October Windows Server updates vs on-prem Exchange: one registry value, set it first

Call: Deploy with care. Install the October Windows Server CU: KB5126651 (2019), KB5126868 (2016), KB5126655 (2022) or KB5126646 (2025). It carries an exploited zero-day fix. But set the registry value below before you do.

What breaks: Microsoft has confirmed this, on release health and on the Exchange team blog. The update limits Endpoint Mapper lookups. On a busy server, the default 512 RPC contexts fill up in about a day, and then:

OWA throws MapiExceptionMdbOffline (ec=1142) and logs Event 1006.
Queues fill with 432 4.3.2 MapiExceptionMdbOffline.
Outlook clients keep mail stuck in Outbox.
Replication and DAG quorum get unhappy.

A reboot clears it until the contexts fill up again. Some servers never see it at all. The October Exchange SU isn’t needed to trigger it.

Fix:

reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f

Restart and monitor. Raise the value if it comes back. Microsoft: don’t uninstall the update, and no fix release is imminent.

This is also the real story behind “Patch Tuesday broke Outlook.” It’s the server, not the client.

Sources: Exchange team blog, Microsoft Exchange article, release health, r/sysadmin thread, October megathread.