Microsoft says enforcement of a specific anti-spam model contributed to Exchange Online mail delays on 4 September 2026, affecting a subset of traffic involving external domains.
Microsoft tracked the incident as EX1467029. A public extract of its service-health record puts the impact between 09:00 and 15:15 UTC. Users may have experienced delays sending and receiving messages from external domains, with intermittent “Server busy” errors across multiple mailboxes.
Poppulo reported increased deferrals for mail sent through protection.outlook.com. Operators on the Mailop mailing list independently reported the same response, accompanied by the suffix codes S77714, S77717 and S77719.
451 4.7.500 Server busy
The 451 response is a temporary SMTP deferral. RFC 5321 classifies 4yz responses as transient failures that an SMTP client should retry. Mail that cannot be transmitted immediately must be queued and periodically retried.
Microsoft initially added affected sending IP addresses to an allow list while it worked to disable the model. During recovery, it identified a different machine-level throttling rule within its anti-spam systems that was still affecting a small number of messages. Microsoft disabled that rule and later marked the incident resolved after extended monitoring.
Microsoft described the anti-spam model as its preliminary root cause. It said it would review model classifications, rule configurations and upstream data pipelines. The public extract checked by emailexpert on 10 September does not yet include the post-incident report Microsoft said would follow.
Related coverage: Microsoft 365 blocks rise 10–15% across SendGrid IPs
