Microsoft 365 Alert – Service Degradation – Microsoft Copilot (Microsoft 365) – MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxes – ONGOING

Announcements Shagufta Naaz

09/02/2026 08:50:00 AM

NHSmail Reference: INC46744691

Microsoft Reference: EX1228240

Issue Status: ONGOING

Issue Description: 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-premise mailbox.

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

Current Update: 10/02/2026 08:36:00 AM –Microsoft has 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 they 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.

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