Proton is great for this.
Discussion
Do you know if they store the connections between your aliases and your main account somewhere readable by them?
that's a good question. the native aliases I would imagine are just wrapped into your mail encryption, but they recently bought simplelogin and that can still function as a standalone app so I'm not sure about its current level of encryption integration.
Proton is nice actually. I am thinking along the way proton is solving the things but over nostr.