A good solution is instead to have payment for delivery of e-mail. If it costs a dollar or even just 50c to send an e-mail, then spammers will bankrupt.
IMO pay-per-delivery will be a start of another business - companies dedicated for spamming - customers pay 25c/email advertisement, and these businesses will put 10 different content into one email and send it at 50c/email, making a profit of $2/email.