Most common causes
- Temporary rate or traffic-management policy
- Reputation or complaint-related deferral
- Infrastructure or destination capacity condition
- A provider-specific diagnostic following 421 identifies the actual temporary cause
What to verify next
- Preserve the full Yahoo response text
- Respect retry and backoff behavior
- Review recent traffic and complaint changes
- Avoid converting a temporary response into an aggressive retry storm
Best diagnostic path
Live analysis
Mail Failure Doctor
Classify the complete rejection and preserve provider-specific diagnostic context.
Open analysis → Live analysisEmail Infrastructure Digital Twin
Map the domain’s public mail infrastructure and provider relationships before remediation.
Open analysis →Why the exact message matters
The same status family can be triggered by different conditions, and providers frequently add diagnostic text that narrows the issue. Use the complete rejection text rather than treating the numeric code as a complete diagnosis.
Provider reference
For the provider-defined meaning and current requirements, review Yahoo Sender Hub SMTP error codes.