It's not clear to me how this is any different than before? Most of my spam that I actually see already has all those things (valid DKIM, one-click unsubscribe link, and a rate limit per sender).

If you really want to fix email spam, create a micro-payments system. One cent for every email you send, the user has two options after they open the email: mark it as spam and keep the penny, mark it as legit and give the penny back. If they don't act on it within a week you get your penny back.

Legit senders won't be harmed because they will get their pennies back, spammers won't be able to afford sending messages anymore. The real interesting part would be stuff like LinkedIn notifications -- if people find them useful they'd give the penny back, but companies would have to decide how many people might actually find it useful for their cost analysis.