Organizations using on-premises Microsoft Exchange Server are experiencing stability issues after the October 2026 Windows Server updates, which Microsoft has confirmed can lead to unreliable Exchange services, especially in environments with heavier workloads. Administrators have reported problems such as connectivity failures with Outlook on the web, disruptions in mail flow, and complications with database availability groups (DAG), all emerging post-update. This bug affects Exchange Servers on Windows Server versions 2016, 2019, 2022, and 2025.
Microsoft has identified that the issue is related to limitations in Endpoint Mapper requests and has recommended a registry-based mitigation to improve capacity and reduce service degradation. The default limit of 512 simultaneous Endpoint Lookup calls may be insufficient under heavy load. Microsoft is investigating the issue and will provide further information. Organizations are advised to maintain critical security updates while implementing additional mitigation measures and closely monitoring affected systems until a long-term solution is found.