Email bounce codes are error messages returned by mail servers when an email cannot be delivered. Understanding these codes helps you troubleshoot delivery issues, improve email campaign performance, and maintain a strong sender reputation. This article provides a detailed overview of traditional and enhanced bounce codes, their meanings, and actionable steps to address them.
When an email fails to deliver, the recipient’s mail server sends a bounce message to the sender, typically including a numeric SMTP status code and a description of the issue. These codes indicate whether the failure is temporary (soft bounce) or permanent (hard bounce), helping you identify and resolve the problem.
Soft Bounces: Temporary issues, such as a full mailbox or server downtime, that may resolve with retries.
Hard Bounces: Permanent failures, such as invalid or non-existent email addresses, require removal from your mailing list.
The table below lists common traditional (4xx, 5xx) and enhanced (e.g., 5.x.x) bounce codes, their explanations, and recommended actions.
Code | Type | Explanation | Action |
421 | Soft | Service not available, closing transmission channel (e.g., server overload or maintenance). | Retry sending after a delay (e.g., 4–24 hours). Check sending rate to avoid exceeding recipient server limits. |
450 | Soft | Mailbox unavailable (e.g., temporarily busy or server down). | Retry after a delay (e.g., 24 hours). If persistent, contact the recipient to confirm their mailbox status. |
451 | Soft | Action aborted due to processing error (e.g., temporary server issue). | Retry later. Monitor for repeated issues, which may indicate a configuration problem. |
452 | Soft | Insufficient system storage or too many messages. | Pause sending and retry after a delay (e.g., 24–48 hours). Advise recipient to clear their inbox. |
500 | Hard | Generic permanent failure (e.g., syntax error or unsupported command). | Check email configuration for errors. Contact your email service provider for support. |
501 | Hard | Syntax error in parameters or arguments (e.g., invalid email format). | Verify and correct the recipient’s email address format. |
502 | Hard | Command not supported by the server. | Ensure your email client uses supported commands. Update settings if needed. |
503 | Hard | Server requires authentication or specific command sequence. | Confirm SMTP authentication settings (e.g., correct credentials). |
550 | Hard | Mailbox unavailable (e.g., does not exist or is blocked). | Remove the email address from your list. Verify the address or contact the recipient via another channel. |
551 | Hard | Recipient not local to the server, and forwarding failed. | Confirm the email address or check for forwarding issues with the recipient’s domain. |
552 | Hard/Soft | Storage allocation exceeded (e.g., mailbox full). | Treat as a soft bounce initially and retry later. If persistent, remove the address. |
553 | Hard | Invalid mailbox name. | Remove the email address from your list and verify its accuracy. |
554 | Hard | Transaction failed (e.g., rejected due to spam filters or blacklisting). | Check sender reputation and email content for spam triggers. Contact recipient server admin if needed. |
5.0.0 | Hard | Address does not exist. | Remove the email address from your list. |
5.1.0 | Hard | Other or undefined mailbox status. | Review bounce message for details and remove the address if invalid. |
5.1.1 | Hard | Bad destination mailbox address. | Remove the email address from your list. |
5.1.6 | Hard | Mailbox has moved or is no longer active. | Remove the email address and verify with the recipient via another channel. |
5.1.7 | Hard | Bad sender’s mailbox address syntax. | Check and correct your sender email address format. |
5.1.8 | Hard | Bad sender’s system address. | Verify your email server configuration and sender address. |
5.2.0 | Hard/Soft | Mailbox issue (e.g., disabled or temporarily unavailable). | Retry if temporary; remove if persistent. |
5.2.1 | Hard | Mailbox disabled, not accepting messages. | Remove the email address from your list. |
5.2.2 | Hard/Soft | Mailbox full. | Retry after a delay. If persistent, treat as a hard bounce and remove. |
5.2.3 | Hard | Message exceeds size limit. | Reduce message size (e.g., compress attachments) and retry. |
5.2.4 | Hard | Mailing list expansion problem. | Check mailing list configuration and remove problematic addresses. |
