Clutter is a feature of Office 365 email that analyzes your emails, and based on your past behavior, determines the messages that you’re most likely to ignore. It then moves those messages to a folder called Clutter so that you can review them later.
Clutter is a great way to automatically filter less frequently read emails from your inbox, however if an email lands in clutter that you expected should have arrived in your inbox, you can simply drag the email back into your inbox.
Microsoft has now turned Clutter on by default. To turn it off, first log in into your email account (the settings for Clutter are in the Outlook Web App).
In the top right corner of the page, go to:
Settings > Options > Mail > Automatic processing > Clutter.
To activate clutter management, choose Separate items identified as Clutter, then Save. You can turn Clutter off anytime by selecting Don't separate items identified as Clutter.
Clutter and junk email are both filtered out before they reach your Inbox. Junk email is evaluated and filtered first, then Outlook processes rules (if you have any). Next, Clutter analyzes the remaining messages and filters out the types of messages that you usually ignore or don't respond to, based on your past behavior.
Please 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.
For more information about how Clutter works please refer to this article.