Exact response pattern
421 4.7.0 [TS04] Messages temporarily deferred due to user complaintsTemporaryYahoo/AOLComplaints
What it means
Yahoo indicates complaint or negative-user feedback pressure.
Likely causes
- High spam complaint rate.
- Poor consent or targeting.
Recommended response
- Stop the affected source/campaign.
- Review complaint loop data.
- Remove problematic acquisition sources.
- Restart only after corrective action.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.