Summary
Email soft bounce retry policies, which dictate how many times and for how long an email sending system attempts to redeliver a message that initially failed due to a temporary issue, significantly influence domain reputation and deliverability. While designed to ensure message delivery despite transient problems, overly aggressive or poorly managed retry policies can paradoxically harm sender reputation by signaling persistent attempts to an unresponsive mailbox provider (MBP). This can lead to increased deferrals, throttling, and a decline in your domain's sending reputation over time.
Key findings
- Standard practice: Many email service providers (ESPs) employ retry policies involving numerous attempts over several days for soft bounces, often in 15-minute intervals. This is considered standard by many in the industry, including major ESPs like Salesforce Marketing Cloud.
- Reputation impact debate: There's a nuanced discussion around whether these retries themselves harm reputation. Some believe that temporary deferrals, when handled appropriately by the ESP, do not negatively impact reputation. Others argue that persistent, unsuccessful attempts, even for temporary issues, can contribute to a decline in sender reputation over time.
- Greylisting: Many soft bounces are due to greylisting, a temporary rejection mechanism used by ISPs to combat spam. Proper retry policies are crucial to pass greylisting and ensure eventual delivery.
- ISP throttling: ISPs often throttle connections or deliveries based on sender reputation. If your retry rate is too high or your reputation is already low, you may experience more frequent deferrals.
- Distinction: It is important to differentiate between soft bounce retry attempts (automated redelivery of a single send) and soft bounce tolerance (when repeated soft bounces across multiple distinct sends lead to an address being suppressed).
Key considerations
- Trust your ESP: ESPs typically have sophisticated MTA configurations based on vast amounts of data and experience, optimizing for various ISP rules. For many, relying on their default retry policies is sufficient.
- Monitor deliverability metrics: While soft bounces are temporary, a consistently high volume or an increase in them can indicate underlying deliverability issues or a declining domain reputation. Pay attention to patterns and bounce codes.
- Understand ISP signals: ISPs use many signals for reputation, and persistently unsuccessful delivery attempts, even temporary ones, can subtly contribute to a negative score over time. Learn more about their behavior from Mailgun.
- Adjust soft bounce tolerance: The soft bounce tolerance setting (how many soft bounces over how many sends before an address is suppressed) is crucial. Default settings by ESPs might be too low for your sending frequency, leading to unnecessary list churn. Consider adjusting this with your ESP.
What email marketers say
Email marketers often express a mix of trust in their ESP's default retry policies and concern over the potential impact on their sender reputation. The discussion highlights a common confusion between automated delivery retries for a single email send and the long-term soft bounce tolerance that determines when a recipient is marked as undeliverable across multiple campaigns. Many marketers prioritize avoiding unnecessary list churn while also being mindful of not appearing overly persistent to MBPs.
Key opinions
- ESP reliance: Many marketers trust their ESPs (like SFMC) to manage soft bounce retry policies, believing these providers have the data and expertise to optimize delivery without harming reputation.
- No reputation impact: Some marketers believe that individual soft bounce retries, particularly for transient errors like a full mailbox or greylisting, do not negatively impact sender reputation, as these are temporary issues.
- Policy standardization: The extensive retry policies of some ESPs are viewed as standard and effective for maximizing delivery, indicating that deliverability teams have refined these policies over time.
- Differing definitions: There is acknowledged confusion and misuse of the terms soft bounce and hard bounce, making clear communication on these topics challenging without precise definitions.
- Focus on tolerance: Marketers are more concerned about soft bounce tolerance (the threshold for marking an address as undeliverable after multiple soft bounces) than individual retry intervals.
Key considerations
- Understand ESP policies: Marketers should be aware of their ESP's specific soft bounce retry policies and soft bounce tolerance settings.
- Review tolerance thresholds: If sending frequently (e.g., several times a week), consider asking your ESP to increase the soft bounce tolerance (e.g., to seven or higher) to prevent premature list churn.
- Time-sensitive content: For time-sensitive campaigns, lengthy retry intervals (e.g., multiple days) might mean emails arrive too late to be relevant. Plan accordingly.
- Focus on the big picture: While retries are managed by the ESP, marketers should focus on overall bounce rate and list hygiene to prevent reputation issues that lead to persistent deferrals.
Email marketer from Email Geeks observes that their ESP's default soft bounce retry policy attempts hundreds of redeliveries over multiple days, raising concerns about potential impacts on domain reputation.
Email marketer from Email Geeks suggests that many ESPs employ extensive soft bounce retry policies without negative reputation impact, comparing it to an MTA checking if a recipient is available rather than sending new messages.
