Summary
Understanding email bounces is crucial for maintaining good sender reputation and optimizing email deliverability. This page explores the distinctions between hard and soft bounces and addresses the critical question of whether to resend emails to hard bounced addresses, considering both traditional definitions and real-world ISP behavior.
Key findings
- Definition nuance: While SMTP 5xx codes are conventionally defined as hard bounces (permanent failures) and 4xx codes as soft bounces (temporary issues), actual ISP responses can blur this line.
- ISP non-compliance: Many email service providers (ESPs) observe 5xx codes that implicitly suggest retrying later, despite the RFC definition of a permanent failure.
- Sender reputation impact: Resending to email addresses that have resulted in a true hard bounce can significantly damage your sender reputation and lead to blocklisting or inbox placement issues.
- Ambiguous hard bounces: Some 5xx errors might indicate temporary technical issues or sender-specific blocks, rather than a permanently invalid address, such as specific reputation-based bounces.
Key considerations
- Automated bounce handling: It is generally recommended to automatically remove hard bounced addresses from your mailing list to protect your sender reputation. While some hard bounces may be recoverable, the risk of continued sending to an invalid address outweighs potential benefits. More information about automated handling can be found on Mailchimp's bounce management guidelines.
- Monitor SMTP codes: Pay close attention to the specific SMTP error codes and descriptions returned, as these can provide insights into whether a 5xx bounce is truly permanent or just a temporary rejection.
- List cleaning practices: Regularly clean your email lists to remove invalid or inactive addresses, even those that produce soft bounces repeatedly, to improve deliverability and avoid spam traps.
- Distinguish between permanent and transient: While RFCs provide guidelines, the practical interpretation of bounce codes by different ISPs means that relying solely on the leading digit (e.g., 5xx for hard) might not always be accurate for determining deliverability.
What email marketers say
Email marketers often face a dilemma when it comes to hard bounces. The common advice is to never resend to a hard bounced address due to the clear negative impact on sender reputation. However, some marketers acknowledge that the reality of bounce codes is not always black and white, leading to nuanced approaches to bounce management.
Key opinions
- Standard practice: Most marketers agree that resending to hard bounces is generally a bad idea because it can harm sender reputation and lead to blocklisting (or blacklisting) issues.
- Ambiguity in codes: Some hard bounce reports might contain errors that are not necessarily unrecoverable, causing confusion about whether an address is truly inactive or simply temporarily unavailable.
- ISP variations: Marketers observe that many receiving mail servers (MX owners) do not strictly adhere to RFC definitions for bounce codes, meaning a 5xx code might sometimes imply a temporary issue that permits retries.
- Importance of real-world data: The practical meaning of bounce codes as interpreted by ESPs, rather than strict RFC adherence, is what ultimately dictates effective bounce management strategies.
Key considerations
- Immediate removal: Despite ambiguities, it is often considered best practice to immediately mark hard bounced addresses as inactive to prevent further damage to sender reputation. This proactive approach helps to maintain domain and IP reputation.
- Beyond SMTP codes: Email marketers should look beyond just the numerical SMTP codes and consider the accompanying descriptive text from the mail server, which can provide clearer context for the bounce reason. SendLayer's guide on hard vs. soft bounces offers insights into fixing such issues.
- ESPs role: It is an ESP's responsibility to interpret and handle bounce responses effectively, accurately classifying them as permanent or temporary based on observed behavior from various mail servers.
- Strategic re-engagement: For soft bounces, a strategy of limited retries is acceptable, but for true hard bounces, focus should shift to re-engaging inactive subscribers through other channels rather than continuous email attempts.
Marketer view
Email marketer from Email Geeks indicates disagreement with the idea of resending to addresses that have indicated permanent failure. This perspective emphasizes protecting sender reputation as a top priority.
Marketer view
Email marketer from Email Geeks affirms the belief that resending to hard bounces is inherently a bad idea due to the detrimental effect on sender reputation. This highlights a common understanding among professionals in the field.
What the experts say
Experts in email deliverability provide a deeper technical perspective on bounce codes, often acknowledging the discrepancy between theoretical RFC definitions and the practical implementation by Internet Service Providers (ISPs). They emphasize that while RFCs offer guidelines, the real-world behavior of mail servers dictates effective bounce management strategies.
Key opinions
- SMTP code interpretation: Experts highlight that 5xx series SMTP codes generally indicate a hard bounce, meaning the specific email will not be delivered to that subscriber in any situation. Conversely, 4xx codes are soft bounces, prompting the Mail Transfer Agent (MTA) to retry delivery later.
