Microsoft 365 Alert – Service Degradation – Exchange Online – Users may have been unable to access their Exchange Online mailboxes using the Outlook desktop client – RESOLVED

Announcements Arpan Das

24/03/2026 16:26:00 PM

NHSmail Reference: INC46781262

Microsoft Reference: EX1260428

Issue Status: RESOLVED

Issue Description: Users may have been unable to access their Exchange Online mailboxes using the Outlook desktop client.

More info: Impacted users may have received a 401 error message.

Final Update: 25/03/2026 16:27:00 PM–Microsoft has confirmed that reverting the change successfully remediated the impact and restored the service.

Scope of impact: Some users attempting to access their Exchange Online mailboxes via the Outlook desktop client may have been impacted.

Root cause: A recent service change introduced an unexpected behaviour that prevented part of the service from operating as designed, resulting in impact.

Next Steps: Microsoft will continue to closely monitor the service to ensure it remains healthy. They are reviewing their deployment procedures to better identify similar issues during their development and testing cycles.