How do email service providers manage soft and hard bounces, bounce codes, and soft bounce tolerance?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 May 2025
Updated 19 Aug 2025
11 min read
Email service providers (ESPs) play a crucial role in managing email bounces, which are messages that cannot be delivered to the recipient. These bounces are categorized primarily into soft bounces and hard bounces, each signaling a different type of delivery issue. How an ESP handles these bounces directly impacts a sender's deliverability and overall reputation. Understanding their internal processes, including how they interpret bounce codes and apply soft bounce tolerance rules, is vital for any sender looking to maintain a healthy email program.
At a fundamental level, when an email fails to deliver, the recipient's mail server sends back an SMTP (Simple Mail Transfer Protocol) bounce code. These codes, often accompanied by a descriptive message, provide insights into why the delivery failed. ESPs then process these codes to determine whether the bounce is temporary or permanent, guiding their subsequent actions for that recipient address. This classification is critical because mismanaging bounces can lead to significant deliverability issues, including being flagged as spam or landing on a blocklist (or blacklist).
The distinction between soft and hard bounces, while commonly used by ESPs, isn't always explicitly defined by technical standards like RFCs. Instead, it's often an internal construct developed by ESPs to safeguard their sending infrastructure and their clients' sender reputations. This means that while there are general conventions, the specifics of how bounces are handled can vary from one ESP to another. Let's delve into how these mechanisms work.
Understanding bounce codes and classifications
When an email bounces, the sending server receives an SMTP status code, typically a three-digit number, indicating the delivery status. These codes are standardized, but their interpretation can vary slightly depending on the receiving mail server and, subsequently, the ESP. The first digit of the code is key to understanding the nature of the bounce.
A 4.x.x code generally signifies a temporary failure, known as a soft bounce. This means the email reached the recipient's server but couldn't be delivered for a transient reason. Common examples include a full mailbox, server downtime, or a temporary block by a spam filter. In these cases, ESPs will typically retry sending the email after a certain period, often with increasing back-off intervals. The idea is that the temporary issue might resolve itself, allowing the email to eventually go through.
In contrast, a 5.x.x code usually indicates a permanent error, which an ESP would categorize as a hard bounce. This often means the email address is invalid, nonexistent, or the domain itself is permanently unreachable. When an ESP receives a hard bounce, it typically ceases all further attempts to send to that address and adds it to a suppression list to protect the sender's reputation. Repeatedly sending to invalid addresses can look like a directory harvesting attack or poor list hygiene, negatively affecting your standing with mailbox providers.
However, it is important to note that not all 5.x.x codes automatically classify as hard bounces by ESPs. For example, a 5.7.1 policy rejection, which might indicate your email was flagged as spam, could be treated as a soft bounce. This is because a future email with different content might pass the spam filter, or if your IP or domain gets delisted from a blacklist (or blocklist), future sends might succeed. ESPs must analyze the specific context of each bounce code, not just the leading digit, to make informed decisions about future sending attempts. You can learn more about how ESPs classify these codes in our guide on how email service providers classify SMTP bounce codes.
SMTP Code
Meaning
ESP Classification (Typical)
4.2.2
Mailbox full
Soft bounce (temporary)
4.4.1
Network connection error
Soft bounce (temporary)
5.1.1
Bad destination mailbox address
Hard bounce (permanent)
5.7.1
Delivery not authorized, message refused
Soft bounce (often policy-based, can be temporary)
Managing soft bounce tolerance
The concept of soft bounce tolerance is an ESP-specific mechanism designed to manage recipients who experience temporary delivery failures. Unlike hard bounces, which lead to immediate suppression, soft bounces allow ESPs to keep trying to deliver messages to an address for a defined period or a set number of attempts. This tolerance is crucial because it gives the recipient's mail server time to resolve transient issues, preventing the premature removal of a potentially valid email address from your list.
ESPs implement soft bounce tolerance differently. Some might have a fixed number of soft bounces (e.g., 3, 7, or even 15) over the lifetime of a subscription, as illustrated by Mailchimp's policy of allowing multiple soft bounces. Others might consider a combination of bounces and engagement, or factor in the time between bounces. For instance, an ESP might convert a soft bounce to a hard bounce only after several consecutive soft bounces occur over a specific time period (e.g., 10-14 days) without any successful deliveries in between. If a delivery succeeds, it typically resets the soft bounce counter, recognizing that the temporary issue has cleared up.
