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

At the top left corner of the page, select the App launcher icon, and then select Mail.

Outlook on the web App Launcher

Click the Gear icon and Select Options

From the Options list select Clutter

Uncheck "Separate items identified as clutter" then click Save