If you looked at the folder structure in an Office365 Mailbox (or older Exchange Mailbox) using a MAPI editor so you could see all the Non_IPM_Subtree and hidden folders around 5 years ago it was a relatively simple picture. Today with the rapid pace of the change in Office365 its a little bit of minefield on new folders and hidden data that is stored and used by various applications in your Mailbox. A good case in point is the Files Hidden folder which Tony explains about in https://www.petri.com/mysterious-files-folder . Microsoft Teams is the new kid on the block in terms of collaboration applications that integrate into Mailboxes and other Office365 workloads. As you chat using the Microsoft Teams client as part of it compliance/discoverability process associated with Microsoft Teams a cloud-based process creates conversation items in a Hidden folder in your Office365 Mailbox called TeamChat (this is a Subfolder of the Conversation history). Because its a hidden folder users won
Pushing the Envelope in Messaging and Office 365 Development