Why don't mailbox providers publish detailed bounce message explanations?

Key findings

• Vague responses: MBPs sometimes intentionally keep bounce messages vague, particularly when they suspect malicious activity or when the issue is beyond simple remediation. The message might be generalized if the sender has done something intentionally bad.
• Resource constraints: Postmaster teams at MBPs are often under-resourced, making it difficult to dedicate time to developing and maintaining comprehensive, dynamic documentation for every conceivable bounce scenario. Publishing and updating such detailed information requires significant effort, which might not be prioritized over other system operations.
• Self-explanatory nature: Many MBPs believe their bounce messages, especially those adhering to SMTP RFC standards, are already clear enough. They expect senders or their Email Service Providers (ESPs) to understand standard error codes and their implications, as discussed in our guide on SMTP bounce codes and their meaning.
• Risk of misuse: Providing overly specific bounce reasons could inadvertently give spammers and bad actors more information, allowing them to refine their techniques and bypass filters more easily. This secrecy helps protect the integrity of the email ecosystem, as outlined in discussions about common email bounce messages.

Key considerations

• Interpret existing data: Senders should focus on thoroughly analyzing the information already provided in bounce messages and postmaster sites. Generic codes can sometimes be cross-referenced with additional context for a more specific diagnosis.
• Improve internal processes: Instead of waiting for MBPs to publish more data, senders and ESPs should invest in better internal training and systems to process and interpret existing bounce information more effectively. This includes understanding the nuances of how ESPs manage soft and hard bounces.
• Professional standards: A higher standard of knowledge among deliverability professionals regarding SMTP RFCs and common error patterns can mitigate the perceived lack of clear explanations.
• Dynamic policies: Recognize that many blocklist and rejection reasons are dynamic and depend on real-time sender reputation, making static documentation quickly outdated. This dynamic nature means that the information might not be explicitly published as it changes frequently, as is the case for blocklist reasons.

Key opinions

• Need for clarity: Marketers believe that a comprehensive list of error meanings would save them considerable time and resources by enabling them to adapt their sending practices more appropriately. The current system often leaves them guessing about the root cause of issues, like when emails bounce due to policy or spam filters.
• Operational efficiency: From a marketer's perspective, clearer bounce messages would directly translate into improved deliverability rates and a better understanding of list hygiene, helping them to manage issues like soft bounce tolerance.
• Reducing support burden: It is argued that by publishing detailed FAQs or documentation, MBPs could reduce the volume of repeat inquiries they receive regarding bounce errors.
• Impact on sender reputation: High bounce rates, often exacerbated by unclear error messages, can severely damage a sender's reputation, potentially leading to blocks. Marketers need to understand these errors to maintain a healthy sender reputation, as explained when discussing how different bounce types affect ESP classification.

Key considerations

• Leverage available resources: Marketers should make the most of what is currently available, such as postmaster sites and common SMTP error code manuals, even if they are not exhaustive. Many bounces contain enough information for initial troubleshooting.
• Engage with ESPs: Work closely with ESPs to get more specific bounce log data and interpretations, as ESPs often have more detailed internal logging and insights into the specific meanings of varied bounce codes.
• Continuous learning: Invest in ongoing education for deliverability teams to better understand SMTP protocols and the general categories of bounce errors. Understanding why email bounces happen generally is crucial.
• Feedback loops: Utilize feedback loops provided by MBPs to receive direct notifications about spam complaints and other issues affecting deliverability, which can indirectly inform about potential bounce triggers.

Key opinions

• Intentional vagueness: Experts suggest that MBPs may intentionally provide vague responses if the sender has engaged in malicious or undesirable behavior. Specific error codes might inadvertently assist spammers in bypassing detection, therefore they aim to provide information only when a sender can do something about the issue.
• Resource allocation: Postmaster and SMTP teams are often under-resourced, making it impractical to constantly update public documentation for every dynamic bounce scenario. The effort required to publish and maintain such comprehensive information is immense and often outweighed by other operational priorities.
• Sender responsibility: The information about bounce issues is often published across various platforms, including postmaster pages and within the bounce messages themselves. Experts feel that senders should be more proactive in reading and understanding these existing resources before seeking further clarification, especially concerning issues like why getting useful support from MBPs is challenging.
• Complexity of policies: MBP policies and blacklists (or blocklists) are complex and dynamic. A single generic bounce code might represent several underlying issues that change based on sender reputation, volume, or content, making a static, detailed explanation difficult to maintain or even counterproductive.

Key considerations

• Read the message: Many bounces are indeed self-explanatory if read carefully. Experts often find that simply repeating the bounce message to a sender can lead to understanding.
• Reference RFCs: Professionals diagnosing SMTP rejections should be highly familiar with relevant RFCs, such as RFC 5321, which document standard SMTP error codes. This knowledge forms the foundation for interpreting bounce messages.
• Beyond FAQs: Instead of merely compiling an FAQ of error codes, experts argue for the need for better training and professional standards in the industry. Understanding how ESPs classify and manage SMTP bounce codes is a critical skill.
• Contextual understanding: Understanding the context of the bounce, including whether it's related to concurrent connections, IP reputation, or content, is more valuable than a generic code explanation. An expert from Word to the Wise notes that the issue is more at the reading end than the writing end.

Key findings

• SMTP RFC definitions: SMTP RFCs, like RFC 5321, define the basic categories and meanings of bounce codes (e.g., 5xx for permanent negative replies, 4xx for temporary failures). These provide a foundational understanding, but do not delve into specific, proprietary reasons for rejections, or why what RFC 5322 says versus what actually works in practice might differ.
• General vs. specific: Documentation often provides a general understanding of error types (e.g., syntax errors, mailbox full) but stops short of explaining the precise, dynamic conditions that trigger these errors at a particular MBP. For instance, a 501 Syntax error means you are violating SMTP by sending illegal commands, but the exact condition that led to it is often left to the sender to diagnose.
• Postmaster sites: Many major MBPs maintain postmaster sites that offer some level of explanation for their deferments and bounces, sometimes including tokens or URLs for more information. However, these sites are typically static and not updated as frequently as internal policies may change.
• Operational secrecy: From an abuse prevention standpoint, publishing overly detailed explanations of filter triggers would likely empower spammers to circumvent security measures. MBPs are naturally protective of their anti-abuse strategies, which influences what information they make public about their systems.

Key considerations

• RFC compliance: Senders should ensure their systems adhere strictly to SMTP RFCs to minimize common protocol-related bounces. A solid understanding of these foundational documents is crucial, as they serve as the universal language of email deliverability. For example, understanding DMARC tags can help with related authentication issues.
• Monitor and adapt: Since many bounce reasons are dynamic and reputation-based, senders must continuously monitor their deliverability metrics and adapt their sending behavior. Relying solely on static documentation might not be sufficient.
• Aggregate information: Compile insights from various sources, including MBP postmaster pages, community forums, and deliverability experts, to build a more comprehensive understanding of specific bounce patterns. This holistic approach can help decipher the underlying reasons for rejections.
• Engage with ESPs: ESPs often have relationships with MBPs and can provide more nuanced interpretations of bounce data based on their aggregate sending patterns and historical interactions. Leveraging their expertise is key.