Microsoft 365 Alert – Service Degradation – Exchange Online – Users may be unable to see the free/busy status of multiple cross-tenant mailboxes in Exchange Online – NEW

Announcements Sharmista Mohanta

17/06/2026 10:04:00 AM

NHSmail Reference: INC46853862

Microsoft Reference: EX1386636

Issue Status: OPEN

Issue Description: 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 Update: 17/06/2026 10:06:00 AM – Microsoft has 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. Their fix to address the regression is currently at 90 percent saturation in the affected environment, and they have 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.

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