[gausswho]

I haven't tried myself but is this insufficiently proper? https://proton.me/support/proton-mail-export-tool

[Avamander]

Last I checked that tool reordered all the headers (which destroys a lot of forensic value amongst other issues) and neither should such a tool be the only way to get (supposedly) good exports.

Both the IMAP bridge and web interface should provide original unmodified emails upon request.