Summary
Asynchronous bounces, also known as late or out-of-band (OOB) bounces, present a unique challenge in email deliverability. Unlike synchronous bounces, which provide immediate feedback during the SMTP transaction, OOB bounces arrive much later, often in a non-standardized format. This makes it difficult for email service providers (ESPs) and senders to accurately associate them with the original email and campaign. While there isn't a universally adopted standard format for these delayed error messages, Variable Envelope Return Path (VERP) is widely recognized as the most effective mechanism for linking them back to specific messages.
Key findings
- No universal standard: There is no widely adopted, uniform format for late bounce error messages across all mailbox providers, leading to parsing complexities.
- VERP's role: VERP is the primary method used by ESPs to associate asynchronous bounces with the original recipient and campaign, by encoding information directly into the return-path address.
- Limited adoption: Despite existing RFCs, like RFC 3464 for Delivery Status Notifications, a common standard for detailed asynchronous bounce formats has not been broadly adopted by mailbox providers.
- Discouraged practice: Sending out-of-band bounces is generally discouraged due to potential for spam backscatter and difficulties in attribution, making their occurrence rare but problematic.
- Custom headers: Some ESPs, in addition to VERP, implement custom headers to try and capture more context about the original email, attempting to overcome the format limitations.
Key considerations
- Handling volume: Despite their rarity, bulk senders must have systems in place to process and interpret asynchronous bounces to maintain list hygiene.
- Impact on reputation: While ignoring a small fraction of OOB bounces might not drastically impact sender reputation, proper handling of all bounce types is crucial for long-term deliverability. Understanding best practices for managing hard and soft bounces is key.
- VERP implementation: ESPs should assess the trade-offs of implementing VERP at scale, considering its utility versus the potential for return-path addresses to attract spam.
- Data capture: Senders should aim to capture as much data as possible from asynchronous bounces, even if it requires custom parsing, to identify unreachable addresses and reduce future sending to them.
What email marketers say
Email marketers often face practical challenges in tracking and understanding asynchronous (late or out-of-band) bounces, primarily due to the lack of a consistent standard for their error messages. While they acknowledge the rarity of these bounces, the inability to easily associate them with specific campaigns or recipients complicates list hygiene and performance analysis. Many rely on mechanisms like VERP, but still encounter situations where additional data is needed to fully diagnose the issue.
Key opinions
- Difficulty in association: Marketers frequently struggle to connect asynchronous bounces to the original email because of varied or missing identifiers.
- VERP as a workaround: VERP is seen as the most reliable, albeit sometimes limited, way to gain insights into the original email that triggered an asynchronous bounce.
- Need for more info: Even with VERP, marketers often wish for additional data or a more structured format to diagnose problems, leading some ESPs to use extra custom headers.
- Limited impact from ignoring: Some bulk senders may discard asynchronous bounces due to the effort involved in handling them, believing the low volume will not significantly affect their sender reputation (though this is not recommended for optimal deliverability).
Key considerations
- Robust bounce processing: Despite the challenges, implementing robust systems to process all bounce types, including the less common asynchronous ones, is vital for maintaining a clean mailing list and avoiding blocklisting.
- Analyzing common bounce messages: While asynchronous bounces lack standard formats, understanding common email bounce messages from synchronous bounces can provide a foundation for interpreting the less structured asynchronous ones.
- Prioritizing synchronous bounces: Given their rarity, marketers should focus most of their bounce management efforts on synchronous bounces, which provide immediate and often more actionable feedback. You can learn more about understanding and handling bounces from industry blogs.
Marketer view
Marketer from Email Geeks indicates the challenge with late bounces is associating them with the original email due to a lack of standard formatting. This struggle is evident across various ESPs' documentation.
Marketer view
Marketer from Email Geeks suggests that VERP appears to be the most reliable way to obtain information on the original email linked to a late bounce. This method helps connect the bounce back to its source.
What the experts say
Email deliverability experts highlight that asynchronous bounces, while adhering to certain RFCs for their structure, often lack consistent adoption by mailbox providers, making them hard to parse automatically. A strong consensus exists that sending out-of-band bounces is generally undesirable due to potential for abuse (like backscatter spam). VERP emerges as the most practical solution for senders to attribute these rare bounces, though it comes with its own set of considerations, such as the risk of attracting spam to VERP-generated return-path addresses.
