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 – RESOLVED

Announcements Shagufta Naaz

09/02/2026 08:50:00 AM

NHSmail Reference: INC46744691

Microsoft Reference: EX1228240

Issue Status: RESOLVED

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

Final Update: Microsoft has 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 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 steps: Microsoft are reviewing our change implementation standards to ensure similar issues are caught and addressed ahead of deployment to prevent this from happening again.