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SMTP Bounce Code 5.7.1: Delivery Not Authorized

SMTP bounce code 5.7.1 indicates that the message was rejected due to security or policy restrictions. The receiving mail server has determined that you are not authorized to send emails to the recipient, often due to spam filtering, authentication failures, or policy violations.

What Does 5.7.1 Mean?

The enhanced status code 5.7.1 follows the SMTP Enhanced Status Code format:

  • 5 = Permanent failure (hard bounce)
  • 7 = Security or policy status
  • 1 = Delivery not authorized, message refused

When you receive a 5.7.1 bounce, it means the receiving mail server has blocked your email due to security policies, authentication issues, or spam filtering. This is a serious issue that requires immediate attention to your email authentication and sender reputation.

Bounce Type

  • Type: Rejection (permanent failure)
  • Category: Content/Auth
  • Action Required: Review email authentication, sender reputation, and content policies

Common Causes

  1. SPF Authentication Failure: Your sending domain's SPF record doesn't authorize your sending IP address
  2. DKIM Signature Missing or Invalid: Your emails lack a valid DKIM signature or the signature verification failed
  3. DMARC Policy Violation: Your email fails DMARC authentication checks
  4. IP Reputation Issues: Your sending IP address is on a blocklist or has a poor reputation
  5. Domain Reputation: Your sending domain has a poor reputation or is flagged as spam
  6. Content Filtering: The email content triggered spam filters or policy violations
  7. Rate Limiting: You're sending too many emails too quickly, triggering rate limits
  8. Recipient Policy: The recipient's organization has policies that block emails from your domain or IP

How to Resolve

For Email Marketers

  1. Check Email Authentication: Verify that SPF, DKIM, and DMARC records are properly configured for your sending domain
  2. Review Sender Reputation: Check your IP and domain reputation using tools like:
    • Sender Score
    • Google Postmaster Tools
    • Microsoft SNDS
  3. Check Blocklists: Verify that your sending IP addresses aren't on any major blocklists:
    • Spamhaus
    • Barracuda
    • SURBL
    • SpamCop
  4. Review Email Content: Ensure your email content doesn't trigger spam filters:
    • Avoid spam trigger words
    • Maintain proper text-to-image ratios
    • Include proper unsubscribe links
    • Avoid excessive use of capital letters or exclamation marks
  5. Warm Up Your IP: If you're using a new IP address, gradually increase sending volume over several weeks
  6. Monitor Bounce Rates: Keep your bounce rate below 2% and complaint rate below 0.1%

For Developers

  1. Implement SPF Records: Ensure your domain has proper SPF records authorizing your sending IPs
  2. Set Up DKIM Signing: Configure DKIM signing for all outgoing emails
  3. Configure DMARC: Set up DMARC policies to protect your domain and improve deliverability
  4. Monitor Authentication: Track SPF, DKIM, and DMARC pass rates
  5. Implement Rate Limiting: Control sending rates to avoid triggering rate limits
  6. Log Authentication Failures: Track which emails fail authentication to identify issues

Email Authentication Checklist

  • SPF record is properly configured and includes all sending IPs
  • DKIM signing is enabled and keys are valid
  • DMARC policy is set (start with p=none for monitoring)
  • Reverse DNS (PTR) records are configured for sending IPs
  • Sending domain matches the From address domain
  • Unsubscribe links are present and functional

Related Bounce Codes

Examples

Example Bounce Message

550 5.7.1 Delivery not authorized
Message rejected due to security policy.

Example Enhanced Status Code

550 5.7.1 <[email protected]>: Delivery not authorized, message refused due to SPF policy

Common Email Provider Responses

  • Gmail: "Message rejected due to domain policy"
  • Outlook/Hotmail: "550 5.7.1 Delivery not authorized"
  • Enterprise: "Message rejected due to security policy"
  • Custom: "SPF authentication failed"

Best Practices

  1. Always Authenticate: Never send emails without proper SPF, DKIM, and DMARC configuration
  2. Monitor Reputation: Regularly check your sender reputation and address issues promptly
  3. Maintain List Hygiene: Keep your email list clean to avoid spam complaints
  4. Follow Best Practices: Adhere to email marketing best practices and CAN-SPAM/CASL/GDPR requirements
  5. Test Before Sending: Use email testing tools to verify authentication before sending campaigns
  6. Gradual Scaling: When increasing sending volume, do so gradually to avoid triggering filters
  7. Respond to Issues Quickly: If you notice authentication failures, investigate and fix them immediately