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

20/03/2026 08:40:00 AM

NHSmail Reference: INC46778121

Microsoft Reference: EX1256020

Issue Status: RESOLVED

Issue Description: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods.

More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client.

Final Update: 02/04/2026 09:03:00 AM– Microsoft telemetry indicates that disabling the impacting updates has significantly reduced the number of errors. They are continuing to monitor the environment to validate no additional mitigation actions are needed.

Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps.

Root cause: A recent change within the Exchange Online service intended to introduce a new virtual account resulted in impact.

Next update by: Microsoft are reviewing the impacting service update to determine why the routing conflict wasn’t identified through their pre-deployment testing processes, so that they can prevent similar instances of impact from occurring in the future.

Additionally, they are implementing additional optimizations on the service and client-sides to further reduce the error counts and make the service more robust.