Microsoft: October Windows Server updates break Exchange sign-in, mail flow

Microsoft has confirmed that Windows Server security updates released this month are disrupting email access and delivery on some on-premises Exchange Server systems. In a weekend update to its Windows release health dashboard, the company said users on Windows Server 2019 and later may be unable to sign in to Outlook on the web or may see messages sent from Outlook sit in the Outbox without being delivered. Exchange may also run into trouble accessing mailboxes during mailbox replication.

Some affected customers are seeing OWA fail with a MapiExceptionMdbOffline error stating the server connection could not be made, with code hr=0x80004005 and ec=1142, along with Event ID 1006 in the application log. According to Microsoft, the October 2026 update added a default cap on how much memory Endpoint Mapper can use when handling Endpoint Lookup calls. For workloads like Exchange Server, the default ceiling of 512 simultaneous Endpoint Lookup calls may be too low under heavy load, the company said.

Microsoft said administrators can raise the maximum number of simultaneous Endpoint Lookup calls through a registry setting, which also increases the memory Endpoint Mapper is permitted to use. In a separate blog post, the company said the reported problems are not tied to installation of the October 2026 security updates for Exchange Server itself and are not a known issue with that Exchange update. Any Exchange version running on affected Windows versions could be hit, Microsoft said, adding that impact will not be uniform: severity and how quickly problems appear depend on server load, and some servers may never encounter an issue.

Microsoft is urging customers not to uninstall the October 2026 Exchange Server security updates while it works on a fix. As a temporary workaround, it pointed to a registry change on affected Exchange Server systems, run from an elevated Command Prompt followed by a reboot of the device, that adds the EpmapSavedContextCap value under HKLM\SOFTWARE\Microsoft\Rpc and sets it to 65536. After the restart, customers should monitor their Exchange environment and raise the Endpoint Mapper memory limit again if the problem returns.

The disclosure arrives in a busy month for Exchange administration. In July, Microsoft announced it will stop shipping extended security updates for Exchange 2016 and 2019 this month, following a six-month extension announced in April 2026 to the original Extended Security Update program.

More hardware news from TechManNews.