Most common causes
- The destination address is on the organization’s block list
- The destination domain is blocked by tenant policy
- A blocked recipient in a multi-recipient message causes the message to be rejected
What to verify next
- Confirm the exact blocked recipient or domain in the NDR
- Review tenant allow/block policy with the organization’s email administrator
- Retest after the policy entry is corrected
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.