Clutter is a Microsoft Outlook feature that attempts to help you filter low-priority email, saving time for your most important messages. The email server keeps track of the email you read and the ones you don’t. Clutter is automatic. As new email comes in, it takes messages you’re most likely to ignore and puts them into the “Clutter” folder. The more you use it, the better it gets. And if you find Clutter isn’t for you, you can turn it off.
It’s important to either closely watch your Clutter folder while it learns your preferences, or disable Clutter, to avoid missing important emails. Information on how to monitor and train Clutter, or how to disable Clutter is available at https://support.csuchico.edu/TDClient/KB/ArticleDet?ID=11701.