Microsoft is currently dealing with an Exchange Online service issue that is causing delays when users send or receive emails from external domains. The incident is being tracked as EX1467029, with Microsoft investigating reports of intermittent “Server busy” errors affecting multiple mailboxes. The problem does not mean that all Exchange Online email services are completely unavailable, but affected users can experience noticeable delays in message delivery.
The issue was acknowledged by Microsoft on September 4, 2026, after reports began appearing about problems with external email communication. Users attempting to exchange messages with people outside their organization may see temporary delivery failures or “Server busy” responses. Microsoft has classified the situation as a service degradation, meaning the service remains available but some users are experiencing interruptions or slower-than-normal performance.
Microsoft’s investigation found that anti-spam protections may be contributing to the problem for a subset of affected users. These protections can result in additional throttling actions, which may increase delays when external messages are being processed. Microsoft is reviewing service telemetry and analyzing the behavior to understand the underlying cause and determine the most effective way to reduce the impact.
The company has since started disabling the specific model involved in the issue, according to the latest service-health update. Microsoft reported that telemetry was showing a decrease in matching and throttling actions as the rule changes spread through the service. The rollout was expected to complete within around 30 minutes of that update, although Microsoft continued to treat the incident as an active service issue while the changes were being applied.
For users affected by the problem, email clients may periodically retry messages that were delayed by the incident. Microsoft has also warned that if an original sender receives a non-delivery report after 24 to 72 hours, depending on the email client or provider, the message may need to be sent again. This means some delayed messages may eventually be delivered automatically, while others could require the sender to resend them.
Microsoft has not indicated that every Exchange Online customer is affected, and the exact number of impacted users or regions has not been disclosed. The company is continuing to review service telemetry and the effect of the mitigation as the changes propagate across Exchange Online. The incident remains a developing service issue, with Microsoft expected to provide further updates as its remediation work progresses.
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