Summary
Email Service Providers manage email deliverability by classifying bounces into two main categories: soft bounces, which indicate temporary delivery issues, and hard bounces, which signify permanent failures. This classification is primarily driven by SMTP bounce codes, with 4.x.x codes typically denoting soft bounces and 5.x.x codes representing hard bounces. While hard-bounced addresses are immediately suppressed from future sends, soft bounces trigger re-delivery attempts. To maintain sender reputation and list hygiene, ESPs implement a 'soft bounce tolerance,' an algorithmic mechanism that monitors repeated soft bounces from an address. If an address consistently produces soft bounces over a set period or after a specific number of attempts, the ESP's system will convert it into a hard bounce and suppress it. The specific criteria for soft bounce conversion and retry routines, including whether successful deliveries or engagement reset bounce counters, vary considerably across different ESPs.
Key findings
- Bounce Classification by Code: Email Service Providers classify delivery failures as either hard (permanent) or soft (temporary) bounces, primarily using SMTP bounce codes, with 4.x.x codes generally indicating soft bounces and 5.x.x codes indicating hard bounces. However, some 5.x.x codes, like policy rejections or blacklisting, may be treated as soft if the issue could be temporary.
- Soft Bounce Tolerance: A key mechanism is 'soft bounce tolerance,' where ESPs monitor repeated soft bounces from an email address and, after a certain number of occurrences or a specific timeframe without successful delivery, convert them into hard bounces. This process prevents endless re-delivery attempts and protects sender reputation.
- Retry Logic: Soft bounces typically trigger re-delivery attempts over a period, while hard bounces lead to immediate cessation of sending and suppression of the email address from future mailings.
- Counter Reset Mechanisms: Some ESPs employ sophisticated logic where a successful delivery or even an engagement signal, like a click, can reset the soft bounce counter for a recipient, providing a more flexible approach to managing temporary issues.
- ESP Policy Variation: The exact handling of soft bounces, including the number of retries, the retry duration, and the specific threshold for converting soft bounces to hard bounces, varies significantly among different Email Service Providers.
Key considerations
- Understand ESP Policies: Email marketers must understand their specific ESP's bounce handling policies, especially soft bounce tolerance, as these mechanisms directly impact list hygiene and deliverability.
- Monitor Bounce Rates: Regularly monitoring bounce rates and types is crucial for identifying potential issues, maintaining list health, and preventing reputation damage.
- Review Soft Bounce Thresholds: For frequent senders, default soft bounce tolerance settings may be too low, potentially removing active subscribers due to temporary issues. Understanding and, if possible, adjusting these thresholds can be beneficial.
- Leverage Engagement Data: While not universally offered, an ESP's ability to factor in engagement, such as clicks, to reset soft bounce counters provides a more robust and subscriber-friendly approach to bounce management.
What email marketers say
11 marketer opinions
Email Service Providers (ESPs) employ sophisticated systems to manage email bounces, distinguishing between temporary 'soft' bounces and permanent 'hard' bounces primarily based on SMTP bounce codes. This nuanced approach, including the implementation of soft bounce tolerance, is crucial for preserving sender reputation and maintaining clean email lists. While hard bounces lead to immediate address suppression, soft bounces prompt the ESP to initiate re-delivery attempts over a defined period. The core of this management lies in the ESP's algorithms, which monitor repeated soft bounces from an address and, upon reaching a set threshold or after a specific timeframe without successful delivery, automatically convert the soft bounce into a hard bounce, adding it to a suppression list. Importantly, successful delivery to an address typically resets its soft bounce counter, showcasing the dynamic nature of these systems. The specific thresholds for soft bounce conversion and retry routines can vary significantly across different ESPs, requiring marketers to understand their provider's unique policies.
Key opinions
- ESP-Driven Bounce Distinction: The differentiation between soft and hard bounces is largely an ESP construct, designed to preserve sender reputation and streamline deliverability, going beyond a strict interpretation of SMTP bounce codes alone.
- SMTP Code Interpretation: ESPs primarily classify bounces using SMTP codes: 4.x.x codes typically denote temporary soft bounces (leading to retries), while 5.x.x codes generally indicate permanent hard bounces (leading to immediate suppression). However, some 5.x.x codes, such as policy rejections or blacklisting, may be treated as soft bounces by ESPs if the issue could resolve over time.
- Soft Bounce Conversion Logic: ESPs employ a 'soft bounce tolerance' mechanism, where an address generating multiple soft bounces over a specific period or after a set number of attempts, without successful delivery in between, is algorithmically converted into a hard bounce and added to a suppression list. A recommended robust logic involves 3-5 soft bounces over 10-14 separate days.
- Counter Reset Upon Success: A crucial aspect of ESP bounce management is that a successful delivery to an email address typically resets its soft bounce tolerance counter, allowing for re-engagement even after temporary delivery issues.
