Summary
Understanding bounce message error codes is critical for effective email deliverability and maintaining a healthy sender reputation. Bounce messages consist of a three-digit SMTP code, enhanced status codes, and free text, with the SMTP code being the primary indicator. A key distinction is between hard bounces (permanent failures like invalid addresses) and soft bounces (temporary issues like full inboxes). Experts emphasize removing hard bounces immediately and retrying soft bounces later. However, interpreting error codes can be complex and inconsistent across ISPs and ESPs, with some not adhering to RFCs or stripping extended codes. Robust bounce handling systems require flexibility, address validation, and regular list cleaning to adapt to these inconsistencies and improve engagement.
Key findings
- SMTP Codes: SMTP codes are a key element, where the first digit indicates success, a transient error, or a permanent error (RFC 5321).
- Hard vs. Soft Bounces: Hard bounces (permanent failures) should be removed immediately, while soft bounces (temporary issues) can be retried.
- ISPs Inconsistency: The meaning of specific error codes can vary across ISPs and ESPs; some may not adhere to RFCs or might strip extended codes.
- Parsing and Classifying: Parsing SMTP codes and classifying bounce reasons (invalid recipient, mailbox full, etc.) is essential for automated handling.
- NDRs provide details: NDRs from Exchange Online provide Enhanced Status Codes
Key considerations
- Robust System: A robust and flexible bounce handling system is necessary to adapt to inconsistencies in error code interpretations.
- Regular Cleaning: Regularly clean the email list by removing hard bounces and unengaged subscribers.
- Address Validation: Validate email addresses to minimize hard bounces and protect sender reputation.
- Segmentation: Segment email lists to improve deliverability by targeting the right audience
- Monitoring: Monitor bounce rates and messages for content/infrastructure issues
What email marketers say
9 marketer opinions
Understanding bounce message error codes is crucial for maintaining a healthy email list and sender reputation. Email marketers emphasize differentiating between hard bounces (permanent failures like invalid addresses) and soft bounces (temporary issues like full inboxes). Hard bounces should be immediately removed from the list, while soft bounces can be retried. Analyzing bounce messages helps identify issues with email addresses, server configurations, or content. Implementing a bounce management system and regularly cleaning the email list are vital for improving deliverability and engagement.
Key opinions
- Hard vs. Soft Bounces: Distinguish between hard bounces (permanent failures) and soft bounces (temporary issues).
- Immediate Removal: Remove hard bounces from the email list immediately.
- Regular Cleaning: Regularly clean the email list to maintain a low bounce rate.
- SMTP Codes: SMTP codes such as 550 indicate permanent failure while 450 are temporary failures.
- Bounce insights: Bounce messages provide valuable insights for improving email deliverability and engagement.
Key considerations
- Bounce Management System: Implement a bounce management system to handle bounce messages effectively.
- Sender Reputation: Prioritize managing your bounce rate to protect your sender reputation.
- List Segmentation: Segment your email lists to target the right audience to improve deliverability.
- Code Variations: Be aware that specific SMTP error codes may have variations between ISPs
- Address Validation: Validate email addresses to ensure emails get delivered
Marketer view
Email marketer from SendGrid shares that understanding bounce codes helps identify and segment problematic email addresses. Implementing a bounce management system based on these codes is crucial for maintaining sender reputation and improving deliverability.
4 Jul 2022 - SendGrid
Marketer view
Email marketer from Reddit explains that you should pay attention to hard bounces (5xx errors) and remove those addresses from your list immediately to avoid damaging your sender reputation. Soft bounces (4xx errors) may be temporary, so you can retry sending to them later.
16 Mar 2024 - Reddit
What the experts say
4 expert opinions
Experts emphasize the importance of understanding bounce messages for effective email deliverability. Bounce messages consist of SMTP codes, enhanced status codes, and free text, with the SMTP code being the primary indicator. A key distinction is between hard (permanent) and soft (temporary) bounces. However, the interpretation of error codes can be complex and inconsistent across different ISPs and ESPs. A flexible bounce handling system is crucial, considering that many ISPs may not adhere to RFCs, and extended SMTP codes may be absent or stripped by some ESPs.
Key opinions
- Bounce Message Structure: Bounce messages include SMTP codes, enhanced status codes, and free text.
