Microsoft 365 Alert – Service Degradation – Exchange Online – Users may be unexpectedly signed out from Outlook on the web when accessing shared mailboxes – RESOLVED

Announcements Sourav Nandi

20/10/2025 08:51:00 AM

NHSmail Reference: INC46659636

Microsoft Reference: EX1173828

Issue Status: RESOLVED

Issue Description: Users may be unexpectedly signed out from Outlook on the web when accessing shared mailboxes.

More info: Users may be unexpectedly signed out from Outlook on the web when accessing shared mailboxes.

Final Update: 28/10/2025 08:10:00 AM – Microsoft have determined that a recent package intended to update the appearance of the Outlook suite header contained a bug that created this impact. A new version of this update, without the bug, was then implemented to update the suite header without any residual impact.

Scope of impact: This issue can potentially affect any user when accessing shared mailboxes from Outlook on the web.

Root cause: A bug contained in a recent package intended to update the appearance of the Outlook suite header created this impact.

Next steps: Microsoft are reviewing the bug in a recent package intended to update the appearance of the Outlook suite header to better understand what led to the error that resulted in impact and prevent similar future occurrences.