What is Error 4xx?

Error 4xx refers to a class of temporary email delivery failures. Common examples include:

  • 421: The server is not available and cannot process your email at the moment.
  • 450: The recipient's mailbox is temporarily unavailable, often due to server-side issues.

These errors are temporary and typically resolve themselves when the email server retries sending the message.

How to Fix Error 4xx?

Follow these steps to resolve and prevent temporary email delivery failures:

  1. Wait for the Email Server to Retry:
    • Most email servers are configured to retry sending emails automatically after a temporary failure.
    • If you receive this error, allow some time for the server to attempt resending the email.
  2. Check Recipient’s Mailbox Status:
    • Contact the recipient to ensure their mailbox is active and not temporarily unavailable (e.g., full or suspended).
  3. Verify Your Server Settings:
    • Ensure that your email server complies with the recipient's server policies.
    • Check for proper DNS configuration, including SPF, DKIM, and DMARC settings.
  4. Avoid Sending in Bulk During High-Traffic Periods:
    • Large-scale email campaigns sent during peak times can overwhelm recipient servers, leading to temporary rejections.
    • Use throttling to space out your email delivery over time.
  5. Monitor for Greylisting:
    • Some servers temporarily reject emails from unknown senders to test if the sending server retries (a process known as greylisting).
    • Ensure your server is set to retry automatically.

Preventing Error 4xx in the Future

  • Maintain a clean email list to avoid sending to inactive addresses.
  • Authenticate your domain with SPF, DKIM, and DMARC to build trust with recipient servers.
  • Monitor your server's reputation and ensure it's not blacklisted.
  • Space out bulk emails using throttling or a reliable email marketing platform.

Temporary failures like Error 4xx are common and often resolve themselves. However, by following these best practices, you can minimize the chances of encountering such issues and ensure smoother email delivery.

 

List of Common SMTP Errors

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