Most common causes
- The recipient address is misspelled or no longer exists
- An alias or proxy address was removed
- Directory synchronization has not produced the expected address
- A migration or domain transition left the sender using an obsolete recipient identity
What to verify next
- Verify the exact SMTP address from the NDR
- Confirm whether the recipient or alias exists in the destination directory
- Check recent migration, alias and directory-sync changes
- Remove obsolete cached recipient addresses from the sending workflow
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.