There is one goal: to eliminate spam. If you want social implications at 10,
50 and 90 percent then you are missing the point. Social implications are
irrelevant. Economic ones are the only ones that drive anything. I don't
care if Joe Blow wants a new spam proof email system - he doesn't control
his mail servers. His ISP does. And his ISP doesn't want to pay for spam.
Even if it costs him 15% more than non-spam email (assuming 80% of email is
spam) that's still significant cost reduction.
Call it social or economic, it's the same thing. So, let's look at
the incentives to the ISP.