Microsoft 365 Alert – Service Degradation – Exchange Online – Users may have been unable to access their Exchange Online mailbox through multiple connection methods – RESOLVED

Announcements Arpan Das

25/05/2026 10:02:00 AM

NHSmail Reference: INC46834496

Microsoft Reference: EX1320296

Issue Status: RESOLVED

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

More info: The affected connection methods consisted of the following:
-Outlook on the web
-Messaging Application Programming Interface (MAPI)

Current Update: 25/05/2026 09:56:00 AM – Microsoft has identified that a portion of the mailbox infrastructure which processes access requests for Outlook on the web and MAPI encountered failures and caused it to perform the expected threshold. They have restarted the affected portion of infrastructure to remediate the impact and confirmed through monitoring our service telemetry that this issue is resolved.

Scope of impact: Users in the United Kingdom who attempted to access their Exchange Online mailbox through Outlook on the web or Messaging Application Programming Interface (MAPI) may have been affected by this issue.

Root cause: A portion of the mailbox infrastructure which processes access requests for Outlook on the web and MAPI encountered failures and caused it to perform the expected threshold.

Next Steps: Microsoft are analyzing the performance data associated with the impacted portion of infrastructure so that they can better understand why it was encountering failures and prevent impact to Exchange Online mailbox access in the future.