Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online

Title: Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online User impact: Users may have been unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online. More info: This impact occurred only when two or more mailbox statuses were requested. Final status: We've completed deployment of our fix across the affected environment and validated with a subset of affected users that they are now able to see the free/busy status of cross-tenant mailboxes in Exchange Online as expected. Scope of impact: Users attempting to access the free/busy status of multiple mailboxes at once may have been impacted. Start time: Thursday, May 14, 2026, at 12:00 AM UTC End time: Monday, June 15, 2026, at 7:00 PM UTC Root cause: A NetCore deployment introduced a regression where headers were being dropped during free/busy status requests for cross-tenant scenarios, resulting in back-end authentication requests failing. Next steps: - We're analyzing the offending NetCore deployment to validate why the potential for impact wasn't caught during our internal testing and validation phases and how we can improve our deployment implementation process moving forward to ensure similar impact scenarios don't occur in the future. This is the final update for the event.

Title: Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online User impact: Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online. More info: This impact occurs only when two or more mailbox statuses are requested. Current status: We've validated that the source of impact was a NetCore deployment, which introduced a regression where headers were being dropped during free/busy status requests for cross-tenant scenarios, resulting in back-end authentication requests failing. Our fix to address the regression is currently at 90 percent saturation in the affected environment, and we've reached out to affected users to begin testing for remediation. Scope of impact: Users attempting to access the free/busy status of multiple mailboxes at once may be impacted. Start time: Thursday, May 14, 2026, at 12:00 AM UTC Root cause: A NetCore deployment introduced a regression where headers were being dropped during free/busy status requests for cross-tenant scenarios, resulting in back-end authentication requests failing. Next update by: Wednesday, June 17, 2026, at 8:30 PM UTC

Title: Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online User impact: Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online. More info: This impact occurs only when two or more mailbox statuses are requested. Current status: We've confirmed that it may take until our next scheduled update for this fix to saturate and remediate impact. We'll monitor the fix deployment and update the expected timeline of remediation as it progresses. Scope of impact: Users attempting to access the free/busy status of multiple mailboxes at once may be impacted. Next update by: Tuesday, June 16, 2026, at 8:30 PM UTC

Title: Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online User impact: Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online. More info: This impact occurs only when two or more mailbox statuses are requested. Current status: We've determined that our previous fix will not effectively remediate impact. We're creating a secondary targeted fix that we expect to address the issue and are confirming the timeline for deployment. Scope of impact: Users attempting to access the free/busy status of multiple mailboxes at once may be impacted. Next update by: Thursday, June 11, 2026, at 8:30 PM UTC