Summary
Mailbox providers generally do not publish detailed bounce message explanations due to a combination of factors. A primary concern is preventing spammers from exploiting detailed information to bypass filters and improve their techniques. The complexity and variability of bounce messages, coupled with the constantly changing email landscape, makes maintaining accurate and up-to-date documentation challenging. Many experts believe that existing resources, such as SMTP RFCs and postmaster pages, often provide sufficient information if senders are properly trained to interpret them. Additionally, some providers intentionally maintain a degree of ambiguity for security reasons or because the technical specifics are implementation-dependent. The lack of detailed explanations also stems from resource constraints within postmaster teams and a perception that the effort required to create and maintain such documentation would not be an effective use of their time, especially when senders often fail to interpret the existing information correctly or seek help even when the answer is readily available in the bounce message itself.
Key findings
- Spammer Prevention: Detailed bounce message explanations could provide spammers with valuable information for bypassing filters and improving their techniques.
- Complexity and Variability: The diverse range of reasons for bounces and the ever-changing email landscape make it difficult to create comprehensive and up-to-date documentation.
- Existing Resources Suffice: SMTP RFCs, postmaster pages, and the bounce messages themselves often contain enough information if senders are properly trained to interpret them.
- Resource Constraints: Postmaster teams often lack the resources to create and maintain detailed bounce message explanations.
- Intentional Ambiguity: Some mailbox providers intentionally maintain a degree of ambiguity for security reasons or because the technical specifics are implementation-dependent.
- Interpretation Issue: A common issue is not the lack of information, but senders’ inability to correctly interpret existing bounce messages.
Key considerations
- Sender Education: Focus on educating senders about interpreting existing bounce messages and SMTP error codes rather than relying on more detailed explanations.
- SMTP Expertise: A deeper understanding of SMTP protocols and standards is essential for diagnosing and resolving email deliverability issues.
- Provider's Balancing Act: Mailbox providers must balance the need for clearer communication with the imperative to protect their systems from abuse and spam.
- Detailed Message Context: When seeking help, senders should provide the complete bounce message and relevant context to allow for accurate diagnosis.
- Prioritization: Postmaster teams must prioritize tasks and allocate resources effectively, weighing the benefits of detailed documentation against other pressing needs.
What email marketers say
9 marketer opinions
Mailbox providers generally do not publish detailed bounce message explanations primarily to prevent spammers from exploiting this information to bypass filters and improve their techniques. The complexity and variability of bounce messages, coupled with the constantly changing email landscape, makes maintaining accurate and up-to-date documentation challenging. Additionally, providers often believe that the current bounce messages are sufficient for those who understand how to interpret them, and that increased clarity would only benefit malicious actors.
Key opinions
- Spammer Prevention: A major reason for not providing detailed bounce explanations is to prevent spammers from learning how to circumvent email filters. Vague messages obscure the exact reasons for bounces, making it harder for malicious actors to refine their methods.
- Message Clarity: Mailbox providers assume current bounce messages are sufficiently clear, but people are just repeating the same content back to the sender. They may perceive the issue as a lack of understanding rather than a lack of detail.
- Documentation Overhead: The dynamic nature of email infrastructure and spam filters requires constant updates to bounce message explanations, creating a significant maintenance overhead for mailbox providers.
- Complexity: The complexity and variance of bounce messages, coupled with the absence of agreed-upon standards, makes it difficult to create a unified and comprehensive documentation.
- Obscurity as Security: Providers use a degree of ambiguity as a security measure to protect their email filtering mechanisms.
Key considerations
- Sender Understanding: Senders need better resources and training to interpret existing bounce messages effectively. Rather than relying on detailed explanations, focusing on understanding the existing information can improve troubleshooting.
- Potential for Misinterpretation: Detailed explanations could be misinterpreted or misused by senders, leading to ineffective or even detrimental actions. A general understanding of SMTP and email protocols is crucial.
- Balancing Clarity and Security: Mailbox providers must balance the need for clearer communication with the imperative to protect their systems from abuse. Finding this balance is a continuous challenge.
- Resource Allocation: Creating and maintaining detailed documentation requires significant resources. Providers must weigh the benefits of improved clarity against the costs of developing and updating these resources.
