Exchange Online outage disrupts email and authentication

Microsoft is investigating a widespread Exchange Online outage that is delaying or blocking email delivery, mailbox searches, administration tasks and authentication for customers. The incident is tracked as EX1464935 in the Microsoft 365 admin center.

The scope remains unclear. Microsoft has not identified the affected regions, so the impact on US organizations cannot yet be determined. Downdetector recorded tens of thousands of user reports involving Outlook email delivery and access to the Exchange Online platform.

Microsoft’s reported symptoms go beyond delayed messages. Customers may also encounter incomplete or failed search results in Exchange Online mailboxes, authentication errors when opening Exchange services, and problems using Exchange administration interfaces. The company also listed intermittent mailbox-operation failures and disruptions to message-delivery workflows.

Organizations that depend on Exchange Online for both mail transport and identity checks may face multiple problems at once. A user might be unable to sign in, while an already authenticated session could still experience delays in mailbox operations or message delivery. Microsoft has not said that the incident is a security breach, and the available information points to a service-configuration failure rather than an attack.

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“We’ve isolated a common failure pattern across affected Exchange Online requests that is associated with authentication and protocol connectivity. We’re analyzing service telemetry to identify the underlying source of impact and validate potential remediation options. Additionally, we’re continuing to assess the scope of impact and monitor for additional affected scenarios.”

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Authentication configuration is at the center of the incident

In an update on August 31, 2026, at 14:29 EDT, Microsoft narrowed the suspected cause to a core authentication configuration used by multiple internal Exchange Online services. The company said it was manually testing individual servers and resetting their configurations to determine whether that would resolve the failures.

“We’ve identified issues related to a core authentication configuration used by multiple internal services within the Exchange Online infrastructure. We’re performing a manual test on the individual server level to reset to configurations to validate if this resolves the issue.”

That wording indicates a server-level remediation process rather than a confirmed, platform-wide fix. Microsoft had not disclosed the configuration change, the number of tenants affected, a recovery target, or whether all symptoms shared the same technical cause.

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The outage follows several other Exchange Online disruptions described in the incident report. In June, Microsoft mitigated a mail-flow pipeline issue affecting customers in North America, Asia-Pacific and Europe. In April, the company resolved intermittent mailbox-access problems affecting Outlook mobile and macOS users, as well as a separate incident involving Outlook on the web, Outlook desktop, Exchange ActiveSync and other Exchange Online connection protocols.

Those earlier incidents don’t establish that the August 31 failure has the same cause. They show why the current authentication detail matters: a shared configuration used by multiple internal services can produce failures across otherwise separate user-facing functions.

Microsoft has confirmed the failure pattern and a suspected authentication-configuration problem, but not the geographic boundary or final remediation status. US customers will need to rely on their tenant’s Microsoft 365 service-health dashboard for confirmation of local impact; the public information does not establish whether a particular tenant or region is affected.