Clutter is Microsoft's new mail filter introduced in 2016
As new email comes in, it takes messages you're most likely to ignore and puts them into the "Clutter" folder.
To disable this feature, login to https://login.microsoftonline.com
At the top left corner of the page, select the App launcher icon, and then select Mail.
Click the Gear icon and Select Options
From the Options list select Clutter
Uncheck "Separate items identified as clutter" then click Save