I think this is an area where functional encryption could help:
https://en.wikipedia.org/wiki/Functional_encryption
This would allow a client to combine a server-provided function that calculates a spam score with their private key such that the resulting function calculates a spam score on encrypted email. The client could then hand that function back to the server so it can perform server-side spam detection.
There are a number of drawbacks, including performance and general questions about the security of such a system. That said, I think this is probably the biggest problem (from the OP):
"The third problem is that spam filters rely quite heavily on security through obscurity, because it works well. Though some features are well known (sending IP, links) there are many others, and those are secret. If calculation was pushed to the client then spammers could see exactly what they had to randomise and the cross-propagation of reputations wouldn't work as well."
Using functional encryption to provide server-side spam detection would still require handing a spam scoring function to the client so they can apply that function to their private key and hand the server a result. This would expose the internals of the spam detection routine to all clients, including spammers.
A difficult tradeoff.
https://en.wikipedia.org/wiki/Functional_encryption
This would allow a client to combine a server-provided function that calculates a spam score with their private key such that the resulting function calculates a spam score on encrypted email. The client could then hand that function back to the server so it can perform server-side spam detection.
There are a number of drawbacks, including performance and general questions about the security of such a system. That said, I think this is probably the biggest problem (from the OP):
"The third problem is that spam filters rely quite heavily on security through obscurity, because it works well. Though some features are well known (sending IP, links) there are many others, and those are secret. If calculation was pushed to the client then spammers could see exactly what they had to randomise and the cross-propagation of reputations wouldn't work as well."
Using functional encryption to provide server-side spam detection would still require handing a spam scoring function to the client so they can apply that function to their private key and hand the server a result. This would expose the internals of the spam detection routine to all clients, including spammers.
A difficult tradeoff.