Most common causes
- The account exceeded outbound sending limits
- The sender is suspected of spam or abusive activity
- The account was compromised and subsequently restricted
What to verify next
- Review account security before attempting to restore sending
- Verify whether the sender exceeded limits or generated unexpected volume
- Have the organization’s email administrator follow the provider remediation process
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 NDR 550 5.1.8 reference.