- Real-world vs. RFC: It is widely acknowledged that ISPs often do not strictly adhere to RFC (Request For Comments) standards, especially concerning Non-Delivery Reports (NDRs). This means that a 5xx code might sometimes be used for what is practically a temporary or soft bounce.
- Technical nuance of hard bounces: Some hard bounces (e.g., 5.2.1 'Mailbox disabled, not accepting messages') might stem from technical issues or sender-specific blocks rather than an actually inactive account. This implies that not all 5xx codes signal a perpetually invalid address.
- Risk of resending: Despite ambiguities, resending to addresses that generate hard bounces (even those with a murky definition) is still considered damaging to sender reputation. For more on this, see how email blacklists (or blocklists) actually work.
Key considerations
- Adaptive bounce processing: ESPs must possess sophisticated systems to interpret diverse and sometimes non-standard bounce responses from various receiving domains, determining the true nature of the bounce (e.g., permanent vs. temporary) regardless of the initial SMTP code.
- Reputation management: The primary goal of bounce management is to protect sender reputation. This often means erring on the side of caution and removing addresses that consistently hard bounce, even if some edge cases might theoretically be recoverable. This is especially true for factors influencing BCL scores.
- Understanding RFC compliance: While RFCs provide foundational rules, it's essential to understand that real-world email systems can deviate. This reality necessitates a pragmatic approach to bounce analysis rather than strict adherence to theoretical definitions. A relevant discussion on this can be found on Word to the Wise.
- Granular bounce classification: Implementing systems that can differentiate between various types of 5xx bounces, identifying those that are truly permanent (e.g., invalid address) versus those that are temporary or policy-based (e.g., block by sender, full inbox), is crucial for advanced deliverability management.
Expert view
Deliverability expert from Email Geeks explains that the core principle is that 5xx series SMTP codes are generally considered hard bounces because they indicate that a specific email will not be delivered to that subscriber, regardless of retries. Conversely, 4xx codes are soft bounces, prompting the MTA to try again later.
Expert view
Deliverability expert from SpamResource comments that Mailgun indicates hard bounces mean that a message cannot be delivered due to an unchanging, permanent reason. They assert that there's nothing a sender can do to fix it, as the delivery simply isn't going to happen.
What the documentation says
Official documentation and industry standards define email bounces based on their permanence and the SMTP codes associated with them. While these definitions provide a foundational understanding, the real-world implementation by various mail servers often introduces complexities that require careful interpretation by senders.
Key findings
- Hard bounce definition: Documentation consistently defines a hard bounce as a permanent failure for an email to be delivered. This typically means the recipient's email address is invalid, does not exist, or the domain is incorrect.
- Soft bounce definition: A soft bounce indicates a temporary email delivery issue. Common reasons include a full inbox, the recipient's mail server being temporarily down or overloaded, or the email message being too large.
- SMTP code categories: RFCs categorize SMTP reply codes, with 5xx series codes signifying permanent negative completion replies (hard bounces) and 4xx series codes indicating temporary negative completion replies (soft bounces).
- Actionable insights: For hard bounces, documentation advises immediate removal of the address from the mailing list. For soft bounces, retries are usually attempted, but persistent soft bounces should also lead to list cleaning.
Key considerations
- Adherence to standards: While RFCs set the standard for SMTP communication, actual server responses can vary. Senders should primarily manage bounces based on their classification as permanent or temporary, which aligns with standard email bounce management best practices.
- Impact on sender reputation: Continually sending to invalid addresses (leading to hard bounces) negatively impacts your sender reputation, increasing the likelihood of future emails being marked as spam or blocked. This is often addressed in email bounce management guides.
- Automated bounce processing: Email service providers (ESPs) are designed to automatically process bounce notifications and take appropriate action, such as suppressing hard bounced addresses, to protect their clients' deliverability. Ensuring proper DMARC, DKIM, and SPF setup helps with accurate bounce responses.
- Categorization accuracy: Accurate categorization of bounces is vital. While most 5xx codes point to permanent issues, some sub-codes might indicate specific, recoverable problems, requiring deeper analysis.
Technical article
Mailchimp documentation clarifies that a hard bounce signals a permanent reason why an email cannot be delivered. It advises that in most cases, such hard bounced email addresses should be automatically removed or cleaned from your audience list to maintain list hygiene and sender reputation.
Technical article
SendLayer documentation states that hard bounce emails are those that have been permanently rejected by the receiving mail server. This means that immediate action is required by the sender, typically involving the removal of the address, as the email cannot be delivered in its current state.
Related resources
15 resources
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