💡 Our Technical Review in summary
1. Summary
A routing change for Autodiscover requests introduced a header duplication issue within Exchange Online. This caused a significant backlog of requests that ultimately timed out or failed. The issue specifically affected organizations operating in hybrid environments that utilize Exchange Web Services (EWS) for MailTips and cross-tenant Autodiscover functionality.
2. Impact
The impact was observed across three primary areas:
- Free/Busy Visibility: Outlook desktop users could not see MailTips for free/busy status, often receiving an error message stating, “we can’t show MailTips right now.”
- Outlook Stability: The Outlook desktop client crashed when users attempted to add new on-premises mailboxes.
- Administrative Tasks: Admins may have been unable to create new on-premises mailboxes, and cross-tenant AutoDiscover requests from hybrid environments failed.
3. Status
Status: Service Restored
Microsoft has successfully deployed a fix to prevent header duplication across the affected Exchange Online infrastructure (versions 15.20.9587.019 and higher). The fix has been validated with affected users, and the service is confirmed as fully operational.
- Incident Duration: January 27, 2026, to February 12, 2026.
- Final Resolution: Deployment of a code fix to address request routing.
- Next Steps: Microsoft is reviewing change implementation standards to prevent similar routing issues in future deployments.
Microsoft Official Issue Details
User Impact
Users’ MailTips may not have reflected free/busy correctly in Outlook desktop and crashes may have occurred.
Status: serviceRestored
Service: Exchange Online
Classification: advisory
Origin: microsoft
Updates
- 2026-02-17 21:06
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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.
- 2026-02-11 19:42
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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: Our monitoring of the deployment of the fix to prevent the header duplication issue indicates that the majority of admins should be experiencing remediation, and we’re beginning to validate impact resolution with a subset of affected admins as the saturation of the fix fully completes.
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 18, 2026, at 2:00 AM UTC
- 2026-02-09 18:51
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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
- 2026-02-07 01:41
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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-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 status: We’ve determined that 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. We’ve confirmed that, in addition to AutoDiscover failures and Outlook crashes, users may experience the MailTips free/busy feature isn’t operating as expected and may see an error stating “we can’t show MailTips right now.” As we develop a fix to address this issue, affected admins may be able to mitigate the impact by recycling the Autodiscover application pool. We anticipate having an estimation for the delivery of the permanent fix to resolve this issue by our next scheduled 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: Monday, February 9, 2026, at 8:30 PM UTC
- 2026-02-06 23:00
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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-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 status: We’re reviewing the impacted request flow and any recent changes to the service to determine the source of the errors and formulate a remediation plan.
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.
Next update by: Saturday, February 7, 2026, at 1:00 AM UTC
- 2026-02-06 21:07
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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-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 status: We’re continuing to analyze system logs and telemetry to determine the source of what we suspect is preventing specific AutoDiscover requests from being processed as expected, causing on-premises service requests to accumulate, and some requests to intermittently fail, leading to 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.
Next update by: Friday, February 6, 2026, at 11:00 PM UTC
- 2026-02-06 19:19
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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-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 status: We’re reviewing system logs to isolate the origin of this issue.
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.
Next update by: Friday, February 6, 2026, at 9:00 PM UTC
- 2026-02-06 19:01
Title: Users’ MailTips may not reflect free/busy correctly in Outlook desktop and crashes may occur when adding mailboxesUser 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-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 status: We’re investigating a potential issue in which admins’ cross-tenant AutoDiscover requests may fail from hybrid environments and Outlook desktop crashes when adding additional on-premises mailboxes, and checking for impact to your organization. We’ll provide an update within 30 minutes.
