Summary
When reviewing bounce reports, encountering blank fields for bounceback messages, SMTP reply codes, and SMTP error codes can be puzzling. These missing details often indicate specific technical scenarios in the email delivery process, ranging from how receiving servers handle undeliverable mail to how sending platforms process bounce notifications. Understanding the underlying reasons is crucial for accurate deliverability troubleshooting and maintaining a healthy sender reputation.
Key findings
- Out-of-band bounces: Blank fields are frequently associated with out-of-band bounces, where the receiving server accepts the email first, then notifies of delivery failure asynchronously via a separate bounce message.
- ESP processing: The sending platform or Email Service Provider (ESP) may not be properly parsing or extracting the relevant information from the raw bounce emails (Delivery Status Notifications or DSNs), leading to empty fields in their reporting dashboard.
- Internal suppression: Some bounces occur before the email even reaches an external SMTP server, due to internal suppression lists or rules within the sending system, meaning no standard SMTP rejection message is generated.
- Autoresponders: Elderly or non-standard autoresponders can generate bounce messages that are difficult for modern bounce handlers to interpret, resulting in blank fields.
Key considerations
- Platform specifics: The specific sending platform or ESP you are using plays a significant role in how bounce data is interpreted and displayed. Reporting capabilities vary widely.
- Raw data access: If blank fields are a concern, seek access to the raw bounce data or consult with your ESP's support to understand the underlying messages they receive.
- Impact assessment: A low percentage of blank bounces (e.g., 0.00001%) might be acceptable, but a higher percentage (e.g., 1% or more) warrants immediate investigation as it could point to serious issues. Understanding common email bounce messages can help.
- Actionable insights: Even without explicit codes, a bounce indicates an undeliverable address. Consider these recipients for removal from your mailing list, especially if they are autoresponders, to protect your sender reputation. More information on why emails bounce back can provide further context.
What email marketers say
Email marketers often face the challenge of interpreting bounce reports provided by their sending platforms. The absence of specific bounceback messages, SMTP reply codes, or SMTP error codes can make it difficult to diagnose deliverability issues. Marketers commonly attribute these blanks to their ESP's data processing or the nature of out-of-band bounces, where the full context of the delivery failure is not immediately apparent during the initial SMTP transaction.
Key opinions
- ESP interpretation: Many marketers believe that blank bounce fields are a consequence of their ESP's system, which is responsible for translating raw SMTP responses into user-friendly dashboard information.
- Asynchronous bounces: There's a common understanding that blank fields often point to asynchronous (out-of-band) bounces, where the rejection happens after the initial mail acceptance.
- Internal suppression: Some speculate that bounces occur due to internal suppression rules within the sending platform, before the email even reaches the recipient's server, hence the lack of SMTP codes.
- Autoresponder impact: There is a general consensus that outdated or specific autoresponders can be a source of these unformatted or blank bounce messages.
Key considerations
- Investigate raw data: Marketers frequently suggest that if blank bounce fields are a significant issue, it's essential to request or access the raw bounce data from the ESP for deeper analysis.
- Quantify the problem: Many advise checking the percentage of bounces with blank fields. A high rate indicates a more serious problem requiring immediate attention, while a very low rate might be dismissed.
- List hygiene: Regardless of the missing details, any bounce signifies a delivery failure, and marketers emphasize removing these addresses, especially autoresponders, to improve email deliverability.
- Platform support: Marketers often rely on their ESP's support to clarify what causes these blank fields and how their system handles such scenarios, as detailed in articles like troubleshooting email bounces.
Marketer view
Marketer from Email Geeks queries why bounce reporting fields are blank, seeking community insight into whether it's due to the receiving server not returning any information or if they are just autoresponders. This highlights a common challenge in understanding bounce specifics.
Marketer view
Marketer from Email Geeks suggests that blank bounce reporting fields often result from the sending platform or ESP's interpretation, or lack thereof, of raw SMTP responses and bounce messages for display on their dashboard.
What the experts say
Experts in email deliverability offer deeper insights into why bounce reporting fields might appear blank. They often refer to the distinction between in-band and out-of-band bounces, stressing that the latter, by its nature, lacks immediate SMTP codes. Furthermore, they highlight the complexities of ESP bounce handlers and the potential for internal suppression mechanisms to prevent emails from ever reaching an external SMTP server, leading to a complete absence of standard error information.