The optimal soft bounce tolerance isn't a one-size-fits-all number. It heavily depends on your sending frequency. If you send daily emails, a low tolerance (e.g., 3 bounces) could lead to rapid suppression of a recipient who is simply on vacation or has a temporarily full mailbox. Conversely, if you send monthly, a tolerance of 3 might be perfectly adequate, allowing three months for the issue to resolve. It's crucial for senders to understand their ESP's specific soft bounce policy and how it might impact their list hygiene and deliverability.
Adjusting the soft bounce tolerance, where possible, or aligning your sending frequency with your ESP's default tolerance, is a key aspect of proactive bounce management. Ignoring this can inadvertently lead to the suppression of valuable, engaged subscribers due to transient issues. You can explore this further in our article on how email soft bounce retry policies affect domain reputation.
Impact on sender reputation and deliverability
Hard bounces and reputation
Hard bounces, indicating permanent delivery failures, have a significant negative impact on sender reputation. Mailbox providers, like Google and Yahoo, view high hard bounce rates as a strong indicator of poor list hygiene. This can suggest that a sender is not maintaining their list effectively or, worse, is sending to purchased or scraped lists, which are often filled with invalid addresses. Such behavior can lead to increased spam classifications and even blocklisting (or blacklisting) of your sending IPs and domains, impacting your overall domain reputation. ESPs proactively suppress hard bounced addresses to protect both their infrastructure and their clients' deliverability.
Effective hard bounce management is non-negotiable. Senders should never attempt to resend to addresses that have hard bounced, as this sends negative signals to mailbox providers and can quickly damage sender trust. The general rule is that a hard bounce means that address should be removed permanently from your active mailing list.
While soft bounces don't immediately damage your sender reputation in the same way hard bounces do, repeated soft bounces to the same address can still signal underlying issues. If an address consistently soft bounces, it could indicate an abandoned inbox, an consistently overloaded server, or an aggressive spam filter that regularly defers your mail. Over time, these can accumulate and contribute to a decline in your sender score. Therefore, ESPs often have a threshold for soft bounces, after which they convert them into hard bounces and suppress the address, as discussed above.
The cumulative effect of soft bounces can sometimes be a silent killer of deliverability. It's not about one or two temporary failures, but a pattern of undeliverability. Mailbox providers track sender behavior closely, and if a significant portion of your emails consistently hits temporary walls, it can raise red flags. This is why ESPs aim to balance the desire to reach recipients with the need to protect their clients' reputations by eventually suppressing addresses with persistent soft bounce issues. For more insights, refer to this guide on managing soft and hard bounces.
ESP best practices and sender responsibilities
ESPs are constantly refining their bounce management algorithms to optimize deliverability. This involves sophisticated analysis of bounce codes, retry logic, and subscriber engagement. A well-managed bounce system ensures that legitimate emails have every chance to reach their destination while quickly removing problematic addresses that could harm sender reputation. For instance, ESPs often use advanced soft bounce suppression logic that considers not just the number of bounces but also the time elapsed, the sender's frequency, and even the recipient's past engagement.
From a sender's perspective, actively monitoring your bounce rates is crucial. While ESPs handle the immediate suppression, understanding the types of bounces you receive can inform your list cleaning strategies and content optimization. For example, a sudden spike in 5.7.1 policy rejections might indicate an issue with your email content or authentication, prompting you to review your messaging or DMARC, SPF, and DKIM records. Regular list hygiene, removing inactive or unengaged subscribers, can preempt many bounce issues. This is covered in more detail in our guide on email list hygiene best practices.
Proactive steps for senders
Validate lists: Regularly clean your email lists to remove invalid or inactive addresses. This is the most effective way to prevent hard bounces.
Monitor reports: Pay close attention to your ESP's bounce reports. Understand the specific bounce codes and messages to diagnose underlying issues.
Segment based on engagement: Isolate less engaged subscribers into separate segments and consider re-engagement campaigns or removal if they continue to soft bounce.
Ultimately, the goal of ESP bounce management is to maximize deliverability and preserve sender reputation. By working in tandem with your ESP and actively engaging in list hygiene, you can significantly reduce bounce rates and ensure your emails reach their intended recipients. More information on managing different types of bounces can be found in our comprehensive article on best practices for managing hard and soft bounces.
