Most common causes
- The SMTP client is configured with the destination hostname as its own HELO identity
- A copied mail-server configuration preserved the wrong hostname
- A relay or appliance generates an invalid EHLO name
What to verify next
- Run Reverse DNS & HELO Auditor for the sending host
- Capture the SMTP conversation and confirm the EHLO value
- Configure a stable fully qualified identity for the sending server
- Verify the corrected identity also resolves consistently where appropriate
Best diagnostic path
Live analysis
Mail Failure Doctor
Classify the complete rejection and preserve provider-specific diagnostic context.
Open analysis → Live analysisReverse DNS & HELO Auditor
Check reverse identity, forward confirmation and SMTP greeting consistency.
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.