think the option of either paying cash or answering a Turing challenge is a likely hybrid evolution.)

My intuition is that ultimately, a relatively volume-independent 'responsible sender' credentialing flat fee (or bond) would dominate rather than per-message fees. After all, the marginal cost of email delivery is inherently small, and the real aim here is to freeze out bad actors, not add friction at every step for every actor. (The anonymity-reducing aspects of such a system are left as a topic for another day.)

But to even find out what mix of filtering/challenges/fees works best, and which are the optimal entities to bear the costs and responsibilities, requires an iterative process of deployment, commerce, and user feedback. That market-learning process shouldn't be preempted by what -- in the DearAOL campaign -- look to me like reflexively anti-economic objections.

I fear, though, that I'm repeating myself and the main event debate is in a couple hours. Perhaps I'll better understand the objections afterwards...