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/prot

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 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.