Summary
The consensus among deliverability professionals is that no major mailbox provider reliably or predominantly sends asynchronous bounces. While they do occur, they are infrequent and typically stem from specific, often complex, scenarios rather than a provider's default behavior. Mailbox providers (MBPs) generally prefer to issue synchronous bounces because they are more efficient, consuming fewer resources and reducing the potential for backscatter.
Key findings
- Rarity: Asynchronous bounces are considered very rare across major mailbox providers.
- Efficiency Preference: MBPs favor synchronous bounces due to lower bandwidth and resource consumption.
- Historical Context: Efforts to minimize backscatter (a form of spam abuse) led many providers to reduce or eliminate asynchronous bounces. Learn more about email backscatter.
- Complex Environments: When asynchronous bounces do occur, they are often observed in complex B2B mailbox setups with multiple layers of protection or due to misconfigured forwarding.
Key considerations
- ESP Processing: Some email service providers may not properly process asynchronous bounces, requiring manual intervention for senders. Understand ESP bounce expectations.
- Testing Challenges: Intentionally generating an asynchronous bounce for testing purposes is difficult due to their infrequent nature and the lack of specific providers that consistently produce them. However, you can learn more about how to generate hard bounces for testing.
- Industry Standard: The industry standard leans heavily towards synchronous bounce reporting for immediate feedback on delivery failures. Read Gainsight's email configuration guidance.
What email marketers say
Email marketers often seek predictable bounce behavior to maintain clean mailing lists. However, the reality of asynchronous bounces presents a challenge, as they are uncommon and not consistently generated by specific mailbox providers. This unpredictability leads to difficulties in automated processing and often necessitates manual intervention for list hygiene.
Key opinions
- Unpredictability: Marketers find it difficult to identify specific mailbox providers that consistently send asynchronous bounces.
- Manual Intervention: Lack of proper asynchronous bounce processing by some ESPs leads to manual list updates, which can be time consuming.
- Testing Limitations: It is not feasible to rely on specific small mailbox providers for consistently generating asynchronous bounces for testing purposes.
- Preference for Sync: Most marketers understand that mailbox providers prefer synchronous bounces for operational efficiency.
Key considerations
- ESP Capabilities: Evaluate your ESP's ability to manage various bounce types effectively.
- Bounce Classification: Properly classifying and managing SMTP bounce codes is essential for list hygiene, regardless of bounce type.
- Testing Workarounds: For specific testing needs, direct injection of an asynchronous bounce message into your MTA system might be a viable workaround.
- Complex Routing: Be aware that over-engineered B2B mailboxes with multiple security layers are more likely to generate asynchronous bounces due to internal SMTP hop failures. Review bounce handling strategies.
Marketer view
Marketer from Email Geeks suggests that many are actively seeking to understand which mailbox providers consistently send asynchronous bounces to better manage their email programs. This indicates a general curiosity in the community about less common bounce behaviors.
Marketer view
Marketer from Delivery Forum notes that identifying specific MBPs for consistent asynchronous bounces is an ongoing challenge for email programs. This difficulty often leads to frustrations when trying to implement automated bounce processing.
What the experts say
Deliverability experts largely agree that asynchronous bounces are a rare occurrence in modern email ecosystems. They confirm that the industry has moved away from this type of bounce due to past issues, such as backscatter, and a general preference for immediate feedback mechanisms. When asynchronous bounces do happen, they are often indicative of intricate or unusual email routing rather than a consistent behavior of major mailbox providers.
Key opinions
- Minimized Use: Most companies have significantly minimized asynchronous bounces to avoid issues like backscatter and spam reflection.
- Efficiency Driven: Mailbox providers fundamentally prefer synchronous bounces when possible, as they are more resource-efficient.
- Edge Cases: Asynchronous bounces are now relatively rare and primarily occur in specific circumstances, such as complex B2B mailbox configurations.
- Forwarding Issues: Poorly implemented email forwarding or certain spam filtering services are common sources of asynchronous bounces.
Key considerations
- Testing Complexity: Crafting realistic asynchronous bounces for testing is challenging and requires careful simulation to mimic genuine bounce messages.Understand bounce message timing.
- DMARC Impact: Failures in authentication protocols like DKIM and SPF within complex email chains can lead to asynchronous bounces, especially when DMARC policies are enforced. Troubleshoot DMARC reports.
- VERP Usage: Using Variable Envelope Return Path (VERP) can reduce the need to specifically track or be concerned about asynchronous bounces because it simplifies bounce processing.
- Historical Causes: The Backscatterer website provides historical context on the origins of such bounces, though its information requires careful consideration of its source.
Expert view
Expert from Email Geeks indicates that asynchronous bounces can occur depending on specific circumstances. This highlights that while rare, they are not entirely eliminated from the email ecosystem.
Expert view
Expert from Spam Resource highlights that modern email infrastructure prioritizes immediate feedback, reducing the need for asynchronous bounces. This shift reflects a move towards more efficient and real-time deliverability insights.
What the documentation says
Official documentation and industry standards consistently highlight the preference for synchronous bounce reporting among major mailbox providers. This approach provides immediate feedback on delivery failures, allowing senders and email service providers to react promptly. While asynchronous bounces are acknowledged, they are typically described as a less common occurrence, often tied to specific technical reasons or deferred delivery scenarios rather than a primary mode of bounce notification.
Key findings
- Synchronous Preference: Most major mailbox providers prioritize generating synchronous bounces, which communicate delivery failures immediately upon initial connection.
- ESP Role: Email Service Providers are expected to implement robust systems for detecting, classifying, and handling various bounce types to assist senders. Learn more about ESP bounce handling.
- Reputation Protection: Frameworks exist to prevent repeated bounces to the same contact, safeguarding sender reputation from unnecessary hard bounces. Recover your domain reputation.
Key considerations
- SMTP Responses: Understanding and parsing SMTP responses is critical for differentiating between various bounce types, including deferred and permanent failures.
- Late Bounce Handling: Protocols for asynchronous bounces (often called late bounces) are less standardized, requiring careful management by senders. Handle asynchronous bounces effectively.
- Resource Optimization: Mailbox providers often design their systems to minimize resource consumption, which includes favoring synchronous bounces whenever possible.
Technical article
Documentation from Gainsight Inc. states that most major mailbox providers primarily generate synchronous bounces, providing immediate failure notification. This approach helps in prompt identification of invalid recipient addresses.
Technical article
Documentation from Kickbox Blog emphasizes that reliable ESPs are crucial for detecting and handling bounces effectively. This is a foundational element of maintaining good sender reputation and deliverability.
Related resources
3 resources
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