Defending Exchange servers under attack

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Securing Exchange servers is one of the most important things defenders can do to limit organizational exposure to attacks. Any threat or vulnerability impacting Exchange servers should be treated with the highest priority because these servers contain critical business data, as well as highly privileged accounts that attackers attempt to compromise to gain admin rights to the server and, consequently, complete control of the network.

https://www.microsoft.com/security/blog/...er-attack/