Yes I know how mail encryption works. The point is that mails get encrypted as soon as they enter mailbox’ servers, so mailbox can never know any contents, so they have nothing to hand over to authorities if they come asking. They do have to know the private key used for this though, that is a risk to consider. Of course, if you want fully secure mail, you need both parties agreeing on encrypting/decrypting the mail. Or just don’t use email, use a secure protocol instead.