Most common causes
- A compromised account or system is generating abusive traffic
- Unexpected bulk traffic crossed Microsoft enforcement thresholds
- A connector or routing path is being abused
- A recently renewed or changed tenant state may need administrative review
What to verify next
- Treat unexpected volume as a potential security incident
- Review compromised accounts, connectors and recent volume changes
- Stop abusive traffic before requesting remediation
- Follow the provider’s administrative recovery path
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 Microsoft Exchange Online NDR reference.