Windows Mail App looses account after updating to windows 10

I am (trying to) administrate my dads computer and he now updated his windows 8 to windows 10. He was using the windows Mail App (the one with the blue square and the white envelope) and after booting windows 10, everything was there except a specific mail account on a private server. The microsoft mail account was automatically migrated.

He was missing some important mails from this private account and since I am more familiar with Thunderbird and the account was coected via IMAP, I set everything up via Thunderbird. The result is, I am missing all mails which where saved in folders via the Mail App.

To cut a long story short, it looks like the mails are still saved as .eml files in

C:Users<user>AppDataLocalPackagesmicrosoft.windowscommunicationsapps_<id>LocalStateLiveCommSOMEWHERE

but when I try to import them with Thunderbird and ImportExportTools, the attachments are all 0 byte.

My question:
Is there a way to import these mails to Thunderbird with working attachments?
Or, as an alteative, import them into the Mail App? I already created the IMAP coection to the account again and I was hoping the mail app will recreate the folder structure but without success.

Thanks in advance for any suggestions!