After installing the October 2026 Windows Server updates, on-premises Exchange servers may show OWA errors, growing mail queues and DAG problems. Microsoft has confirmed the issue and published a workaround that does not require removing the update. Here are the symptoms, the affected updates and a step-by-step fix.
After installing the October 2026 Windows Server updates, on-premises Exchange servers may show OWA errors, growing mail queues and DAG problems. Microsoft has confirmed the issue and published a workaround that does not require removing the update. Here are the symptoms, the affected updates and a step-by-step fix.
Exchange down? Get immediate help from Doğa Network. Our experts can review your server remotely, apply the workaround safely and get mail flowing again. +90 850 888 3642 · hi@doga.network
At a glance
- Problem: Exchange servers may become unstable after the October 2026 Windows Server updates.
- Affected updates: KB5126646 (Server 2025), KB5126655 (Server 2022), KB5126651 (Server 2019), KB5126868 (Server 2016).
- Who can be affected: According to Microsoft, any Exchange Server version running on these Windows Server versions.
- Fix: Do not uninstall the update. Add one registry value and restart the server.
- Status: Microsoft is still investigating; no permanent fix has been released yet.
Symptoms
According to Microsoft's support article, affected servers show:
- Outlook on the web (OWA) fails with:
MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142) - Mail queues grow throughout the day, delivery is delayed and eventually mail flow slows down or stops.
- Database Availability Group (DAG) members can't communicate or establish quorum.
The impact depends on server workload. The issue progresses faster on some servers than others, and some servers may not show any symptoms. Monitor your queues for a few days before concluding you are not affected.
Microsoft's workaround
- Do not uninstall the October 2026 Windows Server updates.
- On each affected Exchange server, run this in an elevated Command Prompt:
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f- Restart the server.
- After the restart, monitor the environment. If the issue recurs, Microsoft advises increasing the limit.
In DAG environments we recommend restarting servers one at a time using maintenance mode to avoid outages during database switchovers.
Why not uninstall the update?
The October updates fix many critical vulnerabilities. Removing them re-exposes the server, which for an internet-facing system like Exchange is a far bigger risk than a mail outage. See our Exchange CVE-2026-70571 advisory for details.
A reminder for Exchange 2016/2019 users
The paid ESU program for Exchange 2016 and 2019 ends with October 2026. Your options are summarized in our Exchange 2016/2019 ESU post.
How can Doğa Network help?
- If you see OWA errors, growing queues or DAG issues, we apply the workaround with same-day remote support and restore mail flow.
- We plan rolling, outage-free restarts for DAG environments.
- If you are not affected yet, we set up queue and system monitoring for early warning.
- Longer term, we plan your move to Exchange SE or Microsoft 365.
Contact us now: +90 850 888 3642 · hi@doga.network
Sources
More articles
October 2026 Patch Tuesday summary: 3 zero-days, 9.8-rated server flaws
Microsoft's 13 October 2026 Patch Tuesday updates fixed roughly 1,400 flaws. An exploited Windows Shell zero-day, 9.8-rated RCEs in DHCP/MSMQ/QUIC, Outlook preview-pane flaws and a final Exchange update, summarized with an SMB action plan.
Read more →
Last month for Exchange 2016/2019: ESU program ends with October 2026
Exchange Server 2016 and 2019 reached end of support on 14 October 2025. The paid Extended Security Update (ESU) program now closes at the end of October 2026, and Microsoft has said there will be no further extension. For servers on ESU, this is the last month of updates. Here is what changes, the risks and your options.
Read more →
13 October 2026: Last patch for Windows Server 2012/R2, Windows 10 ESU year one ends
Patch Tuesday on 13 October 2026 will be the last Extended Security Updates (ESU) release for Windows Server 2012 and 2012 R2. On the same day, year one of the commercial ESU programme for Windows 10 ends, and Windows Server 2022 moves from mainstream to extended support. Here are the dates, the costs and an action plan for SMBs.
Read more →