Microsoft says its October 2026 Windows Server updates can make some Exchange servers unstable. The problem is separate from the month\u2019s Exchange security updates and may affect Exchange Server 2016, 2019, and Subscription Edition running on Windows Server 2016, 2019, 2022, or 2025.<\/p>\n
Symptoms can include Outlook on the web connection failures and growing email queues that slow or stop delivery. The fault may emerge over time, and its severity depends on server workload. Microsoft says some servers may not be affected at all.<\/p>\n
The issue appears to stem from a Windows default that limits Endpoint Mapper to 512 simultaneous Endpoint Lookup calls. Microsoft says that limit may be too low for heavily loaded Exchange workloads.<\/p>\n
Restarting a server may provide temporary relief. Microsoft\u2019s current workaround is to increase the limit using a registry setting. The company advises customers not to uninstall the October Windows Server updates and says it is investigating.<\/p>\n
Some administrators reported repeated failures after installing the updates, with one saying they had spent the weekend working on affected servers. Microsoft has not announced an out-of-band fix.<\/p>\n