I think this might explain a bit of my problem: https://forum.duplicati.com/t/backup-does-not-contain-ost-files/1381/2
So it seems like a solution would be to remove the registry key to allow VSS to “backup” .ost files. (And since .ost files are a bit difficult you need another tool to convert it to .pst)
Maybe this was also the issue for the thread starter, that his file was excluded by VSS in HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\BackupRestore\FilesNotToSnapshot
For me I will use another folder/mailbox in outlook that is using a .pst file so I can move the important email to that folder/mailbox. Cause this was a bit painful.