Microsoft: October Windows Server updates break Exchange sign-in and mail flow

Microsoft confirmed that October 2026 Windows Server security updates cause OWA sign-in and mail delivery failures on some on-premises Exchange Server systems. The issue stems from a new 512-call default limit on Endpoint Mapper memory; Microsoft is working on a fix and offers a registry workaround.

• Issues affect Exchange on Windows Server 2019 and later
• Errors include MapiExceptionMdbOffline and Event ID 1006
• Endpoint Mapper capped at 512 simultaneous lookup calls
• Workaround: edit a registry key and reboot the server