Microsoft 365 Alert – Service Degradation – Exchange Online – Group owners were unable to modify group memberships in the Global Address Book in the classic Outlook desktop client – RESOLVED

04/02/2026 09:00:00 AM

NHSmail Reference: INC46741286

Microsoft Reference: EX1226468

Issue Status: RESOLVED

Issue Description: Group owners were unable to modify group memberships in the Global Address Book in the classic Outlook desktop client.

More info: Distribution Groups may have been able to be modified, but Security Groups may have been affected.

While they worked towards mitigation, affected users may have been able to utilize the new Outlook desktop client or Outlook on the web to perform Distribution Group modifications.

Final Update: 04/02/2026 12:44:00 AM – Microsoft has identified that a recent feature change intended to enable the use of a new API for managing distribution group modifications introduced an issue resulting in impact. They have reverted the affecting change and confirmed that impact was successfully remediated.

Scope of impact: This issue may have affected all group owners modifying certain group memberships through the Outlook Address Book in the classis Outlook desktop client.

Root cause: A recent feature change intended to enable the use of a new API for managing distribution group modifications introduced an issue resulting in impact.

Next Steps: Microsoft are reviewing the affecting change to understand why it resulted in impact to better prevent similar future occurrences.