Using Classic Outlook on Windows for personal email, calendar, and contact management
No offence, but if 29 people have the same problem, on top of the companies of some of the commenters, the correct response is that "we don't know what the problem is and we are working to resolve it".
We know we can reinstall the software, format the drive, change computers, or use other programs. But we don't want to do that...
Just to add, that for me, it is doing it only for specific senders so far.