The ThirdPartyFileProvidersEnabled setting in OWA mailbox policies controls if Exchange Online mailboxes can access services like Drop and Dropbox for attachments. Office 365 tenants need to decide if they want to allow this kind of access. There’s both good and bad in the feature, but it’s easily turned off if you feel the need.
https://office365itpros.com/2019/04/12/owa-thirdpartyfileprovidersenabled-setting/