- Hard vs. Soft Bounces: Distinguish between hard (permanent) and soft (temporary) bounce types for list hygiene.
- Inconsistent Interpretation: The meaning of specific error codes can vary across ISPs and ESPs.
- Lack of RFC Adherence: Many ISPs may not adhere to RFC standards when generating response codes.
- Missing Extended Codes: A significant portion of bounces may lack extended SMTP codes.
Key considerations
- Focus on SMTP Codes: Prioritize the three-digit SMTP response code for initial analysis.
- Flexible Handling: Implement a flexible bounce handling system to adapt to inconsistent codes.
- Resource Awareness: Be aware that many ISPs may not adhere to the RFCs.
- Account for Missing Data: Design bounce handling with the understanding that some ESPs might strip SMTP data.
- Code Variability: Be aware that the meaning of error codes is not fixed and may differ depending on ISP.
Expert view
Expert from Email Geeks explains the three parts of a bounce message: the three-digit SMTP code (defined by RFC 5321), the enhanced mail system status code (RFC 3463), and free text created by the ISP. The SMTP code is the most important, and the rest is additional detail. He recommends keeping bounce handling rules as simple as possible, focusing on the three-digit SMTP response code, and adjusting for specific problems. Also shares links to resources with more information.
1 Sep 2024 - Email Geeks
Expert view
Expert from Word to the Wise shares that bounce handling is complex because the meaning of specific error codes can vary between ISPs and email service providers. Creating a robust bounce processing system needs flexibility to handle inconsistent bounce codes and messages.
29 Aug 2021 - Word to the Wise
What the documentation says
5 technical articles
Documentation emphasizes the importance of SMTP reply codes, particularly the first digit, which indicates the status of the email delivery (success, transient error, or permanent error). Bounce codes are standardized responses indicating delivery failure reasons, categorized as hard bounces (permanent issues) and soft bounces (temporary issues). NDRs provide enhanced status codes with granular details about delivery problems. Bounce classification categorizes bounces by reason, like 'invalid_recipient' or 'mailbox_full,' aligning with SMTP codes for optimal deliverability. Effective bounce message handling requires parsing codes and messages to automate subscriber status updates and maintain clean lists.
Key findings
- SMTP Reply Codes: SMTP reply codes are three-digit numbers indicating success, transient error, or permanent error based on the first digit.
- Hard vs. Soft Bounces: Bounce codes differentiate between hard bounces (permanent failures) and soft bounces (temporary issues).
- Enhanced Status Codes: NDRs in Exchange Online contain enhanced status codes providing detailed information about delivery problems.
- Bounce Classification: Bounces are classified into categories corresponding to specific SMTP error codes to guide handling.
- Automated Handling: Effective bounce message handling enables automated subscriber status updates and list maintenance.
Key considerations
- Initial Digit Analysis: Focus on the first digit of SMTP reply codes to quickly assess the delivery status.
- Categorize Bounce Reasons: Categorize bounce reasons based on the specific bounce classification provided by the sending platform.
- Implement Parsing Logic: Implement parsing logic to extract and interpret SMTP codes and messages.
- Automate List Maintenance: Use bounce message data to automate subscriber status updates and maintain clean lists.
- Utilize Enhanced Codes: Where available use enhanced status codes to provide granular detail about delivery problems
Technical article
Documentation from RFC 5321 explains that SMTP reply codes are three-digit numbers where the first digit indicates success, error, or incomplete status. Codes starting with 2 indicate success, 4 indicate transient errors (retryable), and 5 indicate permanent errors.
13 Jul 2021 - RFC 5321
Technical article
Documentation from Oracle explains that bounce message handling involves parsing the SMTP response codes and messages to determine the reason for the bounce. This allows for automated processes to update subscriber statuses and maintain a clean mailing list.
20 Aug 2023 - Oracle
What are common email bounce messages and what do they mean?
Is mailbox full still a valid email bounce message?
How can I monitor Gmail SMTP response codes for bounce monitoring?
How do I troubleshoot email bounce messages?
How are email bounce rates calculated and what is considered a good bounce rate?
Can 'invalid recipient' bounce messages be false positives and what should I do about it?
