October Windows Server updates break Exchange sign-in and mail flow
Original title: Microsoft: October Windows Server updates break Exchange sign-in, mail flow
A new 512-call Endpoint Mapper limit can stall OWA and delivery; Microsoft offers a registry workaround, not an uninstall.
Microsoft confirmed that the October 2026 Windows security updates can disrupt on-premises Exchange on Windows Server 2019 and later. Users may be unable to sign in to Outlook on the web, outbound messages can remain in the Outbox, and mailbox replication can fail, including MapiExceptionMdbOffline errors and Event ID 1006. The update limits Endpoint Mapper to 512 simultaneous Endpoint Lookup calls. Microsoft says this is unrelated to the October 2026 Exchange security updates and is preparing a fix.
Unconfirmed: Microsoft has not named a fixed Windows update; it says impact depends on Exchange load.
- Any Exchange version running on the impacted Windows releases can be affected.
- Impact varies with server load, and some servers may never show the problem.
- The workaround sets EpmapSavedContextCap to 65536, then requires a reboot.
- Microsoft says customers should not uninstall the October 2026 Exchange updates.
- Extended security updates for Exchange 2016 and 2019 stop this month.
Impact: OWA sign-in, mail delivery, and mailbox replication fail
| Product | Affected |
|---|---|
| Microsoft Windows Server | 2019 and later |
| Microsoft Exchange Server | any version on impacted Windows |
- 1.Set EpmapSavedContextCap to 65536 under HKLM\SOFTWARE\Microsoft\Rpc and reboot.
- 2.Do not uninstall the October 2026 Exchange Server security updates.
- 3.If failures return, raise the Endpoint Mapper limit and continue monitoring.
- Vendors
- Microsoft
From the original article
2 min read at bleepingcomputer.com
Microsoft has confirmed that this month's Windows Server security updates are causing OWA sign-in and mail delivery issues on some Exchange Server systems. [...]
By Sergiu Gatlan · bleepingcomputer.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.