What’s happening
Microsoft has documented a known issue affecting organizations that run on-premises Exchange Server alongside Windows Server. After installing the October 2026 Windows security updates, some Exchange servers have experienced problems with Outlook on the web (OWA) sign-in and with email delivery from Outlook. This is relevant to SMB administrators who still operate on-premises Exchange infrastructure, even if their broader email environment is otherwise built around Microsoft 365.
The issue is published on Microsoft’s official Windows Server release health pages for Windows Server 2025 and Windows Server 2022, under the “known issues” sections for those products. These pages are Microsoft’s standard mechanism for tracking post-update problems, their affected platforms, and available workarounds.
Which update triggered the issue
The problem is tied to the October 2026 Windows security updates:
- Windows Server 2025: OS Build 26100.33650, released as KB5126646 on 2026-10-13
- Windows Server 2022: OS Build 20348.5761, released as KB5126655 on 2026-10-13
Microsoft states that installing the separate October 2026 Exchange Server security updates is not required for this issue to occur — the problem originates from the Windows update itself, not from an Exchange-side patch.
Affected versions
According to the Windows Server release health documentation:
Exchange Server versions reported as affected:
- Exchange Server 2016
- Exchange Server 2019
- Exchange Server Subscription Edition
Windows Server platforms listed as affected:
- Windows Server 2025
- Windows Server 2022
- Windows Server 2019
- Windows Server 2016
Microsoft’s documentation notes that no Windows client platforms are affected — this is an on-premises server-side issue only. The published pages do not list specific Exchange cumulative update (CU) build numbers, so administrators should rely on the behaviour described below rather than build numbers to confirm whether they’re affected.
Documented symptoms
Microsoft describes the following symptoms occurring after the affected update is installed:
- Users may be unable to sign in to Outlook on the web (OWA)
- Emails sent from Outlook may remain stuck in the Outbox and fail to be delivered
- Exchange may have issues accessing mailboxes during mailbox replication
- Email queues may grow without messages being processed
- In some configurations, OWA may fail with the error: MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142), logged as EventID 1006 in the application log
If your organization runs on-premises Exchange and users report sign-in failures to OWA or messages piling up in their Outbox after applying recent Windows updates, these documented symptoms are a reasonable starting point for diagnosis — though a full confirmation should still include checking the specific update installed on the server and reviewing application log entries.
Root cause as documented by Microsoft
Microsoft explains that the October 2026 Windows update introduced a default limit on the amount of memory used by Endpoint Mapper when handling Endpoint Lookup calls. Endpoint Mapper is a Windows component involved in resolving network service endpoints, and workloads like Exchange Server can generate a high volume of these lookup calls.
The default limit introduced by the update allows 512 simultaneous Endpoint Lookup calls. Microsoft notes that this default may not be sufficient for Exchange Server under heavy load, which is consistent with the reported symptoms of stalled mail delivery and growing queues.
Workaround and current status
As of the information published by Microsoft:
- Status: Mitigated (not fully resolved) as of 2026-10-16
- Microsoft describes a registry-based workaround that increases the maximum number of simultaneous Endpoint Lookup calls Endpoint Mapper can process, which in turn increases the memory available to it. This workaround is detailed in a separate Microsoft Exchange Team blog post referenced from the Windows Server release health pages.
- Microsoft Support offers an additional mitigation for affected devices. Microsoft states that organizations needing this should contact Microsoft Support for business directly.
- Microsoft indicates it is still investigating the issue and will update its documentation as more information becomes available. No permanent fix or rollback has been published in the supplied release health documentation at the time of writing.
Because the registry workaround is a configuration change made directly on affected servers, administrators should treat it as a documented Microsoft mitigation rather than a long-term fix, since Microsoft itself has not characterized it as a final resolution.
Practical recommendations for SMB administrators
The following guidance is practical advice based on the documented issue, not an official Microsoft requirement:
- Check your Exchange and Windows Server versions against the list above before assuming you’re affected — this issue is specific to on-premises Exchange Server 2016, 2019, and Subscription Edition running on the listed Windows Server platforms.
- Monitor for the documented symptoms — OWA sign-in failures, stuck Outbox messages, growing queues, and the specific MapiExceptionMdbOffline error with EventID 1006 — rather than assuming any Exchange slowdown is related to this issue.
- Review the Microsoft Exchange Team blog post referenced in the release health page before applying the registry workaround, since registry changes on production mail servers carry risk if applied incorrectly.
- Contact Microsoft Support for business if your organization needs the additional mitigation Microsoft has made available, particularly if the registry workaround doesn’t fully resolve the symptoms in your environment.
- Hold off on assuming a permanent fix exists. Since Microsoft has only marked this issue as “Mitigated” and states it is still investigating, continue monitoring the Windows Server 2025 and Windows Server 2022 known issues pages for updates.
- Keep Exchange server patching current, but be aware that Microsoft has explicitly stated that installing the October 2026 Exchange security updates does not prevent or resolve this particular issue — it is driven by the Windows update, not the Exchange update.
Conclusion
This issue is a reminder that Windows Server updates can affect on-premises workloads like Exchange Server in ways that aren’t always obvious at first glance, since the trigger here is an infrastructure-level change to Endpoint Mapper rather than anything specific to Exchange’s own code. For SMB administrators still running on-premises Exchange, the practical priority is to confirm whether your environment matches the affected Exchange and Windows Server versions, watch for the documented symptoms, and apply Microsoft’s registry workaround or contact Microsoft Support for business if needed. Because Microsoft has only mitigated — not resolved — this issue, it’s worth checking the official Windows Server release health pages periodically for updates before considering the matter closed. As with all operational changes affecting production mail infrastructure, test any workaround in a non-production environment where possible before applying it broadly.