I checked with the mailbox.org team
They have an "encrypted inbox" which is pretty transparent and understandable.
Mail is not encrypted by default. But mostly encrypted in transit. There is a real danger of the provider or law enforcement snooping on your mails.
Their feature allows you to upload your PGP public key there. They receive the mail and pass it through a filter pipeline in RAM only. You could do things like move, forward etc there.
In the end, the mail is encrypted and only stored encrypted in your inbox!
Law enforcement only gets access to the inbox and not RAM content, which means they get nothing.
On Desktop Thunderbird has PGP support, on GrapheneOS OpenKeyChain+Thunderbird Android do the trick. You need to enable PGP for the account and have your private key imported.
Unlike Tuta and Proton, this is a standardized ecosystem. I tried Tuta and Proton for calendar and was pretty dissatified, Tuta being kinda outdated and no offline support, Proton being extremely locked in.