- Variable ESP Policies: The specific retry routines, soft bounce thresholds, and conversion policies vary significantly between different Email Service Providers, emphasizing the need for marketers to understand their chosen platform's unique approach.
Key considerations
- Understanding ESP-Specific Policies: Email marketers must thoroughly understand their Email Service Provider's specific bounce handling mechanisms, including the soft bounce tolerance thresholds and retry routines, as these significantly influence deliverability and list health.
- Optimizing Soft Bounce Tolerance: The default soft bounce tolerance settings of an ESP might be insufficient for frequent senders, potentially leading to the premature suppression of temporarily unavailable but otherwise valid subscriber addresses. Marketers should investigate if these settings can be adjusted or accounted for in their strategy.
- Maintaining List Hygiene and Reputation: Effective bounce management, including the proper classification and suppression of hard bounces and persistent soft bounces, is critical for maintaining a clean email list and protecting sender reputation from being negatively impacted by repeated delivery failures.
- Leveraging Counter Reset Mechanisms: Recognize that successful email delivery typically resets an address's soft bounce counter, highlighting the dynamic nature of bounce management and the importance of continued successful engagement for preventing suppression.
Marketer view
Email marketer from Email Geeks explains that the distinction between soft and hard bounces is an ESP construct, designed to preserve sender reputation. He clarifies that 5.x.x bounce codes generally mean a message should not be retried, with 5.1.1 (invalid recipient) being a common hard bounce, while 4.x.x codes indicate a temporary failure that should be retried. He notes that policy rejections (e.g., 5.7.1 for spam) and blacklisting, despite being 5.x.x, should be classified as soft bounces because future emails might succeed. David suggests that hard bounces are added to a suppression list immediately, while soft bounces are not, but can be converted to hard bounces by ESP algorithms after too many occurrences. He advocates for a more robust soft to hard bounce conversion logic, recommending 3-5 soft bounces over 10-14 separate days with no successful deliveries in between, and emphasizing that successful delivery should reset the bounce counter.
22 Apr 2023 - Email Geeks
Marketer view
Email marketer from Email Geeks explains that soft bounce tolerance is an ESP-specific mechanism where soft bounces remain active on the list for a set number of sends before being converted to hard bounces. He notes that 4.x.x codes typically go into a retry routine, while 5.x.x codes do not result in retries for that specific message. Based on his decade of experience at an ESP, he confirms that a successful delivery typically resets the soft bounce tolerance counter. Neil references Mailchimp's and Act-On's varying soft bounce policies, stressing that a low tolerance, like a default of 3, might be insufficient for frequent senders and that understanding and adjusting this tolerance is crucial for email marketers, yet often overlooked.
14 Dec 2024 - Email Geeks
What the experts say
5 expert opinions
ESPs manage email delivery failures by categorizing them as soft or hard bounces. Hard bounces, indicating permanent issues, lead to immediate address suppression. For temporary soft bounces, ESPs initiate re-delivery attempts. They employ sophisticated logic, known as soft bounce tolerance, to monitor persistent soft bounces. This often involves converting an address to a hard bounce after a specific number of consecutive soft bounces, or if engagement signals, like clicks, are absent, effectively stopping further delivery attempts to problematic addresses. SMTP bounce codes, for example 4.x.x for temporary and 5.x.x for permanent, and diagnostic messages are crucial tools for ESPs to understand the precise reason for a bounce and determine the appropriate action, which can vary significantly between providers.
Key opinions
- Dynamic Soft Bounce Logic: ESPs' approaches to soft bounce handling vary significantly, ranging from simple lifetime counters to more dynamic systems where engagement signals, like clicks, can reset a soft bounce counter. This flexibility helps account for temporary recipient issues.
- Engagement-Driven Conversion: Some sophisticated ESPs will convert a soft bounce to a hard bounce only after a specific number of soft bounces, for example 5, if no engagement has occurred, with a successful engagement resetting the count. This intelligent approach prevents premature suppression of valuable subscribers.
- SMTP Codes Inform Actions: ESPs critically leverage SMTP bounce codes (4.x.x for temporary, 5.x.x for permanent) and associated diagnostic messages to accurately identify bounce reasons, guiding their decisions on re-delivery attempts versus immediate suppression.
- Consecutive Bounce Thresholds: For robust list hygiene, ESPs typically convert an address to a hard bounce after a consistent series of soft bounces, for example 5-7 consecutive times for the same reason, ensuring that persistent temporary issues are eventually treated as permanent.
Key considerations
- Varying ESP Mechanisms: Marketers must recognize that the specific criteria for soft bounce tolerance, including retry logic and thresholds for converting to a hard bounce, are not uniform across ESPs.
- Engagement's Role in Tolerance: Awareness of whether an ESP's soft bounce handling incorporates user engagement, such as clicks, is important, as this can lead to more accurate suppression decisions and potentially retain active subscribers.
