In SMTP servers I've managed for clients we typically block anywhere from 80 to 99.999% (yes 10000 blocked to one success) messages. I'd call that MegaModeration if there was such a term.
And if you think email spam is solved then I don't believe you read HN often as there is a common complaint of "Gmail is blocking anything I send, I'm a low volume non-commercial sender"
In addition email filtering is extremely slow to react to new methods, generally taking hours depending on the reporting system.
Lastly, you've not thought about the problem much. How are you going to rapidly detect the difference between a fun meme that spreads virally versus an attack against an individual. Far more often you're going to be blocking something that's not a bad thing.
I get that no machine learning is 100% perfect which is why it should be used as an indicator rather than the deciding factor.
I have had issues with gmail blocking emails but as you point out it was always because of ip reputation not over zealous Naive Bayes.
[1] https://demos.co.uk/press-release/staggering-scale-of-social...