What the experts say
Deliverability experts generally agree that while soft bounces are by definition temporary, their handling can still impact sender reputation. The key lies in balancing persistence with politeness towards mailbox providers. Overly aggressive or ill-timed retries, even for legitimate soft bounces, can be interpreted as spammy behavior, leading to throttling or reputation degradation. Experts emphasize that ESPs should continuously refine their retry configurations based on current ISP rules and data, ensuring they act as 'good neighbors' in the email ecosystem.
Key opinions
- Reputation impact: Aggressive retry policies, even for deferred messages, can negatively affect domain reputation and deliverability. Continuously hitting an unresponsive server at too high a rate can lead to persistent deferrals.
- ISP rules: Each ISP has unique rules for delivery rates, backoff, and pauses. ESPs' MTA configurations are based on data and experience to optimize for these varied rules.
- Speed of acceptance: The rate at which an ISP accepts emails is directly dependent on the sender's reputation. If deliveries are being throttled or limited, it signals a reputation issue.
- Beyond retries: While retry intervals are important, experts emphasize that soft bounce tolerance (how many soft bounces before an address is suppressed) is also a critical, though ESP-specific, mechanism.
- Prudent default settings: ESPs typically calibrate their default soft bounce and suppression settings to balance effective delivery with being a good neighbor to MBPs.
Key considerations
- Distinguish bounce types: Clearly defining soft bounces from other temporary failures or hard bounces is crucial for accurate analysis and management.
- Avoid relentless retries: An address that consistently generates deferred responses despite numerous retries may indicate a deeper problem and could harm reputation by wasting resources and appearing persistent.
- Align with ISP recommendations: Ideally, retry policies should align with ISP recommendations found in bounce codes, respecting suggested waiting periods before redelivery.
- Proactive list hygiene: Regardless of retry policies, maintaining a clean list and addressing the root causes of persistent soft bounces (e.g., inactive users, low engagement) is paramount for long-term domain reputation.
Expert from Email Geeks asserts that all delivery attempts, including deferred ones, count toward sending behavior, and an overly aggressive retry rate can indeed negatively impact reputation and deliverability by continuously triggering deferrals.
Expert from Email Geeks challenges the notion that deferred messages don't affect reputation by pointing out the industry practice of gradual volume ramp-ups, which is designed to build positive sender reputation.
What the documentation says
Official documentation and industry standards, such as RFCs, define the technical aspects of email delivery and retry mechanisms. While RFCs provide foundational guidelines for mail transfer agents (MTAs) to handle temporary errors, ESP-specific documentation elaborates on how these guidelines are implemented within their platforms. This documentation often highlights that soft bounces are temporary and that retries are standard, but also that persistent failures or high bounce rates ultimately impact sender reputation and deliverability.
Key findings
- RFC definition: The Internet Engineering Task Force (IETF) RFC 5321, section 4.5.4, provides guidelines for MTA retry strategies for transient (temporary) delivery failures.
- ESP-specific policies: ESPs like Salesforce Marketing Cloud outline specific retry schedules (e.g., every 15 minutes for 72 hours) for temporary bounce codes (e.g., 4xx).
- Suppression criteria: Documentation often defines criteria for moving subscribers to a held or undeliverable status based on a number of final bounces over a specified timeframe (e.g., 3 bounces over 15 days).
- Temporary nature: Soft bounces are consistently defined as temporary delivery failures due to issues like full mailboxes, server downtime, or message size limits.
- High bounce rate impact: Documentation from various sources agrees that a high overall email bounce rate can significantly impact email campaign deliverability and sender reputation.
Key considerations
- Adhere to standards: Ensure your sending system or ESP's retry policies are aligned with RFC guidelines to maintain protocol compliance and optimize delivery.
- Monitor configurable settings: Be aware of any configurable settings within your ESP regarding soft bounce thresholds or suppression logic, even if default settings are recommended.
- Manage bounce rates proactively: Understand that even if soft bounces are temporary, their overall volume contributes to your bounce rate, which ISPs monitor to assess your sender reputation. Regular list hygiene is key.
- Distinguish retry vs. tolerance: Documentation helps to clarify the difference between automated retries for a single email and the soft bounce tolerance that affects subscriber status over multiple campaigns.
The Internet Engineering Task Force (IETF) RFC 5321, section 4.5.4, details recommended retry strategies for mail transfer agents (MTAs) when encountering temporary delivery failures.
Salesforce Marketing Cloud documentation outlines that their system attempts email redelivery every 15 minutes for up to 72 hours for temporary (4xx) bounce codes.
