Exact response pattern
421 4.7.0 [TS01] Messages from this IP are temporarily deferredTemporaryYahoo/AOLReputation
What it means
Yahoo is temporarily deferring the IP, usually because of traffic pattern or reputation signals.
Likely causes
- Sudden volume.
- Poor engagement or complaints.
- New IP reputation.
Recommended response
- Reduce volume and concurrency.
- Review complaint feedback and list quality.
- Maintain spaced retries.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.