A friend is still happily using their trusty 2009 MacBook with El Capitan and Apple Mail v9.3 installed. Apple Mail was configured to fetch all their mail from a POP3 account and save them locally. Now Apple Mail has stopped working and immediately crashes everytime it's opened.
Deactivating the POP3 mail account fixes Apple Mail but naturally their old emails aren't displayed any more. The emails are all still there on the disk unter /Library/Mail/V3/POP-"name-of-mail-account" though from where we can open them manually.
Re-adding the POP3-account makes Apple Mail crash again. Adding the POP3-account to Apple Mail under a clean user account works, as soon as I copy over the Library/Mail folder it's back to crashing.
When trying to import the MBOX-folders into Apple Mail I get the error "no valid MBOX folder found".
So, how can I recover the emails (and preferably transfer them into a less ancient mail program such as Thunderbird)? Any help is greatly appreciated!
