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Another reported symptom is growing email queues. Installing the October 2026 Exchange server security updates isn’t requir","workaround":"This issue can be temporarily mitigated by modifying the registry key as described in Exchange servers may become unstable after installation of October 2026 Windows updates . The October 2026 update introduced a default limit on the amount of memory used by Endpoint Mapper when handling Endpoint Lookup calls. For workloads such as Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls might not be sufficient under heavy load. Administrators can use this registry setting to increase the maximum number of simultaneous Endpoint Lookup calls, which increases the amount of memory that Endpoint Mapper can use. 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