Microsoft 365 Alert – Service Degradation – Exchange Online – Some users may be unable to access their Exchange Online mailboxes through some connection methods – RESOLVED

Announcements Arpan Das

19/12/2025 08:51:00 AM

NHSmail Reference: INC46709261

Microsoft Reference: EX1199580

Issue Status: RESOLVED

Issue Description: Users may have been unable to access their Exchange Online mailboxes through some connection methods.

More info: Affected connection methods included:
– Exchange ActiveSync (EAS)
– Messaging Application Programming Interface (MAPI)
– Outlook on the web

Final Update: 19/12/2025 08:55:00 AM – Microsoft were in the process of analyzing service logs to isolate the source of impact, system monitoring indicated that the service had returned to normal health. They have confirmed that the issue is no longer occurring and they will continue to monitor the service to ensure that the problem doesn’t happen again.

Scope of impact: Some users hosted in the Great Britain region attempting to access their Exchange Online mailboxes through some connection methods may have been impacted.

Next Steps: While the impact related to mailbox access self-resolved, Microsoft are reviewing telemetry and other data related to this event to better understand what caused the issue.