Any generic IMAP/SMPT provider + Thunderbird + PGP will provide the same level of security that Proton does - that is assuming they didn’t mess their client-side encryption/decryption or key storage in some way.
And isn’t that the point? I don’t have time nor do I want to learn about PGP and how to encrypt email. Someone sells that service, great. And it is not like I cannot send normal emails to anyone else. They are using the same standard, not some made up version of SMTP (when sending to other servers, I assume any email from client A to client B both being Proton customer never leave their server, so no need for a new protocol).
Proton is doing to e-mail about the same that WhatsApp and Messenger 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
Proton themself provides a way to export emails in a decrypted format. It is even cross platform. https://proton.me/support/proton-mail-export-tool And all they do is open source, here is the code for their mail server: https://proton.me/support/proton-mail-export-tool. They seem to be using ordinary standards, but what do I know?
I cannot agree with you and I do not think your arguments holds, I would even go as far as to say that they are flawed (example being claiming “closed thing” while being fully open source using open standards). It seems to me that they have something that people are willing to pay money for. You are not one of them (nor am I).
I don’t personally use them as an email provider because of the limit on how many domains they allow as a standard.
https://proton.me/support/android
https://proton.me/support/ios-iphone
They do lock you in on handheld devices but that seems to be a consequence of the fact that they are storing all emails encrypted on the server. After reading this link (“[…]Since IMAP can’t decrypt your emails[…]”), I agree that they are just implementing PGP with an extra steps and creating an unneeded layer (the bridge).
The reason I would not compare it to XMPP is because they are still using SMTP. It is when they stop using SMTP or force others to use something else that I would be very worried.