Key opinions
- RFCs exist, but adoption is low: While RFCs, like RFC 3464, specify formats for delivery status notifications, experts note that most mailbox providers do not fully or consistently adopt them for asynchronous bounces.
- Discouraged practice: Experts strongly discourage the practice of sending asynchronous bounces due to the risks of generating backscatter spam and difficulty in legitimate handling.
- VERP as the primary solution: VERP is identified as the fundamental and most effective mechanism for connecting delayed bounces to the specific original message and recipient.
- Low frequency: Asynchronous bounces are exceedingly rare compared to synchronous bounces, which reduces the impetus for further standardization efforts.
- Unified theory elusive: The idea of a comprehensive, unified theory for all bounce handling remains a long-standing challenge, with asynchronous bounces being a particularly tricky part of the puzzle.
Key considerations
- Spam risk with VERP: While VERP is effective, there's a recognized risk that VERP-generated return-path addresses can become targets for spam, requiring careful management by senders at scale.
- Prioritizing synchronous feedback: Mailbox providers are increasingly trying to give feedback synchronously to prevent the need for asynchronous bounces, reinforcing the importance of handling 4xx mail errors properly.
- Need for comprehensive handling: Even though asynchronous bounces are rare, bulk senders must still be equipped to process them to ensure complete list hygiene. This aligns with overall strategies for classifying and managing SMTP bounce codes.
Expert view
Expert from Email Geeks indicates that the format for delayed bounces is indeed defined in an RFC, and they are actively looking for it, suggesting the existence of a standard even if not widely adopted.
Expert view
Expert from Email Geeks states that sending delayed bounces is generally frowned upon for multiple reasons. They suggest that this discouragement might be why less effort is spent on standardizing a format for something that ideally shouldn't be happening.
What the documentation says
Formal documentation, particularly RFCs, provides a framework for email communication, including bounce notifications. RFC 5321 (Simple Mail Transfer Protocol) outlines the basic protocol, while RFC 3464 (Delivery Status Notifications) specifically defines the format for DSNs, which include delayed bounces. Despite these specifications, the practical implementation across diverse mail servers varies, leading to inconsistencies in how asynchronous bounce error messages are formatted and reported. Vendor-specific documentation often details custom approaches, like the use of VERP alongside proprietary headers, to enhance bounce attribution where universal standards fall short.
Key findings
- RFC 3464 defines DSNs: Delivery Status Notifications (DSNs), including those for delayed bounces, have a defined structure in RFC 3464, which specifies their format and components.
- Asynchronous nature: Documentation often clarifies that an asynchronous bounce means the email was initially accepted by the recipient server, with the failure occurring sometime after that initial transaction.
- VERP implementation guidance: While not strictly an RFC, the concept of VERP is widely discussed in technical documentation as a method for reliably identifying the sender and recipient of a bounced message, particularly for out-of-band bounces.
- Vendor-specific enhancements: ESPs may implement additional mechanisms, such as custom X-headers, to augment the information available in DSNs for better tracking and debugging of asynchronous bounces.
Key considerations
- Parsing complexity: Despite RFC definitions, the practical parsing of asynchronous bounce messages can be complex due to variations in implementation and the inclusion of non-standard information. This impacts a sending platform's ability to distinguish hard and soft bounces.
- MIME parts and status codes: DSN messages often contain multiple MIME parts, including machine-readable delivery status data and human-readable explanations, with specific bounce codes indicating the reason for failure.
- Return-path integrity: The stability and integrity of the return-path address are critical for reliable bounce processing, making VERP a robust technical choice for deliverability platforms.
Technical article
Documentation from RFC 3464 (Delivery Status Notifications) states that a DSN message is a Multipurpose Internet Mail Extensions (MIME) message that describes the status of a delivered, delayed, or failed message transfer. It serves as the standard for reporting mail delivery outcomes.
Technical article
Documentation from SparkPost Support regarding Out-of-Band Bounces explains that unlike synchronous rejections, OOB bounces occur when a recipient server initially accepts an email, but a problem (e.g., full mailbox, invalid address) is discovered later, resulting in a delayed bounce notification.
Related resources
5 resources
Related pages
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