Microsoft provides a page resource where the company manages the URLs and IP addresses that are use to service Office 365 to its customers.
Office 365 URLs and IP address ranges
However, Microsoft will be phasing out this resource by October 2, 2018.
After that date, a new web service will be used that publishes Office 365 endpoints to make it easier for customers to evaluate, configure, and stay up to date with changes.
See: Microsoft Phasing Out Office 365 URLs and IP Address Ranges Resource on October 2, 2018
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