Microsoft has acknowledged that the latest Windows Server security updates are leading to sign-in and mail delivery complications for certain Exchange Server systems. In a recent update to its Windows release health dashboard, the tech giant indicated that users may encounter difficulties accessing email or signing into Outlook following the installation of the most recent updates on systems utilizing Windows Server 2019 and beyond.
Reports from affected customers reveal that some Exchange Server OWA instances are failing with a “MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)” error, alongside Event ID 1006 appearing in the application log.
Details of the Update
Microsoft elaborated that, “After installing the October 2026 Windows security update, some on-premises Exchange servers might experience issues accessing or sending email. Users might be unable to sign in to Outlook on the web (OWA), or email messages sent from Outlook might remain in the Outbox and not be delivered. Additionally, Exchange might experience issues accessing mailboxes during mailbox replication.”
The October 2026 update has introduced a default limit on the memory allocated to Endpoint Mapper when managing Endpoint Lookup calls. For Exchange Server workloads, the default cap of 512 simultaneous Endpoint Lookup calls may prove inadequate under heavy load. Administrators have the option to modify a registry setting to increase this limit, thereby allowing Endpoint Mapper to utilize more memory.
Despite these challenges, Microsoft reassured users that the issues reported are not tied to the October 2026 Exchange server security updates, urging customers against uninstalling them. “Issues reported are not related to installation of October 2026 security updates for Exchange Server. This is not a ‘known issue’ related to October 2026 Exchange update. Any version of Exchange that runs on impacted versions of Windows could be affected,” the company stated in a separate blog post.
It is important to note that not all Exchange servers will experience the same level of impact from these updates; the severity and speed of the issues can vary based on the load on individual servers. Some may not encounter any problems at all.
Temporary Workaround
While Microsoft works on a permanent solution, the company has provided a temporary workaround that involves modifying a registry key on the affected Exchange Server systems. To implement this fix, administrators should execute the following command from an elevated Command Prompt on the Exchange Server, followed by a system reboot:
reg add "HKLMSOFTWAREMicrosoftRpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f
Post-reboot, customers are advised to monitor their Exchange environment closely and consider raising the Endpoint Mapper memory limit if issues persist.
In a related note, Microsoft announced in July that it would cease shipping extended security updates for Exchange 2016 and 2019 this month, following a six-month extension that was previously announced in April 2026 as part of the original Extended Security Update (ESU) program.