The cycle of re-engagement and resets
When an email successfully delivers, it typically signals to the ESP that any previous temporary issues for that address have cleared. Most ESPs will then reset the soft bounce counter for that recipient. This ensures that a recipient who was temporarily unavailable, perhaps due to a full mailbox or server maintenance, doesn't get prematurely suppressed once their mailbox is clear. This continuous feedback loop helps ESPs maintain the most accurate status for each subscriber on your list. It is also why it's important to differentiate between block, soft, and hard email bounces.
This reset mechanism means that even if a recipient experiences several soft bounces over a short period, a single successful delivery can give them a fresh start. This approach prioritizes recipient engagement and deliverability over strict, unyielding suppression rules. It acknowledges the dynamic nature of email delivery and recipient availability.
The continuous monitoring of delivery events and their impact on bounce counters highlights the complexity behind ESPs' bounce management systems. They aim to strike a balance: being resilient enough to handle temporary glitches while being strict enough to protect sender reputation from permanent delivery failures. This continuous adaptation ensures that as mail server behaviors evolve, so too do the strategies for effective bounce management, keeping email a viable communication channel.
Views from the trenches
Best practices
Actively maintain your email lists by regularly removing unengaged or invalid contacts to improve deliverability.
Understand your ESP's default soft bounce tolerance and adjust your sending frequency if needed to align with it.
Segment your audience based on engagement to send more relevant emails and reduce overall bounce rates.
Monitor your bounce reports closely, distinguishing between temporary and permanent failures.
Implement email authentication protocols like DMARC, SPF, and DKIM to bolster sender reputation and reduce policy rejections.
Common pitfalls
Assuming all 5.x.x SMTP codes are hard bounces, as some indicate temporary policy rejections or blocklisting.
Ignoring soft bounces, thinking they don't impact sender reputation; repeated soft bounces can lead to suppression.
Not knowing your ESP's soft bounce tolerance, which can lead to premature suppression of valid, but temporarily unavailable, subscribers.
Failing to clean your list after hard bounces, which signals poor list hygiene and can result in blocklisting (or blacklisting).
Not considering the time element in soft bounce tolerance; a fixed number of bounces over a short period can be too aggressive for frequent senders.
Expert tips
Consider a soft bounce threshold that accounts for a minimum number of consecutive unsuccessful deliveries over a specific time period (e.g., 3-5 soft bounces over 10-14 days with no successful deliveries in between).
A successful delivery should always reset the soft bounce counter for a subscriber, reflecting that the previous issue has been resolved.
While RFCs define transient vs. permanent failures, ESPs internally categorize soft vs. hard bounces based on their impact on sender reputation and deliverability.
Be aware that some mail servers might return ambiguous codes (e.g., 4.1.1 for 'no such user'), which can mislead standard bounce classification.
Advocate for more transparent documentation from ESPs regarding their soft bounce tolerance policies, as this is a critical aspect of deliverability.
Expert view
Expert from Email Geeks says most 5.x.x bounces are not hard bounces, as many systems will retry them, despite the 5.x.x code signifying that particular message should not be tried again.
2019-03-20 - Email Geeks
Expert view
Expert from Email Geeks says 4.x.x codes indicate a message should be retried with an increasing backoff interval.
2019-03-20 - Email Geeks
Key takeaways for effective bounce management
Understanding how email service providers manage soft and hard bounces, interpret bounce codes, and apply soft bounce tolerance is fundamental to maintaining strong email deliverability. ESPs act as crucial intermediaries, translating complex SMTP bounce codes into actionable classifications that protect your sender reputation and maximize inbox placement. While hard bounces demand immediate suppression to prevent reputational damage, soft bounces are handled with a more nuanced approach, involving retries and tolerance thresholds based on the likelihood of the issue resolving.
For senders, staying informed about your ESP's specific bounce management policies and regularly monitoring your bounce reports is critical. Proactive list hygiene, coupled with an understanding of how different bounce types impact your sender reputation, empowers you to optimize your email program for long-term success. By appreciating the intricacies of bounce management, you can ensure your messages consistently reach your audience, building trust and engagement.
Google and 
Yahoo, view high hard bounce rates as a strong indicator of poor list hygiene. This can suggest that a sender is not maintaining their list effectively or, worse, is sending to purchased or scraped lists, which are often filled with invalid addresses. Such behavior can lead to increased spam classifications and even blocklisting (or blacklisting) of your sending IPs and domains, impacting your overall domain reputation. ESPs proactively suppress hard bounced addresses to protect both their infrastructure and their clients' deliverability.
