Windows Server security updates disrupt some Exchange email systems

Microsoft confirmed that October 2026 Windows Server security updates can disrupt sign-in and email delivery on some on-premises Exchange systems. Users may be unable to access Outlook on the web, messages may remain in Outlook's Outbox, and mailbox replication may encounter access problems. The updates introduced a default limit of 512 simultaneous Endpoint Lookup calls, which Microsoft said may be insufficient for Exchange under heavy load. While developing a fix, Microsoft provided a registry workaround to increase the limit, followed by a reboot. Impact varies with server load, and some servers may never encounter the problem. Microsoft distinguished the issue from October's Exchange Server security updates.