Marketer view
Email marketer from StackExchange believes the reasons are that there are way too many scenarios for bounces (and there are no agreed-upon standards for it) and email providers don't want to provide spammers with possible hints.
18 Apr 2025 - StackExchange
Marketer view
Email marketer from EmailDeliveryJunkies explains that obfuscation is implemented to keep the information away from email spammers. Specific error information makes it easier for bad actors to exploit systems.
6 Feb 2022 - EmailDeliveryJunkies
What the experts say
8 expert opinions
Experts suggest that mailbox providers don't publish detailed bounce message explanations for several reasons. Existing SMTP response codes and postmaster pages often provide sufficient information, but senders may lack the training to interpret them correctly. Prioritizing clear communication is also resource-intensive for postmaster teams already stretched thin. Moreover, publishing detailed explanations could inadvertently aid spammers in bypassing filters. Sometimes providers are vague on purpose and the problem often lies in the readers ability to understand the explanations as is.
Key opinions
- Existing Documentation: SMTP RFCs and postmaster pages already provide response codes and explanations, though not always in detail. Senders should first consult these resources.
- Resource Constraints: Postmaster teams are often under-resourced, making the creation and maintenance of detailed bounce explanations a low priority.
- Spammer Risk: Detailed explanations could assist spammers in identifying and exploiting vulnerabilities in email filtering systems.
- Interpretation Skills: The core issue may not be a lack of information, but a lack of understanding and training among senders in interpreting existing bounce messages.
- Intentionally Vague: In some cases, mailbox providers intentionally remain vague for security reasons or to prompt senders to investigate deeper issues.
Key considerations
- Sender Education: Investing in sender education and training on interpreting SMTP response codes and bounce messages may be more effective than creating more detailed explanations.
- SMTP Expertise: A better understanding of SMTP protocols and standards is crucial for diagnosing and resolving email deliverability issues.
- Balance of Clarity and Security: Mailbox providers need to find a balance between providing clear explanations and maintaining security against spammers.
- Detailed Message Context: When seeking help, senders should always provide the complete bounce message, as it often contains critical information for diagnosis.
Expert view
Expert from Email Geeks explains that most bounce messages are self-explanatory if read carefully. Many people refuse to answer questions about bounces without the specific message, because the messages often clearly state the issue.
22 Jan 2022 - Email Geeks
Expert view
Expert from Email Geeks explains that a lot of postmaster teams are under-resourced, and website teams have different priorities than SMTP teams, making publishing bounce explanations a low priority. Also, people often ignore published information anyway.
1 Feb 2023 - Email Geeks
What the documentation says
4 technical articles
Documentation suggests that mailbox providers do not publish detailed bounce message explanations for a combination of reasons. SMTP reply codes, as defined in RFC 5321, are intended to provide a general outcome rather than specific details due to the wide variability and implementation-specific nature of failures. Additionally, providers like Google and Microsoft obscure full reason codes to protect against spam and abuse, prioritizing the prevention of insight for malicious actors. The technical and frequently changing nature of these specifics also contributes to a lack of detailed, publicly available documentation.
Key findings
- General Indication: SMTP reply codes are designed to give a general indication of the outcome, not specific reasons.
- Implementation-Specific: Reasons for email failure are often implementation-specific, making detailed general explanations impractical.
- Spam Prevention: Obscuring details protects backend systems and prevents spammers from gaining insights.
- Technical and Changing: The specifics are often highly technical and subject to frequent changes, making documentation difficult to maintain.
Key considerations
- Understanding Categories: Focus should be on understanding the general categories of errors as defined in RFCs.
- Security vs. Clarity: Providers must balance the need for clear communication with the imperative to protect against abuse.
- Reliance on Provider Resources: Senders should rely on the general guidance provided by mailbox providers and their support resources, recognizing that specific details may not be available.
Technical article
Documentation from Google Support explains that to protect their users from spam and abuse, they obscure full reason codes. It is more important to prevent the bad actors from gaining additional insight into what the trigger was.
17 Mar 2025 - Google Support
Technical article
Documentation from RFC 5321 explains that SMTP reply codes are designed to provide a general indication of the outcome of a request, but detailed explanations are not included because the specific reasons for failure can vary widely and are often implementation-specific. The RFC defines the categories of errors, but not the granular details.
21 Oct 2021 - RFC 5321
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