noteworthy wrote:
Whose reputations? Not users'. Instead, it's server admins and email advertisers.

I haven't read this, but generally speaking the problem with using "reputation systems" as a spam filter is the new user initiation problem. If people wanted new users to face hurdles they could easily deploy whitelisting with challenges today. They aren't, either because they are satisfied with the amount of spam they get, or because they perceive that challenging new emailers costs more than spam.

RE: Alberto Mujica: Reputation Management for Email | MIT Spam Conference 2007