Question : How to Fix Error 451 (Local Policy Violation or Greylisting)

What is Error 451?

Error 451 typically indicates that the recipient's server has temporarily rejected your email due to a local policy violation or a process known as greylisting. Common reasons include:

• Greylisting: The recipient's server temporarily rejects emails from unknown senders to test if the sending server will retry.
• Policy Violations: The email content, sender reputation, or other factors may not comply with the recipient's email server policies.

This is a temporary error and usually resolves when the server retries sending the email.

How to Fix Error 451?

Follow these steps to address Error 451:

1. Wait for Automatic Retry:
   • Email servers are typically configured to retry sending emails after temporary failures.
   • Wait for the server to retry automatically, as greylisting often resolves itself after a short delay.
2. Check the Recipient's Email Policies:
   • Ensure your email complies with the recipient's email server policies, such as content guidelines and attachment restrictions.
3. Authenticate Your Emails:
   • Set up SPF, DKIM, and DMARC records to authenticate your domain and improve trustworthiness.
   • Authenticated emails are less likely to be greylisted or blocked.
4. Use a Trusted Sending Server:
   • Ensure your sending server has a good reputation and isn’t blacklisted.
   • Regularly monitor your server’s reputation using tools like MXToolbox.
5. Avoid Rapid Bulk Sending:
   • Sending large volumes of emails to the same server in a short period can trigger greylisting or policy violations.
   • Use throttling to space out emails and reduce the risk of being flagged.

Preventing Error 451 in the Future

• Regularly authenticate your domain with SPF, DKIM, and DMARC records.
• Follow email best practices to comply with recipient server policies.
• Space out bulk emails to prevent overwhelming recipient servers.
• Monitor your sending server's reputation and address any issues promptly.

Error 451 is usually temporary and resolves itself when the server retries sending the email. By following these steps and implementing best practices, you can reduce the chances of encountering this error in the future. For persistent issues, contact your email provider for assistance.

See the List of Common SMTP Errors