October Windows Server updates break Exchange sign in and mail flow

Microsoft has confirmed that the October security updates for Windows Server cause Outlook Web Access sign-in failures and mail delivery issues, BleepingComputer and CyberSecurityNews reported. Techzine Global and BornCity also reported that the October Windows Server updates lead to instability on affected systems. The problem impacts systems that run Windows Server 2019 and later versions, according to BleepingComputer. [1][2][3][4]

The issue causes users to experience trouble accessing email or logging into Outlook, BleepingComputer reported. In some instances, Outlook Web Access fails completely with a specific error message stating MapiExceptionMdbOffline and referencing error code hr equals 0x80004005, according to BleepingComputer. [1]

Windows Server is Microsoft's server operating system, with Windows Server 2025 released on November 1, 2024. As FTMQ Security reported earlier, Microsoft previously released emergency out-of-band updates to patch a high-severity privilege escalation vulnerability in Exchange Server tracked as CVE-2026-96940. [5][9]

Microsoft acknowledged the update issues in a Windows release health dashboard entry over the weekend, BleepingComputer reported. Administrator analysis and workarounds for the Exchange Server issues have begun circulating, according to BornCity. [1][4]