421 4.7.0 [TSS04]
TSS04 Temporary Deferral
Yahoo deferred further messages because the IP is sending more than its reputation allows.
TSS04 is the classic warmup throttle. Yahoo slows deliveries when an IP expands too quickly or after complaint spikes.
Severity
WARNING
Provider
Yahoo Mail
Reference
TSS04
Last Reviewed
2025-06-01
How to Fix 421 4.7.0 [TSS04]
Follow these steps to resolve this Yahoo Mail SMTP error and restore normal delivery.
- ✓Throttle campaigns and resume with smaller hourly caps.
- ✓Check complaint feedback loop and unsubscribe processing for issues.
- ✓Verify SPF, DKIM, and DMARC alignment for every domain in use.
- ✓Scrub recent imports or low-engagement cohorts before resuming.
Helpful Resources
More Yahoo Mail SMTP Codes
Keep these related troubleshooting guides handy while addressing 421 4.7.0 [TSS04].
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