Exact response pattern
421 4.7.0 [TS02] Messages temporarily deferredTemporaryYahoo/AOLRate limit
What it means
Yahoo is applying a temporary traffic or policy limit.
Likely causes
- Burst connections.
- Per-IP or per-domain rate pressure.
Recommended response
- Slow the sending rate.
- Reduce simultaneous connections.
- Monitor acceptance before increasing again.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.