Summary
Out-of-Band (OOB) email bounces frequently appear unformatted or lack standard error codes primarily because the delivery failure occurs after the recipient's mail server has initially accepted the message. These asynchronous bounces are generated by a wide variety of internal systems within the recipient's infrastructure, such as anti-spam filters, forwarding agents, or internal routing components, rather than the initial receiving server. Crucially, these internal systems often do not consistently adhere to or implement the Delivery Status Notification (DSN) standard, leading to the dispatch of simpler, often plain text messages that lack structured formatting or machine-readable error codes. The sheer diversity and varying sophistication of these internal mail flow components further contribute to the inconsistency in OOB bounce reporting.
Key findings
- Post-Acceptance Failure: OOB bounces signify a delivery failure that occurs after the initial SMTP transaction is complete and the email has been accepted by the recipient's server.
- Internal System Origin: Unlike synchronous bounces, OOB bounces are generated by diverse internal mail systems, such as anti-spam or internal routing agents, not the SMTP server that initially accepted the message.
- Non-Adherence to DSN Standards: A primary reason for unformatted OOB bounces is that the internal systems generating them often do not consistently implement or adhere to the Delivery Status Notification (DSN) standard, which specifies structured bounce information.
- Absence of Standard SMTP Codes: These asynchronous bounces typically lack standard SMTP error codes (e.g., 5xx codes) because the failure takes place outside the immediate scope of the initial SMTP conversation.
- System Heterogeneity: The wide variety and varying age of internal mail flow components and third-party integrations contribute significantly to the inconsistent formatting and lack of detailed error codes in OOB bounce messages.
Key considerations
- Automated Processing Challenges: The lack of standardized formatting and error codes in OOB bounces makes their automated parsing and processing by sending systems exceptionally difficult.
- Complex Troubleshooting: Diagnosing the specific cause of an OOB bounce becomes more challenging without structured error information, often requiring manual review of plain text messages.
- Direct Support May Be Needed: In cases of highly unformatted OOB bounces, reaching out to the recipient's email service provider or their support channels may be the only way to gain clarity.
- Understanding RFCs: Familiarity with RFCs like 3463 (Extended Bounce Codes), 3464 (Delivery Status Notifications), and 5321 (SMTP) is crucial for understanding the intended standards, even when OOB bounces deviate.
What email marketers say
6 marketer opinions
Out-of-Band (OOB) email bounces frequently lack standardized formatting or error codes primarily because they are generated asynchronously, after the recipient's mail server has initially accepted the message. These delivery failures originate from diverse internal systems within the recipient's network- such as forwarding services, internal anti-spam filters, or mailbox management systems- rather than the initial SMTP gateway. Critically, these internal components often do not consistently adhere to the Delivery Status Notification (DSN) standard or send back structured SMTP error codes, instead opting for simpler, often plain text, notifications. The sheer variety and differing levels of sophistication among these internal mail flow systems contribute to the inconsistent and unformatted nature of OOB bounce messages.
Key opinions
- Post-Acceptance Failure: OOB bounces occur because the message is initially accepted by the recipient server but then fails delivery later due to an internal issue.
- Internal System Origin: These bounces are generated by various internal systems within the recipient's infrastructure- like forwarding agents, anti-spam filters, or mailbox quota systems- rather than the initial receiving server.
- Non-Standard Adherence: A primary reason for the lack of formatting is that these internal systems often do not consistently adhere to Delivery Status Notification (DSN) standards or standard SMTP error codes.
- Plain Text Messages: OOB bounce notifications frequently appear as simple, unformatted plain text emails, making them challenging for automated parsing.
- System Heterogeneity: The wide variety and varying sophistication of recipient mail systems and internal components contribute significantly to the inconsistency in OOB bounce formatting and content.
Key considerations
- Automation Hurdles: Automated processing of OOB bounces is difficult due to their inconsistent, unstructured nature and lack of standard error codes.
- Manual Interpretation Needed: Email senders often need to manually review these plain text bounce messages to understand the underlying delivery failure.
- Diagnostics Challenges: Pinpointing the exact cause of an OOB bounce becomes complex without standardized error codes or structured formatting.
- Impact on Metrics: The unparseable nature of many OOB bounces can hinder accurate bounce rate calculations and effective list hygiene practices.
- Diverse System Handling: Dealing with OOB bounces requires robust systems capable of interpreting varied, often plain text, messages from diverse recipient mail servers and internal systems.
Marketer view
Email marketer from Validity explains that Out-of-Band (OOB) bounces occur after the initial SMTP transaction is complete and the email is accepted (250 OK). These bounces are often generated by a different, internal system (like a forwarding system or anti-spam filter) rather than the initial receiving server, making them difficult to parse as they frequently lack standard error codes or structured formatting, appearing as plain text emails.
9 Dec 2024 - Validity
Marketer view
Email marketer from Postmark explains that Out-of-Band (OOB) bounces occur when a message is initially accepted by the recipient server but then fails delivery later due to an internal issue. Because these failures happen after the initial SMTP transaction, the bounce message is sent asynchronously and may not adhere to standard SMTP error codes or structured formats, often appearing as a plain text email from a different internal source.
5 Sep 2021 - Postmark
What the experts say
3 expert opinions
Out-of-Band (OOB) email bounces frequently appear unformatted or lack standard error codes because they are generated by internal mail systems after the recipient's server has already accepted the message. Unlike standard SMTP bounces, these internal components- such as forwarding agents or internal spam filters- are often not designed to produce or consistently adhere to Delivery Status Notification (DSN) or other RFC standards for bounce messaging. This results in highly variable, often plain text, notifications that lack structured formatting or machine-readable error codes, making automated processing challenging.
