Most common causes
- Message volume accelerated faster than the sender’s recent pattern
- A new or recently changed IP began sending significant traffic
- SPF or DKIM authenticated traffic crossed a temporary quota or reputation threshold
- A compromised application or credential caused an unexpected burst
What to verify next
- Preserve the complete 4.7.28 diagnostic because Gmail may identify the affected quota
- Pause aggressive retries and allow the temporary condition to cool down
- Compare recent volume, source IPs and authenticated domains
- Investigate unexpected traffic before resuming normal sending
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 Gmail SMTP errors and codes.