On the top right corner of the page, go to Settings > Mail > Automatic processing > Clutter.
Uncheck Separate items identified as Clutter, then Save .
You can turn Clutter on anytime by checking separate items identified as Clutter.
NOTE The Clutter folder remains in Outlook after you turn off the Clutter feature. If you don’t want the folder listed under your Mailbox, move the emails from Clutter to other folders, as appropriate, and then right-click the Clutter folder and choose Delete Folder.