How can you interpret extended SMTP responses (e.g., 5.7.1)?

Interpreting 5.7.1 as an example:

5 (First digit): Permanent failure. Do not retry. Address the underlying issue.

7 (Second digit): Security or policy issue. Related to authentication, authorization, or policy.

1 (Third digit): Delivery not authorized, message refused. Sender lacks permission to deliver to this recipient.

Combined meaning:

"Your message was permanently rejected due to security or policy: you are not authorized to send to this recipient."

Common interpretations:

Sender blocklisted. SPF/DKIM/DMARC failure. Spam filter rejection. Policy-based blocking.

Response:

Check your authentication setup. Review sender reputation. Investigate if blocklisted. May need to contact recipient admin for whitelisting.

Read each digit as narrowing the diagnosis. Class > Subject > Detail.