Status Page

2026-03-24	Status	Description
12:13 PM		Title: Some users may have been unable to access their Exchange Online mailboxes using the Outlook desktop client User impact: Users may have been unable to access their Exchange Online mailboxes using the Outlook desktop client. More info: Impacted users may have received a 401 error message. Final status: We’ve confirmed that reverting the change successfully remediated the impact and restored the service. Scope of impact: Some users attempting to access their Exchange Online mailboxes via the Outlook desktop client may have been impacted. Start time: Tuesday, March 24, 2026, at 5:40 AM UTC End time: Tuesday, March 24, 2026, at 1:00 PM UTC Root cause: A recent service change introduced an unexpected behaviour that prevented part of the service from operating as designed, resulting in impact. Next steps: - We'll continue to closely monitor the service to ensure it remains healthy. - We're reviewing our deployment procedures to better identify similar issues during our development and testing cycles. This is the final update for the event.
12:00 PM		Title: Some users may be unable to access their Exchange Online mailboxes using the Outlook desktop client User impact: Users may be unable to access their Exchange Online mailboxes using the Outlook desktop client. More info: Impacted users may receive a 401 error message. Current status: We’ve identified that a recent service change may be causing impact to occur. We’ve reverted the change to restore normal service and are monitoring service data to confirm the mitigation is working as expected. Scope of impact: Some users attempting to access their Exchange Online mailboxes via the Outlook desktop client may be impacted. This information may be updated as our investigation continues. Next update by: Tuesday, March 24, 2026, at 3:00 PM UTC
11:20 AM		Service Restored Title: Some users’ outbound email messages may display an unexpected sender name in Exchange Online User impact: Users’ outbound email messages may have displayed an unexpected sender name in Exchange Online. More info: When affected users sent email to external recipients, the sender name shown may have been the mailbox DisplayName instead of the configured SimpleDisplayName. Final status: We've confirmed through internal testing that impact is no longer occurring. While no further outbound messages should be impacted, email messages that were already sent with the unexpected sender name will continue to show the sender name used when originally sent. Scope of impact: Some users sending email to external recipients in Exchange Online may have been impacted. Start time: Thursday, March 5, 2026, at 10:00 PM UTC End time: Monday, March 23, 2026, at 6:40 PM UTC Root cause: A recent service update introduced new outbound mail behavior that caused Exchange Online to stop honoring the configured SimpleDisplayName in some outbound email scenarios, resulting in impact. Next steps: - We're reviewing our update testing and validation methods to better identify issues such as this prior to deployment so we can prevent similar impact in the future. This is the final update for the event.

12:13 PM

Title: Some users may have been unable to access their Exchange Online mailboxes using the Outlook desktop client
User impact: Users may have been unable to access their Exchange Online mailboxes using the Outlook desktop client.
More info: Impacted users may have received a 401 error message.
Final status: We’ve confirmed that reverting the change successfully remediated the impact and restored the service.
Scope of impact: Some users attempting to access their Exchange Online mailboxes via the Outlook desktop client may have been impacted.
Start time: Tuesday, March 24, 2026, at 5:40 AM UTC
End time: Tuesday, March 24, 2026, at 1:00 PM UTC
Root cause: A recent service change introduced an unexpected behaviour that prevented part of the service from operating as designed, resulting in impact.
Next steps:
- We'll continue to closely monitor the service to ensure it remains healthy.
- We're reviewing our deployment procedures to better identify similar issues during our development and testing cycles.
This is the final update for the event.

12:00 PM

Title: Some users may be unable to access their Exchange Online mailboxes using the Outlook desktop client
User impact: Users may be unable to access their Exchange Online mailboxes using the Outlook desktop client.
More info: Impacted users may receive a 401 error message.
Current status: We’ve identified that a recent service change may be causing impact to occur. We’ve reverted the change to restore normal service and are monitoring service data to confirm the mitigation is working as expected.
Scope of impact: Some users attempting to access their Exchange Online mailboxes via the Outlook desktop client may be impacted. This information may be updated as our investigation continues.
Next update by: Tuesday, March 24, 2026, at 3:00 PM UTC

11:20 AM

Service Restored

Title: Some users’ outbound email messages may display an unexpected sender name in Exchange Online
User impact: Users’ outbound email messages may have displayed an unexpected sender name in Exchange Online.
More info: When affected users sent email to external recipients, the sender name shown may have been the mailbox DisplayName instead of the configured SimpleDisplayName.
Final status: We've confirmed through internal testing that impact is no longer occurring. While no further outbound messages should be impacted, email messages that were already sent with the unexpected sender name will continue to show the sender name used when originally sent.
Scope of impact: Some users sending email to external recipients in Exchange Online may have been impacted.
Start time: Thursday, March 5, 2026, at 10:00 PM UTC
End time: Monday, March 23, 2026, at 6:40 PM UTC
Root cause: A recent service update introduced new outbound mail behavior that caused Exchange Online to stop honoring the configured SimpleDisplayName in some outbound email scenarios, resulting in impact.
Next steps:
- We're reviewing our update testing and validation methods to better identify issues such as this prior to deployment so we can prevent similar impact in the future.
This is the final update for the event.