Microsoft announced on September 4, 2026, that it will begin blocking mail from Exchange Server 2016 and 2019 installations that do not meet the October 2025 baseline security update. The move forces organizations to upgrade or lose cloud mail connectivity.
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2016 and 2019 installations must meet the October 2025 baseline to reach cloud-hosted inboxes
Microsoft to bounce mail from outdated Exchange servers https://www. theregister.com/on-prem/2026/0 9/04/microsoft-to-bounce-mail-from-outdated-exchange-servers/5294506 Diabolical # microsoft tactics.