Microsoft has recently released a new email management service called "Clutter" which is designed to limit the number of messages delivered to your inbox that you are likely to ignore. The built-in algorithms learn from your habits and move messages of low priority to a folder called "Clutter" in Office 365.
Troubleshooting:
If you would prefer to manage your emails yourself, Clutter can be turned off by following these steps:
Clutter will no longer be moved to the Clutter folder, however those items that had already been moved will remain in that location until you move them elsewhere.