Microsoft is addressing an ongoing disruption in Exchange Online that is preventing emails from being sent to or received from external domains. The issue was first identified at 02:19 AM EDT on September 4, 2026, when engineers began investigating reports of delayed message delivery and recurring Server busyerror notifications. The incident, logged internally as EX1467029, has affected users across multiple regions relying on the cloud-based email service for business communications.
The outage stems from a backend infrastructure problem within Microsoft’s Exchange Online environment, though the company has not disclosed the specific technical root cause. Affected users report that internal email flow remains functional, but messages destined for or originating from outside their organization are queued or rejected with transient failure codes. Microsoft’s service health dashboard indicates ongoing remediation efforts, with engineers applying patches and monitoring system stability in real time. No data loss or security breach has been indicated in the initial assessment.
Microsoft has not provided an estimated time for full service restoration, stating only that teams are working continuously to resolve the issue. The company advises customers to avoid resending messages repeatedly, as this may exacerbate queue backlogs. Instead, users are encouraged to monitor the Microsoft 365 Service Health portal for updates and consider alternative communication methods for time-sensitive correspondence. Historical patterns suggest similar Exchange Online incidents have ranged from under an hour to several hours, depending on the complexity of the underlying fault.
Organizations impacted by the disruption should instruct staff to refrain from bulk email campaigns or large attachments until service normalizes. Microsoft recommends checking individual mailbox sync status and verifying that client applications are updated to the latest version. For critical operations, switching to alternative platforms like Teams or Outlook on the web may offer temporary relief, though these services depend on Exchange Online for certain functions. The firm has pledged to release a post-incident review detailing preventive measures once normal operations resume.
Is my email data at risk during this outage? Microsoft has confirmed that no email data loss or compromise has been detected. Messages are being held in queues and will be delivered once the service stabilizes.
Should I contact Microsoft support for individual cases? For widespread service issues like this, individual support tickets are not necessary unless problems persist after the service health indicator shows resolution. The company is managing the incident centrally.
Will this affect my calendar or contacts sync? Calendar and contact synchronization may also experience delays or failures, as these features rely on the same Exchange Online backend infrastructure as email delivery.