I agree 100% with your ideia. The best path for this would’ve been for them to actually design that system you describe and THEN implement it on Dovecot and Postfix in their own fork or a Dovecot extension / Postfix add-on so others would start using them. Eventually after some times and other providers also optionally supporting the thing an RFC could be written. This is the usual course we see with protocols/extensions and is what should’ve happened here.
I want to share another thing, before Snowden there was Lavabit, they also did “encryption at rest” and the user password involved for some parts of the information and it was proven to be effective. It wasn’t a perfect model but it was certainly better than the havoc Proton did to e-mail by opening the precedent that is okay not to run on standard protocols.
What Proton is doing to e-mail is about the same that WhatsApp, Messenger and others did to messaging - instead of just using an open protocol like XMPP they opted for their closed thing in order to lock people into their apps. People in this community seem to be okay with this just because they sell the “privacy” cool-aid.
server-to-server communication, like for automatic pgp key negotiation, would be nice too.
I’m not sure if this is required. Any decent e-mail server uses TLS to communicate these days, so everything in transit is already encrypted.
Still, Proton has a easy to access data export that doesn’t require a bridge client or subscription or anything. I think that’s required by GDPR.
Yes, they have it because GDPR does require it. It works, but it’s not a real time sync alternative to anything and it is some kind of vendor lock-in.
As I said in other comments, not using standard protocols only makes thing worse. I used iOS as an example, for Android you can get a bridge but that’s just going to be one more thing going for your battery.
Now, consider this, there’s a TON of situation where having a standard SMTP-capable provider is interesting. Maybe you’re running in iOS, maybe you want to have an ESP32 to send a few emails, or some custom software in your computer. All those use cases are impossible or require more coding and more non-standard solutions just because Proton decided to be the first provider ever not to use standard protocols.