Microsoft confirmed that October’s Windows Server security updates can break on-premises Exchange on Windows Server 2019 and later: OWA sign-in fails, mail sits in the Outbox, and mailbox moves can stall.
The cause
The October update added a default cap on Endpoint Mapper memory, 512 simultaneous Endpoint Lookup calls. That can be too low for Exchange under heavy load. Symptoms include “MapiExceptionMdbOffline … ec=1142” and Event ID 1006 in the application log. How bad it gets depends on load; some servers never see it.
What to do
- Don’t uninstall the October Exchange security updates. Microsoft says they’re not the cause.
- Apply Microsoft’s documented registry workaround to raise the Endpoint Lookup limit, reboot, and raise it further if the problem returns.
- Also note: extended security updates for Exchange 2016 and 2019 end this month.
My take
A sensible hardening default that breaks a heavy workload is the classic patching tradeoff. Patch, monitor and keep the workaround handy.
