Microsoft has added a new feature that helps filter your low-priority email. It's called Clutter. Clutter looks at what you've done in the past to determine the messages you're most likely to ignore. It then puts them in the Clutter folder. Clutter will learn which messages aren't important to you.
From time to time, messages maybe incorrectly identified as clutter, and will move the messages to your Clutter folder. You can turn off the Clutter feature.
To Turn Off Clutter
Click the Settings button, and under My app settings, click Mail
Next, under the Mail | Automatic processing options, click Clutter
Uncheck the option Separate items identified as clutter NOTE: The second option Send me notifications about messages that are separated as clutter will automatically uncheck and disappear.