Admin: ‘I spent all weekend on my servers’ after Microsoft breaks Exchange Server with October updates

Microsoft has acknowledged that its October 2026 Windows Server updates can disrupt Exchange Server, causing Outlook Web Access connection errors and long email queues. The issue can leave administrators spending hours restoring service, and its impact varies with server workload.

The affected Windows Server versions are 2016, 2019, 2022 and 2025, while Microsoft says the separate October Exchange security updates are not responsible. The problem stems from a default limit of 512 simultaneous Endpoint Mapper lookup calls, which may be too low under heavy Exchange workloads. Rebooting may help temporarily; Microsoft’s current workaround is a registry change to raise the limit, and it has not committed to an emergency update.