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 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.
Some servers might never see a problem, it added in a separate blog describing the issues.
In a new update to the Windows release health dashboard, Microsoft said the November 2026 Patch Tuesday updates (KB5131080 and KB5131090) address this known issue and advised affected users to install them on all affected systems.
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.
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.
To do that, IT administrators have to run the following command from an elevated Command Prompt on the Exchange Server to increase the maximum number of Endpoint Lookup calls and the amount of memory that Endpoint Mapper can use, and then restart the device: After the system reboots, the admins should monitor the Exchange environment and raise the Endpoint Mapper memory limit if it recurs.
However, Microsoft said that this Outlook on the web (OWA) known issue isn't linked to the October 2026 Exchange server security updates and urged customers not to uninstall them.
Microsoft also shared a temporary workaround that should resolve these issues for those who can't immediately deploy this month's cumulative updates, which requires modifying a registry key on affected Exchange Server systems.
Microsoft: October Windows Server updates break Exchange sign-in, mail flow
What happened
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.
What changed
Some servers might never see a problem, it added in a separate blog describing the issues.
In a new update to the Windows release health dashboard, Microsoft said the November 2026 Patch Tuesday updates (KB5131080 and KB5131090) address this known issue and advised affected users to install them on all affected systems.
Who is affected
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.
Why it matters
Additionally, Exchange might experience issues accessing mailboxes during mailbox replication, Microsoft said at the time.
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.
Technical details
To do that, IT administrators have to run the following command from an elevated Command Prompt on the Exchange Server to increase the maximum number of Endpoint Lookup calls and the amount of memory that Endpoint Mapper can use, and then restart the device: After the system reboots, the admins should monitor the Exchange environment and raise the Endpoint Mapper memory limit if it recurs.
Response
However, Microsoft said that this Outlook on the web (OWA) known issue isn't linked to the October 2026 Exchange server security updates and urged customers not to uninstall them.
Microsoft also shared a temporary workaround that should resolve these issues for those who can't immediately deploy this month's cumulative updates, which requires modifying a registry key on affected Exchange Server systems.
What to watch next
Watch for revised vendor guidance, fixed versions and mitigation updates.