Comment by flupe
3 years ago
The repository for Proton Mail Bridge (which is open source) claims to also host the source for the Import Export app.
Here: https://github.com/ProtonMail/proton-bridge
Briefly looking at the files and code it's hard to tell whether that is still the case, but it's fair to assume Import-Export would reuse most of the machinery behind Bridge.
bridge exposes imap making it easy to download all your data using Thunderbird or another client. I don't know about "export" because imap does what I want and is supported by most providers in the same format.