I lost even more emails, had like 25 folders in my inbox. When emptying the trashcan, all those folders, including the emails, was deleted. And of course they were deleted from the mail server as well. Luckily my gaming-PCs were offline, so I could make a fresh backup and restore everything, via Outlook on Windows. After speaking with Apple Support we tried again. Made a fresh account in the Mac and set the mail up again. Same story. What's remarkable is that Outlook works flawless on the Mac, but not Apples Mail-application.