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

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.