What is the recommended soft bounce suppression logic for email?

Key findings

• Consecutive bounces: A common guideline suggests suppressing an address after 3 soft bounces over a period of at least 2 weeks. This allows for temporary issues to resolve while preventing excessive retries.
• Total soft bounce count: Some systems recommend removing an address after accumulating no more than 5 to 7 soft bounces in total, regardless of the timeframe.
• Timeframe consideration: The duration over which soft bounces are tracked is as important as the count itself. Two weeks is a frequently cited minimum period for tracking these temporary failures.
• Reputation impact: Aggressive soft bounce management is critical for maintaining a positive sender reputation and avoiding blocklists, as persistent attempts to send to problematic addresses signal poor list hygiene.

Key considerations

• Distinguish bounce types: Not all temporary failures should count towards suppression. For instance, rejections due to spam classification might not, while a full mailbox should. Understanding different bounce types and their causes is essential.
• Domain-specific policies: Some domains might have unique error codes or handling policies that require custom suppression logic outside of general rules. Flexibility in your system is key.
• Business needs: The aggressiveness of your suppression logic can be influenced by your specific business goals and the nature of your email campaigns. More aggressive suppression generally leads to better deliverability.
• Regular list cleaning: While suppression logic handles bounces, proactive list cleaning and validation can significantly reduce soft bounce rates overall.

Key opinions

• Consistency is key: Many marketers adopt a consistent policy, such as 3 soft bounces over 2 weeks, as a baseline for suppression to maintain deliverability standards.
• Higher tolerance: Some marketers use a slightly higher tolerance, like 5 or 7 soft bounces, particularly if their campaigns are less frequent or the temporary issues are common with their audience.
• Immediate suppression for hard bounces: There's a strong consensus that hard bounces should lead to immediate, permanent suppression, as they indicate invalid addresses and further sends severely harm reputation.
• Selective counting: Not all temporary failures are treated equally. Marketers often implement logic to distinguish between soft bounces like 'mailbox full' (which count) and 'spam rejection' (which may not count towards suppression thresholds).

Key considerations

• Adaptation to ESP behavior: Marketers must understand how their email service provider (ESP) handles bounces and align their suppression logic accordingly for optimal deliverability.
• Impact on engagement: While aggressive suppression improves deliverability, marketers also consider the potential loss of legitimate subscribers who might only experience temporary issues.
• Monitoring bounce rates: Continuous monitoring of soft bounce rates helps marketers identify trends and adjust their suppression logic as needed to maintain a healthy sending environment.
• User experience: Implementing double opt-in and other list hygiene practices upstream can reduce the number of problematic addresses entering the list, minimizing future soft bounces, as suggested by Shopify's blog.

Key opinions

• Reasonable aggression: Experts generally consider at least three consecutive rejections over a period of two weeks as a reasonable threshold before suppressing a recipient, balancing forgiveness for temporary issues with protection against reputation damage.
• Contextual definitions: The terms soft and hard bounces can be ambiguous, and experts advise establishing agreed-upon definitions internally to ensure consistent handling and accurate reporting.
• Data-driven decisions: ESPs or senders seeking to be more aggressive with their suppression logic should base such decisions on their specific data and business needs, rather than solely on general guidelines.
• Beyond simple counts: Sophisticated suppression logic goes beyond simple bounce counts, considering the specific bounce codes and reasons, such as differentiating between a full mailbox and a spam rejection.

Key considerations

• Not less aggressive: While flexibility exists for being more aggressive, experts advise against being less aggressive than the commonly accepted thresholds (e.g., 3 bounces over 2 weeks), as this could significantly harm sender reputation.
• Complexity of error handling: The deliverability landscape is complex, with varying error messages and handling by different mail providers. A robust system accounts for these exceptions rather than relying on a single general rule.
• Monitoring retry policies: Understanding how soft bounce retry policies affect domain reputation is vital. Excessive retries can quickly lead to blocklisting.
• Proactive list maintenance: Beyond bounce handling, experts advocate for proactive list validation and hygiene to minimize the initial number of soft bounces encountered.

Key findings

• M3AAWG guidelines: The Messaging, Malware and Mobile Anti-Abuse Working Group (M3AAWG) recommends suppressing an address after at least 2 soft bounces over a minimum of two weeks. This is a foundational guideline in email deliverability.
• ESP-specific rules: Email Service Providers (ESPs) like Klaviyo often have their own defined soft bounce suppression policies, such as suppressing an address if it soft bounces more than 7 consecutive times.
• Consecutive vs. total: Some documentation distinguishes between a number of consecutive soft bounces (e.g., 3 to 7) and a total accumulation of soft bounces before suppression, reflecting different approaches to managing temporary failures.
• Temporary suppression periods: Certain documentation, like Bloomreach's updates, outlines a staged approach, where addresses are temporarily suppressed for a set period (e.g., 14 days) after initial soft bounces, before potential permanent suppression.

Key considerations

• Impact of email size: Documentation often advises keeping email sizes below certain thresholds (e.g., 110KB for Klaviyo) to mitigate soft bounces related to recipient mailbox capacity or network issues.
• Proactive list hygiene: Official guides recommend regular list cleaning and the use of practices like double opt-in to reduce both hard and soft bounces, thereby improving overall deliverability and inbox placement.
• Bounce code analysis: Documentation typically references SMTP bounce codes (e.g., 4xx for soft bounces) as the technical basis for categorizing and handling different bounce reasons. This detail is critical for accurate suppression logic and for troubleshooting high soft bounce rates.
• Reputation management: Documentation often links effective bounce management directly to sender reputation. Failing to suppress soft bounces can lead to degraded reputation and increased chances of emails being blocklisted, emphasizing the need for robust logic as per M3AAWG's Sender Best Practices.