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Microsoft confirms October 2026 Windows Server updates break Exchange OWA sign-in and mail flow

1 fonte(s) · 1 atualização(ões) · publicado 19/10/2026 05:58 · coletado há 1 dia

Microsoft has confirmed that the October 2026 Windows Server security updates cause Outlook on the web (OWA) sign-in failures and mail delivery problems on some on-premises Exchange Server systems. Affected hosts run Windows Server 2019 and later. Messages sent from Outlook may stay in the Outbox, and Exchange may have trouble accessing mailboxes during mailbox replication. Microsoft says the problem is not caused by the October 2026 Exchange security updates and urges customers not to uninstall them. It has published a temporary registry workaround while it works on a fix.

Technical Details

  • The October 2026 Windows update introduced a default limit on the memory Endpoint Mapper uses when handling Endpoint Lookup calls. The default of 512 simultaneous calls may be too low for Exchange under heavy load.
  • Reported symptoms include the OWA error "MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)" and Event ID 1006 in the application log.
  • Impact varies with Exchange server load. Some servers may never see the problem, and any Exchange version running on an affected Windows version could be hit.

Remediation

  • Run reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f from an elevated Command Prompt on the Exchange server, then reboot.
  • Monitor the environment after the restart and raise the Endpoint Mapper memory limit again if the problem recurs.
  • Do not uninstall the October 2026 Exchange security updates.

Linha do tempo

  1. bleepingcomputer.com ↗ · pub. 19/10/2026 05:58 · coletado 19/10 17:53 primeiro registro

    Microsoft confirms October 2026 Windows Server updates break Exchange OWA sign-in and mail flow

    ler a análise desta fonte

    Microsoft has confirmed that the October 2026 Windows Server security updates cause Outlook on the web (OWA) sign-in failures and mail delivery problems on some on-premises Exchange Server systems. Affected hosts run Windows Server 2019 and later. Messages sent from Outlook may stay in the Outbox, and Exchange may have trouble accessing mailboxes during mailbox replication. Microsoft says the problem is not caused by the October 2026 Exchange security updates and urges customers not to uninstall them. It has published a temporary registry workaround while it works on a fix.

    Technical Details

    • The October 2026 Windows update introduced a default limit on the memory Endpoint Mapper uses when handling Endpoint Lookup calls. The default of 512 simultaneous calls may be too low for Exchange under heavy load.
    • Reported symptoms include the OWA error "MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)" and Event ID 1006 in the application log.
    • Impact varies with Exchange server load. Some servers may never see the problem, and any Exchange version running on an affected Windows version could be hit.

    Remediation

    • Run reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f from an elevated Command Prompt on the Exchange server, then reboot.
    • Monitor the environment after the restart and raise the Endpoint Mapper memory limit again if the problem recurs.
    • Do not uninstall the October 2026 Exchange security updates.