@PublicWolf
To be clear: using Fastmail is basically just saying, “feh, email isn’t designed for E2E encryption, I just have to trust my provider.” Which I think is the correct answer, but…just to be clear.
Tuta attempts to solve the same problem as Proton Mail, but is much much more explicit about where the E2E encryption boundary lives. That makes it more annoying, but probably also more secure in practice (because you’re very clear about what is and isn’t encrypted).