Some users may experience issues when accessing their Exchange Online mailbox via one or more connection method

A post-incident report has been published.

A post-incident report has been published.

A post-incident report has been published.

Title: Some users may have experienced issues when accessing their Exchange Online mailbox via one or more connection method User impact: Users may have experienced errors when accessing their mailbox via multiple Exchange Online connection methods. More info: Users may have experienced errors or failures when attempting to access their mailbox. Impacted connection methods included, but were not limited to: - Outlook on the web - Outlook desktop - Exchange ActiveSync - Other Exchange Online connection protocols Final status: We've confirmed that service has remained healthy for an extended period of time. Our monitoring indicates the issue is no longer occurring for the affected users. Scope of impact: Impact was specific to some users attempting to access their mailbox via any of the connection methods detailed above. Start time: Monday, March 16, 2026, at 9:30 AM UTC End time: Monday, March 16, 2026, at 1:30 PM UTC Root cause: A small number of Exchange Online backend servers experienced intermittent network communication failures within their high availability groups. When these servers were unable to communicate their required health signals, built-in protection mechanisms automatically initiated failovers to healthy servers to prevent a broader outage. While only a limited portion of backend capacity was affected, multiple failovers occurred in close succession, which temporarily reduced overall service quality. Availability for Outlook on the web was further impacted by an increase in application pool crashes caused by a code bug within specific network call path that led to a threading issue. Next steps: - For a more comprehensive list of next steps and actions, please refer to the Post Incident Report document. We'll provide a preliminary Post-Incident Report within two business days and a final Post-Incident Report within five business days.

Title: Some users may experience issues when accessing their Exchange Online mailbox via one or more connection method User impact: Users may experience errors or failures when accessing their mailbox via one or more Exchange Online connection methods. More info: Users may experience errors or failures when attempting to access their mailbox. Impacted connection methods include but may not be limited to: - Outlook on the web - Outlook desktop - Exchange ActiveSync - Other Exchange Online connection protocols Current status: While telemetry continues to show the issue is no longer occurring for affected users, we're continuing to monitor service health to assess whether any additional actions are required to ensure sustained recovery. Scope of impact: Impact is specific to some users attempting to access their mailbox via any of the connection methods detailed above. Next update by: Monday, March 16, 2026, at 4:30 PM UTC

We’re continuing to monitor service health to identify whether any additional actions are required to ensure longstanding recovery. Our telemetry continues to indicate the issue is no longer occurring for affected users. This quick update is designed to give the latest information on this issue.

Title: Some users may experience issues when accessing their Exchange Online mailbox via one or more connection method User impact: Users may experience errors or failures when accessing their mailbox via one or more Exchange Online connection methods. More info: Users may experience errors or failures when attempting to accessing their mailbox. Impacted connection methods include but may not be limited to: - Outlook on the web Current status: Internal reports and service monitoring telemetry indicate that the issue is no longer occurring for affected users. We’ll continue to monitor service health to ensure ongoing stability. Scope of impact: Impact is specific to some users attempting to access their mailbox via any of the connection methods detailed above. Next update by: Monday, March 16, 2026, at 3:30 PM UTC

Title: Some users may experience issues when accessing their Exchange Online mailbox via one or more connection method User impact: Users may experience errors or failures when accessing their mailbox via one or more Exchange Online connection methods. More info: Users may experience errors or failures when attempting to accessing their mailbox. Impacted connection methods include but may not be limited to: - Outlook on the web Current status: We’ve identified a backend issue that’s preventing some services from responding as expected, resulting in mailbox access delays. We’re investigating this behavior across regions to better understand the impact and determine next steps. We’re seeing signs of service recovery and will share additional updates as our investigation continues. Scope of impact: Impact is specific to some users attempting to access their mailbox via any of the connection methods detailed above. Next update by: Monday, March 16, 2026, at 3:30 PM UTC

Title: Some users may experience issues when accessing their Exchange Online mailbox via one or more connection method User impact: Users may experience errors or failures when accessing their mailbox via one or more Exchange Online connection methods. More info: Users may experience errors or failures when attempting to accessing their mailbox. Impacted connection methods include but may not be limited to: - Outlook on the web Current status: We’re currently reviewing service telemetry data to help identify the root cause and next steps required to mitigate impact. Scope of impact: Impact is specific to some users located in Europe attempting to access their mailbox via any of the connection methods detailed above. Next update by: Monday, March 16, 2026, at 1:30 PM UTC

Title: Potential issues accessing mailboxes via one or more connection methods User impact: Users may experience errors or failures when accessing their mailbox via one or more Exchange Online connection methods. Current status: We're investigating a potential issue and checking for impact to your organization. We'll provide an update within 60 minutes.