email should stay free.  mainly becuase there would be way too much hassel for people to sign up for hotmail, gmail, and school, and work accounts.  and what about mailing lists?  can i anonymously pay for email?  if i pay for email, am I thus gaurenteed certain rights as a consumer?  given that zombienet sends a lot of email, would some people be charged $10 or more  per week for unknowningly sending email.  I'm fairly sure if such was the case, the maker of the OS would not be liable (in either linux, MacOS, or Windows), even though it can be argued it is as much their fault as the consumers if not more.

this idea will not stop spam at all, it will just charge the unwitting consumers running zombieboxes for it.

realistically, a better idea would be a non-spammers list.  people who can see if they are receiving email from others on the non-spammers list.  but i don't know if this is possible given the current email schema.