Most common causes
- Two routing rules or connectors send the same message back to each other
- Hybrid routing points a domain to the wrong side of the environment
- A forwarding or transport rule recreates a path already traversed
- MX or smart-host changes created a circular route
What to verify next
- Use Email Route Visualizer on the complete headers
- Identify repeating hosts or domains in the Received chain
- Compare connectors, transport rules and smart-host destinations
- Break the circular path before resending the message
Best diagnostic path
Live analysis
Mail Failure Doctor
Classify the complete rejection and preserve provider-specific diagnostic context.
Open analysis → Live analysisEmail Route Visualizer
Turn Received headers into an ordered path to expose loops and unexpected intermediaries.
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.