Two simple principles are key for Office 365 network connectivity. Keep things simple and get user data to a Microsoft edge node as quickly as possible. However, some organizations insist on using VPNs and proxy servers to interfere with Office 365 network traffic, which slows things down and makes users unhappy. It’s strange that people haven’t gotten the message after so many years…
https://www.petri.com/network-principles-office-365-connectivity
