Clutter is an Office 365 email filtering feature designed to move low priority messages out of your Inbox and into their own folder titled Clutter. This feature will learn your email reading habits to determine the messages you're most likely to ignore and then moves those messages to the Clutter folder. Most of the mail going into the folder will probably be bulk mail (advertisements) and messages from mailing lists. However, you will want to periodically check the Clutter folder as it may move legitimate email into this folder. You will also want to delete emails from the Clutter folder once in a while, as these emails do count against your storage space.
You can improve Clutter's accuracy in filtering your messages by training it. Drag any messages you deem to be "clutter" from you Inbox into the Clutter folder. Drag any message that is important to you out of the Clutter folder back into your Inbox. The more you utilize the Clutter feature, the more accurate it will be at identifying low priority items in your Inbox.
Or you can just turn off this feature if you do not want your email filtered. Scroll down for instructions.
For more information on using Clutter, please visit these two Microsoft web pages:
If you would prefer that Clutter not filter your email you can turn it off. Here are the steps: