Proton emails are stored in an encrypted form that goes beyond the simple authentication that is part of the POP/IMAP specifications
Proton does have open-source bridges/proxies, so they aren’t hiding these details from us
Perhaps Thunderbird could be enhanced to support the Proton features directly?
For disappearing messages to work, your conversation partner has to promise they won’t take photos of their screen, and they have to promise to use an app that actually implements the feature instead of just pretending to, and the app developers have to promise to have implemented the code to delete a message when the service says it should
Is there actually a cryptographically-sound and physically-complete method for ensuring that a message is only legible for a temporary duration once it leaves your own device and is delivered to someone elses?