October Windows Server Updates Cause Exchange Email Issues

Microsoft has confirmed that October 2026 Windows Server security updates can cause Exchange email issues on some on-premises systems. The problems include failed webmail sign-ins, undelivered messages and difficulties accessing mailboxes during replication.

The issue affects Exchange deployments running on Windows Server 2019 and later. However, its severity depends on server workload, and some installations may experience no disruption.

Users Face Failed Sign-Ins and Stalled Messages

After administrators install the affected Windows updates, users may lose access to Outlook on the web, also known as OWA. Meanwhile, messages sent through Outlook can remain in the Outbox instead of reaching recipients.

Some affected systems also display a mailbox connection error:

MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)

Additionally, administrators may find Event ID 1006 in the application log. Exchange can also encounter mailbox access problems during mailbox replication.

These symptoms may emerge at different speeds across environments because server load influences the impact.

A New Endpoint Mapper Limit Triggers the Problem

Microsoft traced the disruption to a change in the October Windows security updates. The updates introduced a default limit on memory consumption when Endpoint Mapper handles Endpoint Lookup calls.

The default allows 512 simultaneous calls. However, busy Exchange environments may need more capacity, making that limit insufficient under heavy load.

Consequently, servers handling heavier workloads may encounter problems sooner. Less demanding environments, meanwhile, might never reach the threshold that triggers disruption.

Exchange Security Updates Are Not Responsible

Microsoft says the Exchange email issues stem from the underlying Windows updates, rather than October’s Exchange Server security updates.

Therefore, the company urges administrators to keep the Exchange security updates installed. Removing those updates would not address the identified cause.

The issue can affect any Exchange version running on an impacted Windows Server release. As a result, administrators should assess the operating system and workload when investigating symptoms.

Microsoft Provides a Temporary Registry Workaround

While Microsoft develops a fix, it recommends increasing the Endpoint Mapper limit through a registry setting.

Administrators should run the following command from an elevated Command Prompt on the affected Exchange server:

reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f

They must then restart the server for the change to take effect.

The setting increases the permitted number of simultaneous Endpoint Lookup calls. In turn, it allows Endpoint Mapper to use more memory.

After restarting, administrators should monitor mailbox access, sign-ins and message delivery. If the problem returns, Microsoft advises increasing the limit further.

Exchange 2016 and 2019 Approach the End of Extended Updates

Separately, Microsoft plans to stop providing extended security updates for Exchange Server 2016 and 2019 this month.

The deadline follows a six-month extension announced in April 2026. Organizations still using those releases therefore face an additional support deadline alongside the current Windows update disruption.