What is the domain "nrb.footprintdns.com"??

The domain nrb.footprintdns.com is owned by Microsoft and represents servers in Microsoft datacenters. It is used for telemetry purposes that comply with Microsoft privacy commitments that you can read about at: https://products.office.com/en-us/business/office-365-trust-center-privacy. Data sent to this domain is used to identify network connectivity and performance issues and to support improvements to the service. We have plans to rename it to be more descriptive, and to publish it at the Office 365 IP Address and URL publishing site at http://aka.ms/o365ip. If you have any problems using Office 365 related to this, please raise a support incident so that we can assist at https://support.office.com/