Outlook on the Web Failing or Mail Stuck in the Outbox? A Symptom Checker for Exchange Servers After October’s Windows Updates

If your company still runs its own Exchange server and people started complaining about email after Patch Tuesday, the server may not be broken in the usual sense. Microsoft now says a change inside this month’s Windows Server security updates can starve busy Exchange servers of a resource they depend on, and it has published a registry tweak to get them working again while a permanent fix is prepared.

BleepingComputer reported Microsoft’s confirmation, and the details below also draw on the Exchange team’s own write-up.

Check the symptoms

Users cannot get into Outlook on the web (OWA)

Sign-ins fail, and in some cases OWA shows a MapiExceptionMdbOffline error (hr=0x80004005, ec=1142) while Event ID 1006 appears in the server’s Application log.

Mail sits in the Outbox or queues keep growing

Messages sent from Outlook may never leave the Outbox, and Microsoft describes mail queues swelling as the day goes on until delivery slows to a crawl.

Database availability group trouble

Organizations running a DAG have reported members that cannot reach quorum, and mailbox access can falter during mailbox replication.

A reboot helps, but only for a while

Restarting the server can make the problem disappear temporarily, which is a clue that a resource is being exhausted over time.

The cause in one paragraph

The October update put a new default ceiling on how much memory the Windows Endpoint Mapper may use when answering Endpoint Lookup calls, capped at 512 simultaneous calls. Exchange makes heavy use of those lookups, so a loaded server can hit the ceiling. Microsoft lists the Windows updates involved as KB5126646 (Windows Server 2025), KB5126655 (2022), KB5126651 (2019) and KB5126868 (2016). Exchange Server 2016, 2019 and Subscription Edition can all be affected. The worse the load, the sooner trouble appears, and some servers may never notice.

The workaround Microsoft recommends

From an elevated Command Prompt on each affected Exchange server, Microsoft says to raise the Endpoint Mapper limit with this command and then restart the server:

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

After the restart, keep watching OWA, the mail queues and the Application log. If the symptoms return, Microsoft says to raise the value further. It reports that this has resolved the customer cases it has worked on so far.

What not to do

Do not uninstall the October Windows Server updates. Microsoft is explicit about this; they carry security fixes, and the registry change addresses the side effect.
Do not blame the Exchange security update. Microsoft says this month’s Exchange Server security updates are not the cause. The problem comes from Windows, so any Exchange version on an affected Windows build can be hit.
Do not assume one quiet server means all is well. Because the impact depends on load, your busiest server may show problems days before the others.

What this means for your company

Start by finding out whether you run Exchange on your own hardware at all. Many small and mid-size firms moved to hosted email years ago, but a surprising number still keep an on-premises server for a line-of-business app, a hybrid setup or simply because no one retired it. If you do, have whoever manages it confirm which of the updates above are installed, apply Microsoft’s workaround in a planned window rather than in a panic, and document the change so it can be revisited when the permanent fix ships.

This is also a reminder about the calendar. BleepingComputer notes that Microsoft said it will stop shipping extended security updates for Exchange 2016 and 2019 this month. A server that needs a registry tweak to stay up today and will soon stop receiving security fixes deserves a migration plan, whether that means a supported Exchange version or a move to hosted mail.

Need help checking your Exchange servers after this month’s updates, or planning the move off an aging version? Reach out to Layer 7 Data Solutions to discuss how we’ve helped companies similar to yours.