With regards to HashCash, the explanations I've seen didn't persuade me (the counter arguments most often cited have been made by some researchers in some IMHO

With regards to HashCash, the explanations I've seen didn't persuade me (the counter arguments most often cited have been made by some researchers in some IMHO very weak papers). To me it seems that the real reason why it's not being used is that ISPs would have to allocate resources, which would make them less competitive (who wants to offer free email then pay for a lot of infrastructure, or deal with the mess explaining that users have to pay for better delivery). The CPU cost could be moved to the clients, but somehow no IMAP based mail clients seem to ever have implemented it (chicken and egg problem?), and JavaScript has not been an efficient way to do it. Also, I guess the big ISPs have an easy enough time (in their view?) analyzing trust, which is probably a better solution in general if applicable. The problem is that it's easier to judge (newly created) internal users (accounts) than external ones (random IPs). I would think improving the mail system would be a research topic, but perhaps not interesting enough from the point of view of researchers (would it be best done in social sciences or computer science?, and there are many real-world variables, taking time to collect and verify, and working out simulations to predict possible remedies).