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

    From CrudeSausage@crude@sausa.ge to comp.sys.mac.advocacy,comp.os.linux.advocacy,alt.comp.os.windows-11 on Mon Oct 19 09:56:38 2026
    From Newsgroup: alt.comp.os.windows-11

    <https://www.bleepingcomputer.com/news/microsoft/microsoft-october-windows-server-updates-cause-exchange-issues/>

    <https://shorturl.at/maJ3F>

    Microsoft: trust us to run your servers.
    Microsoft: pay no attention to the fact that you can't log in or send e-mail

    Microsoft has confirmed that this month's Windows Server security
    updates are causing OWA sign-in and mail delivery issues on some
    Exchange Server systems.

    In a new Windows release health dashboard update over the weekend,
    Microsoft said users may have trouble accessing email or signing in to
    Outlook after installing the latest updates on systems running Windows
    Server 2019 and later.

    In some cases, affected customers are also reportedly seeing Exchange
    Server OWA failing with a "MapiExceptionMdbOffline: Unable to make
    connection to the server. (hr=0x80004005, ec=1142)" error and Event ID
    1006 in the application log.

    "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," it said.

    "The October 2026 update introduced a default limit on the amount of
    memory used by Endpoint Mapper when handling Endpoint Lookup calls. For workloads such as Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls might not be sufficient under heavy load.
    Administrators can use this registry setting to increase the maximum
    number of simultaneous Endpoint Lookup calls, which increases the amount
    of memory that Endpoint Mapper can use."

    However, Microsoft says these known issues aren't linked to the October
    2026 Exchange server security updates and urges customers not to
    uninstall 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 impacted," Microsoft added
    in a separate blog describing the issues.

    "Not all Exchange servers will be impacted by installation of those
    updates equally; the severity and speed of impact varies based on
    Exchange server load. Some servers might never see a problem."

    While it works on a fix, Microsoft also shared a temporary workaround
    that should resolve these issues by modifying a registry key on affected Exchange Server systems.

    To do that, you have to run the following command from an elevated
    Command Prompt on the Exchange Server and then reboot the device:

    reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t
    REG_DWORD /d 65536 /f
    After the system restarts, customers should monitor the Exchange
    environment and raise the Endpoint Mapper memory limit if it reoccurs.

    In July. Microsoft also announced that it will stop shipping extended
    security updates for Exchange 2016 and 2019 this month after a 6-month extension announced in April 2026 to the original Extended Security
    Update (ESU) program.
    --
    CrudeSausage
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