E-Mail Clutter. This is a new feature in Office 365.
Clutter moves your low priority messages out of your Inbox so you can easily scan for important messages. Clutter analyzes your email habits, and based on your past behavior, it determines the messages that you’re most likely to ignore. It then moves those messages to a folder called Clutter, where you can review them later.
This is different that junk mail in that you probably never want to review junk unless you are looking for something that might be there by mistake. Clutter is more about the messages that you might want to read later. This can include group messages like newsletters, bulletins, and ads from companies you do business with. It tries to learn from your habits and it improves with time.
If you don’t want to use it, it is easy to turn off.
Log into Office365 on the web: http://portal.office365.com
Click to enter your mail account.
Right click on the Clutter folder and look for settings
Then turn it off in settings