Most common causes
- The sending IP has low reputation
- The IP is new and has little positive history
- The outbound path changed to an unexpected IP
- The sending infrastructure is associated with abusive traffic
What to verify next
- Confirm the actual outbound IP in message and SMTP evidence
- Verify PTR, authentication and sender identity consistency
- Review whether the IP recently changed or was newly introduced
- Follow Microsoft’s remediation path when infrastructure is otherwise legitimate
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.