There are some things that will work.
A major provider carries email for a lot of people and can tell if mail is spam if
- the people have no intersecting interests
- they mostly receive it at the same time
- a number of users mark it as spam (nearly all users who regularly mark anything as spam)
Google is obviously doing this and some other for-pay providers too, is my guess. I'd pay for a way to be able to test my email headers against such a service without actually running my email through their servers.
Also, you can hire people to actually read email subject lines and decide whether email is spam. Probably a small number of people could make a huge difference and I'd propose that the cost of such a system could easily be borne by government, or be covered by a very low fee.
As first line of defense, you can make a someone lenient automatic system that blocks out common keywords/patterns in email. This would probably cover 98% of spam and could be tweaked by an end user (for example anything about Trump, CNN, gambling, Gwen Stefani or hot tubs is spam 100% for sure). A central repository for such keywords/patterns could be very useful to end users. Personally I have a number of accounts some of which are old and combined they send me a huge amount of spam, so I am considering what to do about it. The above would be a big help.