Microsoft fixes Exchange sign-ins broken by October Windows Server updates
Original title: Microsoft fixes Exchange sign-ins broken by Windows Server updates
November Patch Tuesday updates KB5131080 and KB5131090 lift an Endpoint Mapper memory cap on Server 2019 and later.
Microsoft says October 2026 Windows Server security updates broke Outlook sign-in and mail delivery on some on-premises Exchange servers running Windows Server 2019 and later. A new default memory limit on Endpoint Mapper Endpoint Lookup calls can block Outlook on the web, leave messages in the Outbox, and disrupt mailbox replication, with impact varying by server load. Microsoft says the problem is not caused by the October Exchange security updates. The November 2026 Patch Tuesday updates KB5131080 and KB5131090 fix it; a registry change is offered as a temporary workaround.
- Microsoft acknowledged the known issue in mid-October.
- Not every Exchange server is affected equally, and some may never fail.
- Workaround sets EpmapSavedContextCap to 65536, then requires a reboot.
- Admins should watch the environment and raise the limit if the fault returns.
Impact: Users cannot sign in to OWA or send mail
| Product | Affected | Fixed in |
|---|---|---|
| Microsoft Windows Server | Windows Server 2019 and later after the October 2026 updates | KB5131080 and KB5131090 |
| Microsoft Exchange Server | some on-premises servers after those Windows updates | — |
- 1.Install the November 2026 updates KB5131080 and KB5131090 on affected systems.
- 2.If updates cannot be deployed, set EpmapSavedContextCap to 65536 and reboot.
- Vendors
- Microsoft
From the original article
2 min read at bleepingcomputer.com
Microsoft has fixed Outlook sign-in and mail delivery issues on Exchange Server systems after installing the October 2026 Windows Server security updates. [...]
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.