You may have noticed a folder in your Outlook email called Clutter. Clutter is a feature designed to move “low priority” mail out of your Inbox and into its own folder. The idea is that it saves you time when scanning through your mail. Clutter sorts your email based on how you’ve handled emails in the past and then continues to learn as from you as you sort your email. For example, if you drag an email from Clutter to your Inbox it will learn not to identify emails from that sender as Clutter.
You can learn more about Cutter here. Please note that the page says it’s for Outlook 2016 for Windows but it’s almost exactly the same as Outlook 2016 for Mac.