Office 365 (Apple Mail) - Undeletable Folders Appearing in Apple Mail
This document describes how to handle unrecognized folders in Apple Mail for Office 365 accounts.
Users have reported that folders that they did not create are appearing in their Apple Mail account, these folders may be called "Yammer Root" or "Files". These folders are created by Office 365 and are harmless. There is no way to delete them.
If a folder that you did not create appeared in Apple Mail, a way to check if it was created by Office 365 is to try to delete the folder. If you are presented with Error: Distinguished folders cannot be deleted, the folder is harmless and should not appear either in the web client (Office 365) or on the desktop application (Outlook 2016 for PC or Mac.)