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Advice and answers from the Autoklose Team
Most common Non-Delivery Reports (NDRs) or Bounce messages

Email bounce codes indicate the reason an email was not successfully delivered. They provide good input on potential problems with email accounts, message content, and contact lists.

Autoklose uses AI to automatically recognize bounces and put them in the Do Not Email list (DNE). As long as in the DNE list, such contacts will not be reached again.

Here is the list of most common NDRs their explanation and potential resolution:

1. Invalid Recipient Address

  • Error Example: 550 5.1.1 User unknown
  • Explanation: The recipient’s email address either doesn’t exist, was deleted, or was mistyped. This error is common when a domain or email account is inactive.
  • Resolution:
    1. Double-check the email address for typos.
    2. Confirm with the recipient that the address is correct.
    3. If it’s an old contact, request an updated email address.

2. Mailbox Full

  • Error Example: 552 5.2.2 Mailbox full
  • Explanation: The recipient has reached their email storage limit and cannot accept new messages.
  • Resolution:
    1. Notify the recipient through an alternative communication method.
    2. Suggest they clear their mailbox or increase their storage quota.
    3. Try resending the email after some time.

3. Blocked by Spam Filters

  • Error Example: 550 5.7.1 Message rejected due to content restrictions
  • Explanation: The email was flagged as spam because of its content, attachments, or sending server's reputation.
  • Resolution:
    1. Avoid using spammy words like "Free," "Act Now," or excessive punctuation (!!!).
    2. Use a professional email signature.
    3. Ensure your email domain has proper authentication (SPF, DKIM, DMARC).
    4. Test your email content with spam-checking tools like Mail Tester.

4. Relay Access Denied

  • Error Example: 554 5.7.1 Relay access denied
  • Explanation: The sending server is not permitted to relay messages through the recipient’s server. This often happens with misconfigured email servers.
  • Resolution:
    1. Ensure your email server is authenticated and authorized to send emails to the recipient.
    2. Update your server's settings to prevent open relay configurations.
    3. Contact your email provider or IT team to resolve server authorization.

5. Recipient Server Unavailable

  • Error Example: 451 4.4.0 Temporary server error
  • Explanation: The recipient’s email server is temporarily down or unreachable due to maintenance, network issues, or overloading.
  • Resolution:
    1. Wait and try resending the email later.
    2. If the issue persists, notify the recipient through other means to inform their IT team.

6. Invalid Email Format

  • Error Example: 554 5.6.0 Invalid message content
  • Explanation: The email message doesn’t follow proper formatting rules. This may include missing headers, corrupted encoding, or unsupported characters.
  • Resolution:
    1. Use a reputable email client to draft and send emails.
    2. Ensure the email includes essential headers like "From," "To," and "Subject."
    3. Avoid using unsupported or special characters in the subject line or email body.

7. Attachment Size Exceeded

  • Error Example: 552 5.3.4 Message size exceeds fixed maximum
  • Explanation: The email’s size, usually due to attachments, exceeds the recipient’s server limit. Common limits are 10–25 MB.
  • Resolution:
    1. Compress large files or use a file-sharing service (e.g., Google Drive, Dropbox, or WeTransfer).
    2. Inform the recipient and confirm their server’s attachment size limit.

8. Authentication Issues

  • Error Example: 535 5.7.8 Authentication credentials invalid
  • Explanation: The sender failed to authenticate with the email server, often due to incorrect login credentials or misconfigured settings.
  • Resolution:
    1. Verify your email account credentials (username/password).
    2. Ensure your outgoing mail server (SMTP) settings are correct.
    3. Contact your IT support or email provider to reset credentials or check configurations.

9. Domain Not Found

  • Error Example: 550 5.1.2 Domain not found
  • Explanation: The domain in the recipient’s email address doesn’t exist or can’t be resolved via DNS (Domain Name System).
  • Resolution:
    1. Double-check the domain spelling (e.g., gmail.com vs. gmal.com).
    2. If the domain is correct, confirm with the recipient that it’s active.
    3. Notify your IT team if DNS resolution issues persist on your side.

10. Blocked Sender/Blacklisted IP

  • Error Example: 554 5.7.1 Rejected: IP address is blacklisted
  • Explanation: Your IP address or domain is blacklisted by the recipient’s server, often due to spam complaints or poor sender reputation.
  • Resolution:
    1. Check your domain/IP reputation using tools like MXToolbox or Talos.
    2. Remove your IP/domain from blacklists by following instructions provided by the blacklist authority.
    3. Strengthen email authentication with SPF, DKIM, and DMARC records.
    4. Limit bulk emails and avoid sending spam-like content.

11. Too Many Emails Sent

  • Error Example: 452 4.5.3 Too many recipients
  • Explanation: The sender exceeded the recipient server’s limits for the number of recipients or emails sent within a specific time frame.
  • Resolution:
    1. Limit the number of recipients per email and the rate of sending (provide more time during the day for campaigns to send).
    2. Wait before resending or spread out emails over time.

12. DNS Failures

  • Error Example: 550 5.4.4 Unable to resolve domain
  • Explanation: The sending server could not resolve the recipient’s domain due to DNS issues.
  • Resolution:
    1. Check your DNS settings and ensure they’re configured correctly.
    2. Verify the recipient’s domain is active and reachable.
    3. Contact your IT team or email provider to troubleshoot DNS issues.

13. Email Loop Detected

  • Error Example: 500 5.4.6 Routing loop detected
  • Explanation: The email is stuck in a loop between two servers due to a misconfiguration in routing or forwarding.
  • Resolution:
    1. Check your server’s forwarding rules to ensure they don’t create loops.
    2. Update any DNS or MX records causing repeated redirections.
    3. Contact IT support to resolve server misconfigurations.

14. Policy Violation

  • Error Example: 550 5.7.0 Policy-related rejection
  • Explanation: The email violates the recipient’s server policies, such as restrictions on bulk emails, marketing messages, or attachments.
  • Resolution:
    1. Review the recipient’s email policy (if available) and adjust your email accordingly.
    2. Ensure your emails are GDPR, CAN-SPAM, or similar compliance-aligned.
    3. Contact the recipient’s IT team to discuss policy exceptions if necessary.

15. Exceeding email provider sending limit
When emails bounce due to exceeding your email provider's sending limit, it typically indicates that you've sent more emails than allowed within a given time frame. This is common with free or lower-tier email accounts, and email accounts with a bad reputation (spam complaints, authentication issues - SPF, DKIM, DMARC, etc).

  • Error Examples:
    1. 421 4.7.0 [Temporary Deferral] Sending Limits Exceeded
      • Explanation: The sending limit has been temporarily reached. Try again later.
    2. 452 4.2.2 Mailbox Full or Sending Limit Exceeded
      • Explanation: Sending was restricted due to exceeding daily or hourly limits.
    3. 550 5.4.5 Daily Sending Quota Exceeded
      • Explanation: The provider has blocked further emails because you surpassed the daily quota.
    4. 554 5.7.1 Too Many Messages Sent or Too Many Recipients
      • Explanation: The number of recipients in your email or the volume of emails sent exceeds the provider’s allowed limits.
  • Resolution:
    1. Consider upgrading to a business or premium plan with email provider that offers higher sending limits.
    2. Distribute your emails over time to stay within hourly or daily limits.
    3. Ensure your list is clean and only includes engaged users to minimize unnecessary emails.
    4. Know your provider's limits and monitor your sending activity to avoid exceeding them.

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Happy klosing! :)

Your autoklose.com team!

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