Key opinions
- Post-SMTP Transaction Failure: Out-of-Band bounces occur after the initial SMTP transaction, meaning the recipient's mail system accepted the message, but then failed to deliver it internally.
- Internal System Origin: These bounces originate from internal mail systems, such as anti-spam filters or forwarding agents, not the initial SMTP server that first received the email.
- Non-Standardized Output: Unlike Feedback Loop (FBL) emails which use ARF, Out-of-Band bounces lack a standard format, frequently appearing as plain text messages without consistent Delivery Status Notification (DSN) codes.
- Design Limitations: The internal systems generating Out-of-Band bounces are often not designed to produce standard SMTP bounces or strictly adhere to RFC guidelines for bounce messages.
- Inconsistent Extended Codes: While some Out-of-Band bounces may include extended bounce codes, their formatting and presence are not standardized across different systems.
Key considerations
- Automated Parsing Challenges: The lack of consistent formatting and standard error codes in Out-of-Band bounces makes them exceptionally difficult for automated systems to parse and categorize effectively.
- Manual Analysis Required: Email senders often need to manually review the plain text content of OOB bounces to decipher the reason for non-delivery and take appropriate action.
- Engage Recipient Support: For particularly unformatted or unclear Out-of-Band bounces, contacting the recipient's email service provider or IT support may be necessary to gain clarity on the issue.
- Reference RFC Standards: While Out-of-Band bounces often deviate, understanding RFCs 3463 (Extended Bounce Codes) and 3464 (Delivery Status Notifications) provides valuable context on the intended standards for bounce messaging.
Expert view
Expert from Email Geeks explains that Out-of-Band (OOB) bounces do not have a standard format, unlike ARF which is used for FBL emails. She notes that while OOBs can have extended bounce codes, their formatting is not standardized and suggests reaching out to Proofpoint support regarding specific unformatted OOBs, as it does not appear to be a standard configuration. She also provides links to RFC 3463 and RFC 3464 for further information on bounce codes.
29 Aug 2022 - Email Geeks
Expert view
Expert from Spam Resource explains that Out-of-Band (OOB) bounces often occur after the initial SMTP transaction, when the recipient's mail system accepts a message but then fails to deliver it to the mailbox. These bounces are generated by internal systems, not the initial SMTP server, which often results in poor formatting, a lack of standard Delivery Status Notification (DSN) codes, or plain text messages, as they do not always conform to RFC standards for bounce messages.
18 Feb 2025 - Spam Resource
What the documentation says
4 technical articles
Out-of-Band (OOB) email bounces commonly appear unformatted or lack specific error codes primarily because the delivery failure occurs after the recipient's mail server has initially accepted the message. These asynchronous notifications originate from diverse internal systems within the recipient's email infrastructure- such as anti-spam solutions, internal routing components, or mailbox management systems. Crucially, these varied internal components often do not consistently adhere to the Delivery Status Notification (DSN) standard or generate standard SMTP error codes, resulting in simpler, frequently plain text, bounce messages that lack structured formatting or machine-readable information.
Key findings
- Post-SMTP Processing Failures: OOB bounces signal a delivery failure occurring after the initial SMTP transaction has completed successfully, meaning the recipient's server initially accepted the email.
- Internal System Origin: These asynchronous bounce messages are generated by various internal components within the recipient's mail infrastructure, such as anti-spam engines, internal routing agents, or mailbox limiters, rather than the initial mail gateway.
- Lack of DSN Adherence: A primary reason for the unformatted appearance of OOB bounces is that the internal systems generating them often do not consistently implement or adhere to the Delivery Status Notification (DSN) standard (RFC 3464), which defines structured bounce information.
- Absence of Standard SMTP Codes: OOB bounces typically lack the standard SMTP error codes (e.g., 5xx series) because the failure occurs outside the immediate, synchronous SMTP conversation, in subsequent internal processes.
- System Heterogeneity: The sheer variety, age, and diverse design of internal mail flow components and third-party integrations contribute significantly to the inconsistent formatting, plain text nature, and lack of detailed error codes in OOB bounce messages.
Key considerations
- Automated Parsing Challenges: The highly variable and often unformatted nature of OOB bounces significantly impedes automated parsing and categorization by email sending platforms.
- Manual Review Necessity: Email senders frequently must manually analyze the plain text content of OOB bounce messages to deduce the underlying cause of delivery failure.
- Impact on Deliverability Metrics: Difficulty in consistently parsing OOB bounces can obscure precise bounce rate calculations and hinder effective list hygiene efforts, affecting overall deliverability insights.
- Understanding System Diversity: Senders need to recognize that OOB bounces originate from a wide array of internal systems, each with its own non-standardized reporting, requiring a flexible approach to bounce handling.
Technical article
Documentation from RFC Editor (RFC 3464) defines the standard for Delivery Status Notifications (DSNs), which provide structured, machine-readable information about delivery failures. The RFC implicitly explains that when Out-of-Band bounces appear unformatted or lack specific error codes, it's often because the internal systems generating these asynchronous notifications do not consistently implement or adhere to this DSN standard, resulting in simpler, less detailed, non-standardized bounce messages.
14 May 2023 - RFC Editor
Technical article
Documentation from Microsoft Learn, in its discussion of Non-Delivery Reports (NDRs), highlights that while Microsoft Exchange attempts to provide detailed failure messages, the sheer variety of internal mail flow components and third-party integrations (like anti-spam or internal routing agents) can lead to situations where bounce messages, particularly for Out-of-Band scenarios, originate from systems that don't always generate standard DSNs, resulting in less structured or plain text notifications instead of formatted error codes.
24 Apr 2024 - Microsoft Learn
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