Comment by j-bos

Trusting them is almost guaranteed, but it doesn't have to be, sort of. The clients are opensource so you literally clone, audit, and run the clients locally.

Full disclosure, I use Proton and overall trust them so unless I see strong evidence of abuse or lies on their part I'm inclined to post contextualizing comments on stuff like this, b/c well I don't wanna host my own mail server, at least not in prod.