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).
Yes, that’s precisely the problem there. You can use PGP with any generic IMAP provider and that will work just fine with handheld devices. There are multiple mail clientes capable of doing and all your mail is still encrypted on the server. Proton just made an alternative implementation that forces you into proprietary systems because it’s more convenient for them.
Those kinds of setups with servers encrypting your mail and still delivering over IMAP are fairly easy to implement, here’s an example. They simply decided to go all proprietary.
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 e
On a generic mail system SMTP is used in two places: 1) from your mail client to your provider and 2) between your provider and other providers. Proton is NOT using SMPT for the first step, making it non-standard and much more closed.
Some people can’t because they need updated proofing tools and that version no longer has updates.