- Interpreting Bounce Codes: While ESPs handle the technical interpretation, understanding that SMTP bounce codes are the foundation for their soft-hard bounce distinctions is key for comprehensive deliverability knowledge.
- Strategic Soft Bounce Conversion: The practice of converting consistently soft-bouncing addresses to hard bounces is a critical strategy for maintaining sender reputation and a clean, high-performing email list.
Expert view
Expert from Email Geeks confirms that soft bounce handling varies significantly between ESPs, with some using a high lifetime threshold (e.g., 20 bounces) and others a simple counter. He shares his preference for a system that converts a soft bounce to a hard bounce after 5 soft bounces without any clicks, where a click would reset the counter. Matt explains that this approach effectively accounts for temporary issues like vacations or abandoned accounts by incorporating engagement signals, rather than relying on a fixed, lifetime number of bounces.
24 Jul 2024 - Email Geeks
Expert view
Expert from Spam Resource explains that Email Service Providers (ESPs) manage soft bounces, which are temporary delivery issues, by re-attempting delivery over a period. If these messages continue to soft bounce after several attempts, the ESP will convert them to a hard bounce and cease future sending, effectively demonstrating soft bounce tolerance. Hard bounces, indicating permanent delivery failures, lead to immediate suppression.
10 Apr 2024 - Spam Resource
What the documentation says
6 technical articles
Email Service Providers (ESPs) handle delivery failures by distinguishing between soft and hard bounces, a classification primarily driven by interpreting SMTP bounce codes. Soft bounces, which signify temporary issues, trigger multiple re-delivery attempts over a set duration. In contrast, hard bounces, indicative of permanent problems, lead to the immediate suppression of the recipient's address. A critical aspect of this management is the 'soft bounce tolerance' mechanism: ESPs monitor repeated soft bounces from an individual address, and if the issue persists beyond a predefined threshold or timeframe, they automatically convert it to a hard bounce and add it to a suppression list, thereby safeguarding sender reputation and ensuring efficient list hygiene.
Key findings
- SMTP Code-Driven Classification: Email Service Providers (ESPs) classify delivery failures as either hard or soft bounces, relying heavily on SMTP codes; 4xx codes typically indicate temporary soft bounces, while 5xx codes signify permanent hard bounces.
- Temporary Bounce Retries: Soft bounces prompt ESPs to retry sending the email multiple times over a defined duration, giving the temporary delivery issue a chance to resolve itself.
- Persistent Soft Bounce Conversion: To maintain list hygiene and sender reputation, ESPs employ a 'soft bounce tolerance' mechanism, converting an email address to a hard bounce if it consistently generates soft bounces over a set period or after a specific number of attempts.
- Hard Bounce Suppression: Upon receiving a hard bounce, ESPs instantly suppress the associated email address, preventing any further delivery attempts and safeguarding the sender's reputation.
- Variations in ESP Rules: While the core principles are consistent, the specific parameters for retry duration, the number of attempts, and the thresholds for converting soft bounces to hard bounces differ among Email Service Providers.
Key considerations
- Familiarize With ESP Policies: Marketers should thoroughly understand their Email Service Provider's specific policies for bounce management, including their soft bounce tolerance thresholds and retry logic, as these directly impact deliverability and list health.
- Maintain Strict List Hygiene: Actively managing bounces, particularly by ensuring hard bounces and consistently soft-bounced addresses are suppressed, is vital for maintaining a clean email list and protecting sender reputation.
- Leverage Bounce Data: ESPs provide detailed bounce notifications and SMTP codes; utilizing this information is key for diagnosing specific delivery issues and making informed decisions about recipient list management.
- Deliverability Impact: The effectiveness of an ESP's bounce management system directly correlates with overall email deliverability, as unmanaged bounces can significantly harm sender reputation.
Technical article
Documentation from SendGrid explains that they classify bounces into two categories: hard bounces and soft bounces. Hard bounces are permanent delivery failures, leading to the email address being added to their suppression list immediately. Soft bounces are temporary issues, and SendGrid will attempt re-delivery for a period before considering it a hard bounce if the issue persists. They process various SMTP bounce codes, such as 5xx codes for hard bounces and 4xx codes for soft bounces, to determine the bounce type and appropriate action.
3 Jul 2024 - SendGrid Documentation
Technical article
Documentation from Mailchimp explains that they categorize bounces into hard, soft, abuse, and block bounces. Hard bounces are permanent failures and are immediately removed from the audience to protect sender reputation. Soft bounces are temporary issues, and Mailchimp will retry sending for a set period. If a soft bounce persists after multiple retries, it can be converted to a hard bounce. They use SMTP codes to determine the exact type of bounce and apply the corresponding action, such as automatically cleaning hard-bounced addresses.
16 May 2023 - Mailchimp Help
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