Summary
Email Service Providers (ESPs) play a crucial role in managing email deliverability, particularly concerning bounces. They distinguish between soft and hard bounces, interpret SMTP bounce codes, and implement specific soft bounce tolerance rules. These mechanisms are vital for maintaining a sender's reputation and ensuring messages reach their intended recipients. Understanding how these systems work helps marketers optimize their campaigns and improve inbox placement.
Key findings
- Categorization: ESPs classify bounces into soft and hard categories to differentiate between temporary and permanent delivery issues. Learn more about hard and soft email bounces.
- SMTP codes: The 4.x.x codes indicate transient (temporary) failures, prompting retries, while 5.x.x codes signify permanent failures. However, some 5.x.x codes (like policy rejections or blocklist listings) might still be treated as soft bounces by ESPs.
- Soft bounce tolerance: This is an ESP-specific setting that determines how many consecutive soft bounces an email address can incur before being converted into a hard bounce and suppressed from future sends. What is a reasonable soft bounce tolerance?
- Reputation preservation: The primary goal of bounce management by ESPs is to protect the sender's reputation by preventing repeated attempts to deliver to invalid or problematic addresses.
Key considerations
- Varying policies: Soft bounce tolerance and classification rules differ significantly between ESPs, leading to varied handling of delivery failures.
- Frequency sensitivity: The effectiveness of soft bounce tolerance is heavily influenced by sending frequency; a low tolerance with high frequency can prematurely suppress valid contacts.
- RFC versus ESP: The distinction between soft and hard bounces is an ESP-specific construct, not explicitly defined in RFCs, which only classify failures as transient or permanent.
- Resetting tolerance: A successful delivery to a previously soft-bouncing address should ideally reset its soft bounce counter. Mailchimp's policy serves as an example of this approach, as described in their Soft vs. Hard Bounces documentation.
What email marketers say
Email marketers often navigate the complexities of bounce management with varying levels of insight into their ESP's specific configurations. While the fundamental distinction between soft and hard bounces is generally understood, the underlying logic, particularly around soft bounce tolerance, can be less transparent. Marketers emphasize the practical impact of these settings on list hygiene and campaign reach, advocating for more clarity and control over how bounces are handled to maintain healthy subscriber lists and sender reputation.
Key opinions
- Lack of awareness: Many email marketers are unaware of their ESP's specific soft bounce tolerance settings, which can lead to unforeseen list suppression.
- Tolerance impact: A low soft bounce tolerance, especially with high sending frequency, can prematurely remove valid subscribers from a list.
- Frequency alignment: The optimal soft bounce tolerance should ideally be adjusted based on the sending frequency of campaigns to prevent legitimate bounces (like full mailboxes) from becoming permanent suppressions.
- ESP transparency: There's a desire for ESPs to be more transparent and provide configurable options for soft bounce tolerance, as highlighted in SendLayer's article on hard and soft bounces.
Key considerations
- Managing campaigns: Understanding and managing bounce types is crucial for the success of email campaigns. Explore best practices for managing hard and soft bounces.
- Suppression logic: The point at which an ESP suppresses a user after multiple soft bounces should be carefully considered to avoid losing engaged subscribers who experience temporary delivery issues. Best practices for soft bounce suppression logic.
- Retry vs. tolerance: It's important to distinguish between the MTA's retry interval for 4.x.x errors and the ESP's soft bounce tolerance, which governs suppression across multiple sends.
- Impact of absence: Subscribers temporarily unable to receive emails (e.g., full mailbox, vacation) risk premature suppression if the soft bounce tolerance is too strict, impacting potential future engagement.
Marketer view
Marketer from Email Geeks suggests that many 5.x.x bounces are not hard bounces, implying that systems might still retry them.
Marketer view
Marketer from Mailchimp explains their soft bounce policy, noting a higher tolerance for active subscribers, allowing up to 15 soft bounces for engaged users.
What the experts say
Experts in email deliverability offer a nuanced perspective on bounce management, often distinguishing between the technical definitions provided by RFCs and the practical implementations by ESPs. They highlight that while SMTP bounce codes (4.x.x and 5.x.x) indicate transient or permanent failures, the classification into 'soft' and 'hard' bounces is largely an ESP construct designed to protect sender reputation. Experts emphasize the need for sophisticated bounce handling that accounts for recoverable issues and avoids premature suppression.
