With Office 365, Microsoft has added a folder called ‘Clutter’. This folder needs some explanation to reduce the confusion. Please read the linked article that explains how clutter works, how to turn it off if you don’t want to use it and how to make it work better for you. The clutter folder can be a great tool, just make sure you check daily to see what emails are there and move them back to the inbox if these are emails you need and delete the ones you don’t on a regular basis.
To find out more about clutter please click on the reference from the official Microsoft support link.