Clutter is a new feature of Office 365 Exchange. Disabled by default, Clutter learns from your actions to determine messages you are likely to ignore. Once clutter is enabled, as new, less important messages arrive, they are automatically moved to your Clutter folder. It does this by leveraging Office Graph’s sophisticated machine learning techniques to learn which emails a user is likely to ignore, delivering a personalised user experience.

To enable Clutter, log into OWA via https://mail.office365.com and go to Settings -> Options -> Automatic Processing -> Clutter and choose “Separate items identified as clutter”