User tries to establish a sync relationship with a site from a computer that isn't a member of your organization's domain ...

User tries to establish a sync relationship with a site from a computer that isn't a member of your organization's domain or is a member of a domain that hasn't been added to the list of domains (called the safe recipients list)that can access document libraries in your organization. The sync relationship is not allowed, and the user's computer is blocked from syncing, downloading, or uploading files on a document library.