Microsoft Fixes Exchange Sign-Ins Broken by October 2026 Windows Server Updates
Microsoft has resolved Outlook sign-in and mail delivery problems affecting on-premises Exchange Server systems after the installation of the October 2026 Windows Server security updates. The issues impacted systems running Windows Server 2019 and later, causing users to be unable to sign in to Outlook on the web or send emails that remained stuck in the Outbox. Microsoft traced the root cause to a new default memory limit imposed on the Endpoint Mapper when processing Endpoint Lookup calls. The company confirmed that the November 2026 Patch Tuesday updates KB5131080 and KB5131090 fully address the known issue. A temporary registry-based workaround was also provided for administrators unable to deploy the cumulative updates immediately. Microsoft emphasized that the problems were unrelated to the October 2026 Exchange Server security updates themselves.
BleepingComputerNov 11, 2026Translated from English