User impact: Users’ MailTips may not have reflected free/busy correctly in Outlook desktop and crashes may have occurred.

More info: This issue was specific to the Outlook desktop client in organizations with a hybrid environment that used Exchange Web Services (EWS) for MailTips. Users may have noticed that the MailTips free/busy feature wasn’t operating as expected and may have seen an error stating “we can’t show MailTips right now.”

The Outlook desktop crashes occurred when users attempted to add a new on-premises mailbox.

Admins may have been unable to create new on-premises mailboxes, and their cross-tenant AutoDiscover requests general may have failed from hybrid environments.

While we worked on deploying the fix, admins may have been able to mitigate the impact by restarting the Autodiscover app pool.

Final status: We’ve validated with a subset of affected users that the fix to prevent the header duplication issue has fully resolved the impact.

Scope of impact: Any admins’ cross-tenant AutoDiscover requests may have failed from hybrid environments, which caused MailTips free/busy not to show and crash for users’ Outlook desktop clients when they added additional on-premises mailboxes.

Start time: Tuesday, January 27, 2026, at 12:30 AM UTC

End time: Thursday, February 12, 2026, at 6:00 PM UTC

Root cause: A routing change for Autodiscover requests introduced a header duplication issue which caused a backlog of requests that ultimately failed or timed out and resulted in the impact.

Next steps:
– We’re reviewing our change implementation standards to ensure similar issues are caught and addressed ahead of deployment to prevent this from happening again.

This is the final update for the event.