EFF's legal director Cindy Cohn, a legendary free-speech litigator (Cindy argued the Bernstein case that legalized crypto in America) has written a great essay for EFF's Deep Links about the reasons that this will fail to solve spam, and punish free speech:
Email being basically free isn't a bug. It's a feature that has driven the digital revolution. It allows groups to scale up from a dozen friends to a hundred people who love knitting to half-a-million concerned citizens without a major bankroll.Link
Email readers and senders will both lose, because the incentives for Yahoo, AOL, and Goodmail are all wrong. Their service is only valuable if it "saves" you from their spam filters. In turn, they have an incentive to treat more of your email as spam, and thereby "encouraging" people to sign up.
Even email senders who just want to reach Dad@aol.com may eventually be in trouble. Once a pay-to-speak system like this gets going, it will be increasing difficult for people who don't pay to get their mail through. The system has no way to distinguish between ordinary mail and bulk mail, spam and non-spam, personal and commercial mail. It just gives preference to people who pay.