October Windows Server Updates Disrupt Exchange Sign-In and Mail Flow

Microsoft says the October 2026 Windows security updates are causing email access and delivery problems on some on-premises Exchange Server systems running Windows Server 2019 or later.

In a Windows release health dashboard update, Microsoft said users may be unable to sign in to Outlook on the web (OWA), while messages sent from Outlook may remain in the Outbox. Some systems have also reportedly shown a “MapiExceptionMdbOffline” error and Event ID 1006 in the application log. Microsoft says Exchange may also have trouble accessing mailboxes during mailbox replication.

Microsoft attributed the issue to a default limit of 512 simultaneous Endpoint Lookup calls handled by Endpoint Mapper. Under heavy load, that limit may be insufficient for workloads such as Exchange Server. The company said impact varies with server load, and some servers may not experience problems.

Microsoft said the issue is not related to the October 2026 Exchange Server security updates and advised customers not to uninstall them. It is working on a fix and provided a temporary registry-based workaround for affected systems, followed by a reboot. After restarting, administrators should monitor the Exchange environment and increase the Endpoint Mapper memory limit if the problem returns.