Exchange Online outage causes email delays, 'Server busy' errors

An ongoing Exchange Online outage caused delays in email delivery to and from external domains, with users encountering intermittent 'Server busy' errors affecting multiple mailboxes. Microsoft tracked the incident as EX1467029 and noted that anti-spam protections may have contributed to the impact for some users. The company has resolved the issue for new emails and is monitoring email queues and service telemetry to confirm full resolution. The root cause remains under investigation, and no timeline for full remediation was initially provided. The outage did not involve a disclosed vulnerability or exploit but was a service disruption impacting mail flow.

Users experienced delayed email delivery and intermittent 'Server busy' errors when sending or receiving emails from external domains in Exchange Online. Some emails initially impacted may require resending if non-delivery reports (NDRs) were received after 24-72 hours. The issue affected multiple mailboxes and was aggravated by anti-spam protections, causing further delays. The outage caused noticeable user impact but did not involve a security breach or exploitation of a vulnerability.

Microsoft has resolved the issue for new emails and is performing final validations to confirm full resolution. Users who received NDRs after 24-72 hours should resend affected emails. No additional user action is required at this time. Microsoft continues to investigate the root cause and will provide updates as available.