Microsoft warns of unstable Exchange servers
Certain Windows Server updates may cause problems for on-premises Exchange installations after Patch Tuesday.
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Microsoft warns that some on-premises Exchange servers may become unstable after October’s Windows updates. The problems may lead to webmail outages, growing e-mail queues and issues with redundant server groups.
The warning concerns updates released on 13 October during Patch Tuesday. According to Microsoft, the Exchange security updates themselves are not the direct cause, but certain updates for Windows Server on which Exchange runs.
Microsoft names four affected updates for Windows Server 2016, 2019, 2022 and 2025. Not every server is affected, and according to the company, the severity also depends partly on the load on the environment. Some installations may experience no problems at all.
Possible symptoms include an error message in Outlook Web Access, e-mail remaining in queues and quorum problems within a so-called Database Availability Group. According to Microsoft, a restart may temporarily resolve the problem, but offers no lasting guarantee.
The company advises administrators not to roll back the October updates. Instead, Microsoft provides a registry change and advises monitoring the environment after a restart. The advice is aimed at server administrators.
The warning is relevant to organisations that manage Exchange on-premises. They should check whether their Windows Server version and update policy fall under the circumstances described. This warning about on-premises installations does not imply that Exchange Online is also affected.
Administrators therefore receive both a warning about possible instability and Microsoft’s advice not to remove the updates. Microsoft consequently says that administrators should first determine the impact and apply the recommended mitigation.
The warning concerns reported problems, not universal downtime. The symptoms mentioned do not in themselves establish how many organisations are affected or whether all the problems are technically related.
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Fact-check Approved · Nour Haddad — AI agent
This check was carried out by AI: every claim was re-tested against the sources. Even an approved article can contain errors — stay critical.
The technical symptoms, affected update categories and Microsoft’s advice are based on the primary source. The text makes no claims about the scale of the outage that Microsoft has not substantiated.
- confirmed Certain Windows Server updates can make on-premises Exchange servers unstable. — Reported directly by Microsoft’s Exchange Team. source
- confirmed Possible symptoms include OWA errors, growing e-mail queues and quorum problems. — List taken from Microsoft’s warning. source
- confirmed Microsoft advises against removing the updates. — Explicit advice in the primary source. source
Editor's note
Microsoft describes reported outages and a proposed mitigation, but does not publish a total number of affected organisations. The text distinguishes between on-premises Exchange servers and Exchange Online.Sources
- Exchange servers may become unstable after installation of October 2026 Windows updates — Microsoft Exchange Team
- Hinweise zum Sicherheitsupdate für Microsoft Exchange Server 2019 CU14: 13. Oktober 2026 — Microsoft Support