5.3.0 | Hard | Undefined mail system status. | Review bounce message for details and contact server admin if needed. |
5.3.1 | Hard/Soft | Mail system full. | Retry after a delay. If persistent, contact recipient server admin. |
5.3.4 | Hard | Message too big for system. | Reduce message size and retry. |
5.4.0 | Hard | Undefined network or routing status. | Check network settings and retry. Contact server admin if unresolved. |
5.4.1 | Hard | No answer from host. | Verify recipient domain and retry. Remove if persistent. |
5.4.2 | Hard | Bad connection. | Check network connectivity and retry. Contact server admin if needed. |
5.4.3 | Hard | Routing server failure. | Contact recipient server admin to resolve routing issues. |
5.4.4 | Hard | Unable to route. | Verify recipient domain and remove if invalid. |
5.4.5 | Hard/Soft | Network congestion. | Retry after a delay. Monitor for repeated issues. |
5.4.6 | Hard | Routing loop detected. | Contact recipient server admin to resolve routing configuration. |
5.4.7 | Hard | Delivery time expired. | Remove the email address and verify recipient details. |
5.5.0 | Hard | Undefined protocol status. | Check email client settings and contact server admin. |
5.5.1 | Hard | Invalid command. | Update email client to use supported commands. |
5.5.2 | Hard | Syntax error. | Correct email syntax and retry. |
5.5.3 | Hard | Too many recipients. | Reduce the number of recipients and retry. |
5.5.4 | Hard | Invalid command arguments. | Verify command arguments in your email client settings. |
5.6.0 | Hard | Undefined media error. | Check email content (e.g., attachments) and retry. |
5.7.0 | Hard | Undefined security status. | Review security settings (e.g., SPF, DKIM) and retry. |
5.7.1 | Hard | Delivery not authorized, message refused. | Check sender reputation and email content for spam triggers. |
5.7.2 | Hard | Mailing list expansion prohibited. | Verify mailing list permissions and configuration. |
5.7.3 | Hard | Security conversion required but not possible. | Update security protocols and retry. |
5.7.4 | Hard | Security features not supported. | Ensure compatibility with recipient server’s security settings. |
5.7.5 | Hard | Cryptographic failure. | Check encryption settings and certificates. |
5.7.6 | Hard | Cryptographic algorithm not supported. | Update to a supported algorithm and retry. |
5.7.7 | Hard | Message integrity failure. | Verify email content and security settings. |
Mail servers may include custom messages alongside bounce codes, providing specific details about the failure. Always review these messages for additional context, as they may indicate unique issues like spam filtering, domain-specific policies, or temporary outages.
Analyze Bounce Messages: Identify the code and any custom message to determine the issue’s nature (soft or hard bounce).
Remove Hard Bounces: Immediately remove invalid or non-existent email addresses to prevent further delivery attempts and protect your sender reputation.
Retry Soft Bounces: Schedule retries for soft bounces after a delay (e.g., 24–48 hours). Limit retries to avoid being flagged as spam.
Monitor Sender Reputation: Use tools like Postmark, SendGrid, or Return Path to track bounce rates and address issues promptly.
Validate Email Lists: Employ email verification services to check addresses before sending.
Ensure Compliance: Follow anti-spam guidelines, include unsubscribe options, and avoid content that triggers spam filters.
Implement Double Opt-In: Confirm email addresses during signup to reduce invalid entries.
Clean Your List Regularly: Remove inactive or bouncing addresses to maintain list hygiene.
Authenticate Emails: Use SPF, DKIM, and DMARC to improve deliverability and avoid rejections.
Monitor Server Health: Ensure your email server is not blacklisted and is properly configured.
Test Emails: Send test messages to verify content, formatting, and delivery settings.
Email bounce codes are essential for diagnosing and resolving email delivery issues. By understanding traditional and enhanced codes, you can take targeted actions to improve deliverability, reduce bounce rates, and maintain a strong sender reputation. Regularly monitor bounce messages, clean your email lists, and adhere to email best practices to ensure successful communication.
For further assistance, contact your email service provider or reach out to our support team.