Key opinions
- ESP constructs: Soft versus hard bounces are primarily ESP constructs, developed to preserve sender reputation, rather than direct definitions from RFCs.
- SMTP code interpretation: While 4.x.x codes suggest retries, not all 5.x.x codes indicate a permanent hard bounce. For instance, policy rejections (e.g., 5.7.1) or blocklist listings should often be treated as soft bounces because the issue might be temporary or remediable. What do different SMTP bounce codes mean?
- Suppression criteria: An optimal soft bounce conversion to hard bounce should ideally involve multiple consecutive failures over a sufficient period (e.g., 3-5 bounces over 10-14 days), with no successful deliveries in between.
- Reputation risk: Repeatedly sending to invalid or bouncing addresses can negatively impact sender reputation, leading to mail deferral or filtering to spam.
Key considerations
- Sender reputation: The way ESPs handle bounce retry policies can significantly affect domain reputation and overall deliverability. How do soft bounce retry policies affect reputation?
- Implementation quality: The quality and logic of soft bounce tolerance implementation vary widely among ESPs, impacting how effectively temporary issues are managed.
- Dynamic tolerance: A more sophisticated approach might involve a dynamic soft bounce tolerance that resets upon successful delivery, preventing premature suppression of transient issues.
- List hygiene: Consistent hard bounces or recurring soft bounces to the same address can signal poor list hygiene, which can severely damage sender credibility and inbox placement (blocklist/blacklist).
Expert view
Expert from Email Geeks notes that most 5.x.x bounces do not necessarily represent hard bounces, challenging common assumptions about bounce classification.
Expert view
Expert from Spam Resource emphasizes the criticality of accurate bounce code classification to avoid mistakenly suppressing valid recipients.
What the documentation says
Official documentation from various email service providers and technical sources clarifies the distinctions between temporary and permanent delivery failures, as outlined by SMTP response codes. While RFCs focus on the technical definition of transient (4.x.x) and permanent (5.x.x) errors, ESP documentation often translates these into their own 'soft' and 'hard' bounce classifications. This documentation typically provides guidelines on how their systems handle retries, suppressions, and soft bounce tolerance, often linking these policies to subscriber activity and overall sender reputation management.
Key findings
- RFC definitions: RFCs delineate between transient failures (4.x.x codes, indicating a temporary issue that may resolve) and permanent failures (5.x.x codes, indicating an unresolvable issue). For specific details, refer to What RFC 5322 says vs. what actually works.
- ESP interpretation: ESPs map these SMTP codes to their internal 'soft' and 'hard' bounce classifications, with varying levels of granularity and specific logic for converting soft to hard bounces.
- Tolerance rules: Documentation often outlines the number of soft bounces or the time period over which they are tolerated before an address is suppressed.
- Activity influence: Some ESPs adjust soft bounce tolerance based on subscriber activity; for example, Mailchimp provides different thresholds for active versus inactive subscribers.
Key considerations
- Parsing responses: The ability to accurately parse SMTP responses is fundamental for correctly identifying and categorizing hard and soft bounces. How to parse SMTP responses for bounces.
- Varied documentation: Despite the importance of bounce management, detailed documentation on specific soft bounce tolerance logic and configuration options can be inconsistent across ESPs.
- Policy nuance: Documentation often highlights the nuances of policy rejections (e.g., 5.7.1), which are technically permanent failures but are frequently treated as soft bounces by ESPs because the issue may be with content, not the recipient address itself.
- Customization: Some ESPs, as per their documentation, allow users to customize soft bounce tolerance, providing greater control over list hygiene tailored to sending patterns.
Technical article
Documentation from Mailchimp states that soft bounces are temporary delivery issues, distinct from hard bounces, and are handled with specific retry logic.
Technical article
Documentation from SendLayer details the various reasons for both hard and soft email bounces, along with troubleshooting methods to improve deliverability.
Related resources
5 resources
Related pages
What is the difference between hard and soft email bounces and how should they be managed?
What is a reasonable soft bounce tolerance in email marketing?
What are the best practices for managing hard and soft bounces in daily email campaigns?
What is the best practice for determining how many soft bounces before suppressing a user?
How do email soft bounce retry policies affect domain reputation and deliverability?
What do different SMTP bounce codes mean for email deliverability and blocks?
How to parse SMTP responses to identify hard and soft bounces?
What RFC 5322 Says vs. What Actually Works
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