Elsewhere in the thread gsreenivas mentions that the solution is too expensive for spammers and so they haven't had a problem with that yet, so we've actually moved to the original solution to spam: charge for sending e-mail*. The original proposal was to charge per email but an annual subscription would have the same effect in the right market (for now.)
Strictly, that's dynamic IPs. The Venn diagram may almost be circular, but not quite. Part of the point being that it's much easier to block static IPs, while a spammer on a dynamic IP will evade any blocks when their IP address changes.
My in-laws outbound email broke recently because, being on a dynamic IP, they'd been set up to send through their ISP's smarthost. Then they changed ISP :P.
This is even missing a trick: unless its politically indicated, AWS does not ban its customers for spamming, high-rate crawling or other abuses, causing its IP ranges to be widely blocked.
So now Amazon gets to market segment and sells a trusted email service, too!
1. all IPs are the same, email for everyone
2. residential IPs cannot into email cause spam
3. consumers don't get into IPs anymore cause CGNAT
3. AWS collects all the other available IPs and rents them out
4. consumers can now get IPs again, rented from AWS
5. ...
6. profit !