you're right its not an unsolved problem, its an unsolvable problem.

There is little point in encrypting mails between apple users, if the keys are kept on the apple servers. What would it achieve after all? Apple, and by extension every nation of importance would still have access.

the only reward you'd get from that is getting locked into the apple ecosphere even stronger, because now you won't be able to read your mail archive anymore unless you use an apple device.

And you obviously can kiss search goodbye. There is no way to search encrypted data after all, unless you decrypt it.