Microsoft 365 Alert – Service Degradation – Exchange Online – Users may have been unable to access their Exchange Online mailboxes via Outlook on the web or MAPI – RESOLVED

28/05/2026 09:25:00 AM

NHSmail Reference: INC46836596

Microsoft Reference: EX1323831

Issue Status: RESOLVED

Issue Description: Users may have been unable to access their Exchange Online mailboxes via Outlook on the web or MAPI.

More info: Users may have experienced errors or failures when attempting to access their mailboxes. Impacted connection methods included:

– Outlook on the web
– Messaging API (MAPI)

Final Update: 28/05/2026 09:25:00 AM– While Microsoft were in the process of analyzing the affected infrastructure, 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: Impact was specific to some users located in the United Kingdom that attempted to access their mailboxes via Outlook on the web or MAPI.

Root cause: A portion of service infrastructure responsible for facilitating user access to Exchange Online mailboxes was operating in a suboptimal state and caused impact.

Next Steps: Microsoft are continuing to review the portion of affected infrastructure to identify the reason it was operating in a suboptimal state, so that they can determine how to prevent similar impact to user mailbox access from occurring in the future.