it's all about the float.
Tim Bray advocates an economic approach to the spam problem, where senders pay a nominal fee to an "SMTP4All, Inc." to have their mail whitelisted by recipients. Sounds great in theory, but impossible in practice. First, you need a major ISP to be on board to get to critical mass, but then you'd have a first-mover problem: If any one of AMY (AOL, Microsoft, Yahoo) did it first, the others (and consumers, and the press) would cry foul -- "why should our users have to pay to send mail to your system?" Second, if the market exists and there's money to be made (especially on the float), then there's bound to be more than one player jumping in, which leads to interop problems that put the current challenge / response mess to shame. With current c/r systems, all users have to spend is time to get whitelisted by their friends. In a heterogenous pay-to-send world, users would have to pony up money or credit to get whitelisted with several different brokers. Assuming that network effects would condense the market to a reasonably-sized oligopoly, you then have today's equivalent of IM (border conflicts, anyone?) with an even stronger economic disincentive (the float) to interop.