I like Rauch's idea, though I think he undersestimates the technical challenges. I guess, though, that if the incenentive is strong enough to get people to upgrade their software (and hardware?) to comply with such a system, then it'll sell. If not, we keep the current system until the next idea comes along.
As long as the idea's out there, here's my suggestion to imporove it. Allow my "fee" to go up as my mailbox fills up. When it's empty, a half cent will get you in, but when there's fifty messages awaiting my attention, it better be really important. In which case you'll pay the whole Lincoln.