There is a new “feature” for Office 365 users called “Clutter” which is turned on by default and may result in your missing some important emails. From the documentation:
“Call it a little less than junk. Call it filler. Call it graymail. In the end, it’s mail you’ve probably opted into and read sparingly.”
The feature works fine so long as users are aware they have a “Clutter” folder and know to either turn it off or check it from time to time.
Here is lots more information about the service and instructions on disabling it if you won’t be using it: