What is Clutter?
Clutter can help you filter low-priority emails, saving time for your most important messages. The email server keeps track of the emails you read and the ones you don't. Once turned on, 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.
How to turn Clutter on or off
Microsoft have created a Clutter help page which explains how to turn on or off Clutter from your web based email.