If there were a good, user-level solution out there, we wouldn't be so worried about passing anti-spam legislation. ISPs can filter and block hosts who send almost nothing but spam, but at the user level there isn't much you can do that also wouldn't affect legitimate e-mail.
Quotes about user-level (2 Quotes)
The security aspects are extremely important to enterprises. We need to make sure those are properly dealt with. We also need to have user-level control so that presence information is only available to certain people.