Microsoft has confirmed that the October Windows Server security updates can disrupt on-premises Exchange email servers, leaving staff unable to sign in to Outlook on the web and email stuck in the outbox.

• What it is: The updates cap the number of simultaneous connections Windows allows, and busy Exchange servers can exceed that cap.
• Who is affected: Businesses running their own Exchange Server 2016, 2019 or Subscription Edition on Windows Server 2016, 2019, 2022 or 2025 with the 13 October updates installed. Exchange Online (Microsoft 365) is not affected.
• Symptoms: Web mail sign-in failures, growing mail queues, and database copies failing to stay in sync. A restart may help temporarily.
• What to do: Microsoft has published a registry workaround that raises the limit, followed by a server restart. Do not uninstall the updates. Microsoft lists the issue as mitigated, not yet fixed.

If you run Exchange on your own servers and have seen mail problems since 13 October, contact us and we will apply the workaround. Source: Cyber Security News.