Microsoft has acknowledged that the October 2026 Windows Server security updates can interfere with email access and delivery on some on-premises Exchange Server systems.
The issue applies to Exchange environments running on Windows Server 2019 and later, according to BleepingComputer’s October 19, 2026 report. Microsoft says any Exchange version installed on an affected Windows release could encounter the problem, although the effect varies with server load and some systems may never show symptoms.
Users may be unable to sign in to Outlook on the web (OWA). Messages sent through Outlook can remain in the Outbox without being delivered, while Exchange may also have difficulty accessing mailboxes during mailbox replication.
Endpoint Mapper limit may be insufficient under heavy load
Microsoft attributes the behavior to a change in the October 2026 Windows security update. The update introduced a default restriction on the memory available to Endpoint Mapper when processing Endpoint Lookup calls.
The default allows 512 simultaneous Endpoint Lookup calls. Microsoft says this may be inadequate for Exchange Server under heavy load. A registry setting can increase the maximum number of simultaneous calls and, consequently, the amount of memory Endpoint Mapper can use.
Affected customers have reported OWA failures accompanied by this error:
MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)
Event ID 1006 may also appear in the Windows Application log. These customer-reported signatures give administrators specific indicators to compare with OWA, mail-flow or mailbox-replication problems in their environments.
Microsoft says the severity and speed of the impact depend on Exchange server load. The issue is therefore not expected to affect every server equally.
Microsoft provides a temporary registry workaround
Microsoft is developing a fix. In the meantime, its temporary workaround is to change the Endpoint Mapper configuration on each affected Exchange server.
Administrators should open an elevated Command Prompt on the server and run:
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f
The Exchange server must then be rebooted. After it restarts, Microsoft advises administrators to monitor the environment and increase the Endpoint Mapper memory limit if the problem returns.
Microsoft has also separated this Windows Server issue from the October 2026 Exchange Server security updates. The company says the Exchange updates do not cause the reported behavior and urges customers not to uninstall those Exchange updates.
Until the permanent fix is available, administrators running Exchange on Windows Server 2019 or later should watch for OWA sign-in failures, Outlook messages remaining in the Outbox, mailbox-access problems during replication, the MapiExceptionMdbOffline error and Event ID 1006.
