421 4.7.0 [TS01]
TS01 New or Cold IP Throttle
Yahoo deferred the connection because the sending IP has little or no history.
TS01 appears during IP or domain warmup. Yahoo pauses delivery until it sees strong engagement and consistent authentication.
Severity
WARNING
Provider
Yahoo Mail
Reference
TS01
Last Reviewed
2025-06-01
How to Fix 421 4.7.0 [TS01]
Follow these steps to resolve this Yahoo Mail SMTP error and restore normal delivery.
- ✓Warm new IPs with small batches of highly engaged recipients.
- ✓Distribute sends over several hours instead of one burst.
- ✓Keep From domains, DKIM selectors, and envelope senders consistent.
- ✓Monitor Yahoo complaint feedback loop results daily while ramping.
Helpful Resources
More Yahoo Mail SMTP Codes
Keep these related troubleshooting guides handy while addressing 421 4.7.0 [TS01].
Still experiencing Yahoo Mail delivery issues?
Run your domain through our free deliverability tools or book a session with Bento's deliverability specialists.