[dfabulich]

Today, Gmail and other major providers do this by tracking sender reputation at the domain level. If users mark your emails as spam, that hurts your domain's reputation. If a domain's reputation gets low enough, Gmail won't deliver mail from that domain.

It's better than nothing (much better than nothing, IMO), but but spam will continue to be an issue as long as we want/expect to receive incoming mail from strangers, and as long as we plan not to have a centralized authority for email identities.

[inigyou]

It has the fuzzy false positive problem too: suppose a million Gmail users sign up for your newsletter with email confirmation and all. Two months later you send a newsletter. A thousand of those will be annoyed by it despite explicitly subscribing and will report it as spam. Boom, you're now banned from Gmail.