Microsoft’s October 2026 security updates for Windows Server have caused unexpected issues for Exchange Server administrators, leading to disruptions in email services. The updates, released on October 13, 2026, can interfere with mailbox access, delay outgoing messages, and disrupt database replication in on-premises Exchange environments. Microsoft acknowledged the problem and provided a temporary registry workaround on October 16 while continuing to investigate the root cause.
The affected updates include Windows Server 2025 (KB5126646), Windows Server 2022 (KB5126655), Windows Server 2019 (KB5126651), and Windows Server 2016 (KB5126868). Exchange Server 2016, 2019, and Subscription Edition (SE) installations may experience problems, though not all deployments are impacted. Symptoms range from Outlook on the web sign-in failures to growing mail queues and database replication issues.
Microsoft attributes the problem to a default memory limit introduced in the October updates, which restricts the Windows RPC Endpoint Mapper to 512 simultaneous Endpoint Lookup calls. This limit may be insufficient for high-demand Exchange workloads. The company recommends increasing the limit via a registry command rather than uninstalling the updates, as doing so would remove critical security fixes.
Administrators are advised to apply the workaround by setting the EpmapSavedContextCap registry value to 65,536 and restarting the server. Microsoft reports that this temporary solution has resolved issues in some cases but emphasizes that further monitoring and adjustments may be necessary. The company’s release-health dashboard lists the issue as mitigated as of October 20, 2026, with ongoing investigations.