Microsoft has resolved an Exchange Online outage that delayed emails sent to and received from external domains. The incident also triggered intermittent “Server busy” errors for users across multiple mailboxes.
Exchange Online outage affected external email flow
Microsoft first acknowledged the issue at 02:19 AM EDT on September 4 under incident reference EX1467029.
The Exchange Online outage affected users attempting to exchange messages with external domains. Some emails were delayed, while others returned intermittent “Server busy” errors.
Microsoft said email clients should periodically retry messages that were affected. However, senders who receive a non-delivery report after 24 to 72 hours may need to send the message again.
Anti-spam systems may have increased delays
Microsoft’s early investigation found that anti-spam protections may have worsened the disruption for some users.
The company reviewed service telemetry to identify the underlying cause and determine the most effective mitigation. It did not disclose the number of affected users or the regions involved.
Microsoft classified the problem as an incident, which typically indicates a noticeable impact on customers.
Microsoft says new emails are working again
In an update issued later on September 4, Microsoft said it had resolved the issue for new emails.
The company reported that its telemetry showed normal service performance. It also said organisations had confirmed that the errors were no longer occurring.
Microsoft continues to monitor email queues and complete final checks. Messages delayed during the incident may still require time to clear.
Exchange Online has experienced several service disruptions in recent months. Earlier incidents affected mailbox access, calendar connectivity, authentication and email delivery across multiple regions.
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