Microsoft says October Windows updates can break on-premises Exchange email
Original title: Microsoft Confirms October Windows Updates Can Break Exchange Server Email Services
Four October 13 Windows Server updates can stall mail and webmail; Microsoft says do not uninstall them.
Microsoft confirmed that Windows Server security updates released on October 13, 2026, can disrupt some on-premises Exchange Server 2016, 2019, and Subscription Edition systems. Users may be unable to sign in to Outlook on the web, messages can stick in the Outbox, and mail queues or Database Availability Group quorum can fail. The cause is a default limit of 512 simultaneous Endpoint Lookup calls in Windows, not the separate October Exchange patches. Microsoft lists the issue as mitigated, with a registry workaround, and says its investigation is ongoing.
Unconfirmed: Microsoft lists the issue as mitigated, not resolved, and says its investigation remains ongoing.
- Linked updates are KB5126646, KB5126655, KB5126651, and KB5126868.
- Watch for MapiExceptionMdbOffline and Application log Event ID 1006.
- Workaround sets EpmapSavedContextCap to 65536, then requires a restart.
- October 2026 is the last scheduled ESU release for Exchange 2016 and 2019.
Impact: Webmail blocked and messages left undelivered
| Product | Affected |
|---|---|
| Microsoft Exchange Server 2016 | some on-premises systems |
| Microsoft Exchange Server 2019 | some on-premises systems |
| Microsoft Exchange Server Subscription Edition | some on-premises systems |
| Microsoft Windows Server 2025 | KB5126646 |
| Microsoft Windows Server 2022 | KB5126655 |
| Microsoft Windows Server 2019 | KB5126651 |
| Microsoft Windows Server 2016 | KB5126868 |
- 1.Do not uninstall the October 13 Windows security updates.
- 2.Set EpmapSavedContextCap to 65536 under HKLM\SOFTWARE\Microsoft\Rpc and restart.
- 3.Contact Microsoft Support if the published workaround is not enough.
- Vendors
- Microsoft
From the original article
2 min read at cybersecuritynews.com
Microsoft has confirmed that Windows Server security updates released on October 13, 2026, can disrupt Exchange Server email services, blocking webmail access and leaving messages undelivered. The issue affects some on-premises systems running Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition. Microsoft has published a workaround and advises administrators not to uninstall […]
By Guru Baran · cybersecuritynews.com
A short excerpt; the article and its images belong to the publisher. Summary, key facts and assessment above are AI-written from the article and can be wrong.