Key opinions
- In-band vs. Out-of-band: Experts delineate between in-band rejections (during SMTP) and out-of-band bounces (post-acceptance), noting that only in-band typically provides immediate SMTP codes.
- DSN parsing: The ability of an ESP's system to correctly parse Delivery Status Notifications (DSNs) is crucial. Malformed or non-standard DSNs can result in blank fields.
- Silent rejections: Some recipient mail servers are intentionally configured to offer minimal or no bounce details, especially if they detect suspicious sending patterns, leading to 'silent' rejections or spam trap related issues.
- Legacy systems: Older mail systems and autoresponders may not adhere to modern bounce messaging standards, making it challenging for contemporary ESPs to extract structured data.
Key considerations
- Source of failure: Understanding whether the bounce occurred during the SMTP transaction or asynchronously is key to diagnosing the issue, as discussed in articles like what different SMTP bounce codes mean.
- Monitoring trends: Tracking the volume and patterns of blank bounce messages can reveal underlying deliverability issues, such as problems with specific recipient domains or shifts in mailbox provider policies.
- Reputation impact: Even if the details are missing, these are still failed deliveries that can negatively impact sender reputation. Senders should still remove these addresses from their lists.
- RFC compliance: Variations in how mail servers implement RFCs (Request for Comments) can lead to inconsistencies in bounce reporting, meaning not all servers will provide the same level of detail.
Expert view
Deliverability Expert from Spamresource.com highlights that understanding SMTP error codes is crucial for diagnosing delivery issues. They note that a missing code suggests a problem within the bounce reporting process itself rather than necessarily the delivery attempt.
Expert view
Deliverability Expert from Word to the Wise explains that out-of-band bounces frequently lack immediate SMTP reply codes because the message was initially accepted by the receiving server, with the subsequent bounce notification being a separate, later communication.
What the documentation says
Official documentation and technical standards, such as Request for Comments (RFCs), define the expected behavior of email systems during delivery failures. While these documents specify how SMTP reply codes and Delivery Status Notifications (DSNs) should be formatted and transmitted, real-world implementations can vary. This variability, combined with factors like silent discarding by recipient servers or non-standard bounce messages, explains why bounce reporting can sometimes lack complete information.
Key findings
- SMTP standards: RFC 5321 (SMTP) mandates that receiving SMTP servers must return an SMTP reply code in response to commands, providing a status on the transaction outcome.
- DSN format: RFC 3464 defines the extensible message format for Delivery Status Notifications (DSNs), which are the primary mechanism for asynchronous bounce reporting.
- Enhanced status codes: RFC 3463 provides a set of enhanced status codes for DSNs to offer more specific reasons for delivery failures, aiming for detailed feedback.
- Implementation variations: Despite RFCs, practical implementations of mail server behavior, especially concerning error reporting, can differ, leading to inconsistent bounce message details.
Key considerations
- Non-delivery reports (NDRs): Documentation confirms that NDRs, or bounce messages, indicate delivery failure, but their content can vary significantly depending on the interacting mail servers.
- Silent discarding: Some systems might accept messages but then silently discard them without generating a formal bounce notification, a practice not explicitly covered by standard bounce codes.
- Mailbox provider specifics: Major mailbox providers often have their own specific rules for when and how they issue bounce details. For example, Google Postmaster Tools might not show all bounce details if the error is not a recognized SMTP rejection code or if it is an asynchronous bounce.
- Error code ranges: General SMTP error codes (e.g., 4xx for transient, 5xx for permanent) are standard, but the specific diagnostic information often depends on the server's configuration and the nature of the failure. More on bounce/block codes can be found here.
Technical article
RFC 3464 documentation describes the standardized, extensible format used for Delivery Status Notifications (DSNs), which are critical for conveying information about email delivery attempts.
Technical article
RFC 5321 (SMTP) documentation states that a receiving SMTP server is required to provide an SMTP reply code in response to commands such as RCPT TO, indicating the status of the transaction during mail delivery.
Related resources
4 resources
Related pages
Why are Out-of-Band (OOB) email bounces unformatted or lack error codes?
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Why don't mailbox providers publish detailed bounce message explanations?
What do different SMTP bounce codes mean for email deliverability and blocks?
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