Microsoft fixes Exchange sign-ins broken by Windows Server updates

Microsoft has fixed Outlook sign-in and mail delivery issues on Exchange Server systems after installing the October 2026 Windows Server security updates.

Microsoft acknowledged this known issue in mid-October, confirming that users had trouble accessing email or signing in to Outlook after installing the latest updates on systems running Windows Server 2019 and later.

As explained at the time, the sign-in and mail flow issues are caused by a new default limit on the amount of memory Endpoint Mapper uses when handling Endpoint Lookup calls, introduced by the October 2026 updates.

"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," Microsoft said at the time.