User impact: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxes.

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

The Outlook desktop crashes occur when attempting to add a new on-premises mailbox.

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

While we work on deploying the fix, admins may be able to mitigate the impact by restarting the Autodiscover app pool.

Current status: We’ve developed a fix that will prevent the header duplication issue from occurring and have begun deploying the fix to the Exchange Online versions 15.20.9587.019, 15.20.9632.004, 15.20.9611.008, and 15.20.9564.022. As the fix saturates, admins will begin to gradually notice impact remediation, and we expect that the fix will complete deployment by the time of our next update.

Scope of impact: Any admins’ cross-tenant AutoDiscover requests may fail from hybrid environments, causing MailTips free/busy not to show and crashing for users’ Outlook desktop clients when adding additional on-premises mailboxes. This information may be updated as our investigation continues.

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

Root cause: A routing change for Autodiscover requests introduced a header duplication issue causing a backlog of requests that ultimately fail or timeout, resulting in the impact.

Next update by: Wednesday, February 11, 2026, at 8:00 PM UTC