· Incident
Degraded outbound delivery to Microsoft 365 recipients
Resolved · Sample incident to show the format · Duration 48 minutes · Affected: outbound delivery (Gateway, Hosted) · Not affected: inbound, archive, console
Timeline (Pacific)
- Monitoring detected deferral rate > 5% to outlook.com / Microsoft 365 recipients from one IP pool. On-call paged.
- Investigating. Microsoft returning 451 4.7.650 (IP reputation throttling) on pool B. Mail queued, not lost.
- Mitigating. Traffic shifted to pools A and C; warm-up pacing applied to B.
- Deferral rate back under 0.5%; queued mail delivered.
- Resolved. Monitoring for 24 hours.
Impact
About 1,900 messages to Microsoft-hosted recipients were delayed by 10–45 minutes. No messages were lost or delivered unencrypted. Locked-delivery notifications were unaffected.
Root cause
A burst of new-customer traffic exceeded the warm-up pacing on pool B after an automation change the previous day.
Corrective actions
- Pacing limits now enforced per pool regardless of automation (done).
- Alert threshold lowered from 5% to 2% deferrals (done).
- Post-incident review published within 5 business days (this page).
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