Neither the sender nor the recipient made an active decision that the respective mail should be encrypted with pgp.
That’s the issue here.
GPG will never be that simple to use. Personally, I hate GPG with such passion, but I'm fully aware it's the best thing we have for asynchronous messaging.
If it was some other email provider that did the same, I'd 100% agree. But with Protonmail, GPG is pretty much the entire reason we're all aware of their existence.
I just tested it myself, and while there is a visual indicator of that automatic key lookup happening, there's zero way to disable it, nor to even verify which key it picked or to display the key hash.
It seems like a terrible solution halfway between convenience/opportunistic encryption and actual security, combining the downsides of either (a high risk of sending messages the recipient won't be able to decrypt and a moderate risk of encrypting to a key other than the expected one).
Though.. note that WhatsApp doesn't have any documentation on their key discovery either, because WhatsApp is not a federated system. Achieving automatic end-to-end encryption in a federated system, when it wasn't there before, isn't so easy. So there's bound to be some growing pains, but we're still trying :)
I understand Protonmail's approach but I also understand - and share - OP point of view.
You have a sign out saying encrypted email here, and are mad someone finally read it.
Remove the sign, or change it to foo+secrets@mydomain.com
Op is mad he left the sign out saying come in and play when people came in to play way after his party ended.
Except, it is. You can’t control of everyone out only some will send you encrypted email.
But if an email provider purportedly bringing email encryption to the non-GPG-trained masses does, it's a different story.
The presence of a key on a key server is in no way or form an explicit opt-in to encrypted email. That's what the CNAME is for.
This is seriously like telling somebody that you can be contacted on a particular phone number and then getting upset that you're getting phone calls. "Who uses the phone these days," you complain, despite having set into motion the sequence of events that led to you getting a phone call! Worse, in this analogy, it's not like you even said "only text this number"; you explicitly applied metadata saying that you would be open to a phone call.
Telling you to use a particular phone number - say through a business card, a contact page or a profile - is the analogy matching configuration of your domain to publish your intent, similar to how your domain also specify where you mail server is.