alt Hacker News...would those "overreach instinct" expand to "handing over access an overreaching and likely corrupt EU or US prosecutor"? (I don't care about 5eyes etc, spyies will spy me, I just don't want stuff to be easily and unexpectedly draggable in a court case, or am email used as bolt-key to access other things to get blocked by a prosecutor's regulation...)
If your threat model includes the USA government then you can only go with obscurity, honestly - preferably self hosted with a completely locked down system that cannot initiate any network communication besides on the relevant mail protocol ports, completely immutable filesystem beyond the mail data with encryption at rest
And with all of that they'll still be able to pwn you through network equipment which relays your mail, eg some router or switch which they backdoored and mirrors all traffic to their datacenter.