Why are deliverable users getting hard bounces and marked as undeliverable?
Michael Ko
Co-founder & CEO, Suped
Published 18 May 2025
Updated 17 Aug 2025
8 min read
It can be incredibly frustrating to see a user, whom you know for a fact is deliverable, suddenly get hit with a hard bounce. The common understanding is that a hard bounce signifies a permanent delivery failure, an unchangeable issue like a non-existent email address. So, when these seemingly valid email addresses hard bounce, and then mysteriously become deliverable again after you manually intervene, it challenges the very definition we rely on.
This paradoxical situation highlights a critical nuance in email deliverability: the distinction between what a mailbox provider reports and how your Email Service Provider (ESP) interprets that report. While a hard bounce typically implies a permanent issue, there are several scenarios where a deliverable address might still receive such a bounce, leading to it being incorrectly marked as undeliverable.
Understanding hard bounces and ESP classifications
The core of the confusion often lies in how ESPs classify bounces. The terms 'hard bounce' and 'soft bounce' are largely constructs made by ESPs to simplify their reporting and internal rules. In reality, a mailbox provider (like Mailchimp) sends back a specific SMTP error code that indicates a delivery failure. Your ESP then translates this code into a 'hard' or 'soft' bounce based on their internal logic and algorithms. Most of the time, this classification is accurate, but it's not foolproof.
Sometimes, temporary issues at the recipient's mail server or network can trigger a 5xx rejection code, which an ESP might then misinterpret as a permanent (hard) bounce. This leads to the email address being suppressed and marked as undeliverable, even if the issue was transient. This can be particularly frustrating when you know the user is expecting your emails and reaches out to you, proving their address is active.
Aggressively removing recipients after the very first 5xx rejection, as some ESPs do, can prematurely suppress valid contacts. While it is important to maintain a clean list to protect your sender reputation, a nuanced approach that considers the specific bounce message is often more effective than a blanket policy.
It's essential to remember that while ESPs generally classify bounces correctly, misclassification does occur. This is why having access to the raw rejection messages from the recipient mailbox provider is crucial. These messages provide the actual reason for the bounce, allowing for a more informed decision about whether an address is truly permanently undeliverable.
Common culprits behind false undeliverables
Several factors can cause a truly deliverable user to experience a hard bounce. One common cause is a temporary system outage or overload at the recipient's mailbox provider. During these periods, the server might return a permanent failure code (like Gmail or Yahoo Mail) even if the mailbox itself is valid and will accept mail later.
Spam blocking is another significant factor. If your sending IP or domain lands on a public email blacklist (or blocklist), or is caught by an aggressive spam filter, mail servers might reject your emails with a hard bounce. These rejections can sometimes be temporary, especially if the listing is short-lived or the content-based block is resolved. Monitoring blocklists is key to identifying and addressing these issues promptly.
Mailbox providers, especially smaller ones, might also be less reliable than larger counterparts like Outlook. They might experience more frequent outages or simply have different, sometimes stricter, bounce classification rules. This can lead to a 'user not found' error even for an active mailbox if their system is experiencing a momentary glitch, or if your IP or domain is on a shared blacklist (or blocklist) they consult.
Furthermore, a hard bounce might occur if the recipient's domain itself (rather than just the email address) has an issue, such as it no longer existing or having DNS problems. While the user might still technically be 'deliverable' if they are reachable through other means, the email cannot reach them through that specific address due to a domain-level problem. This is distinct from a user not existing, and requires a different approach to diagnose.
Practical steps to investigate and resolve
The most crucial step in understanding why deliverable users receive hard bounces is to inspect the raw rejection message from the recipient's mail server. Your ESP should provide these details in their bounce logs. This message, often a 5xx SMTP reply, will give you the precise reason for the rejection, which may differ from your ESP's generalized hard bounce classification. For example, a 'USER_NOT_FOUND' might sometimes be a temporary catch-all error.
Example SMTP bounce message (USER_NOT_FOUND)text
550 5.1.1 <recipient@example.com>: Recipient address rejected: User unknown in local recipient table
Once you have the raw message, you can determine if the bounce is truly permanent or a temporary glitch. If it's a known, active user, and the message indicates a temporary issue, you might consider re-activating them. If your ESP's policy prevents this, you should open a dialogue with them. You can present your findings to advocate for a more nuanced approach to bounce handling, especially for those cases where a legitimate user is being inadvertently suppressed. Sometimes ESPs can adjust their internal logic for specific bounce types.
If manual re-activation is an option, it is a way to verify if the address is truly deliverable. This is what helps confirm that the initial hard bounce was a false positive. Continuously monitoring your deliverability metrics and engagement rates can also help you identify patterns where false hard bounces might be occurring, especially with specific domains or mailbox providers.
Traditional hard bounce handling
Upon receiving any 5xx SMTP error, the address is immediately marked as permanently undeliverable and suppressed from future sends. This is often the default or mandatory policy for some ESPs.
Pros: Minimizes risk to sender reputation by quickly removing potentially invalid addresses. Reduces sending costs associated with bouncing emails.
Cons: Can prematurely suppress valid, engaged contacts due to temporary issues or ESP misclassification. Leads to missed communication opportunities and customer frustration.
Nuanced bounce management
Bounce classification is based on analyzing the specific SMTP error message and context. Temporary issues are handled differently from truly permanent failures. Manual review for known active users is possible.
Pros: Maximizes reach to deliverable subscribers. Improves customer experience by ensuring continuity of communication. Provides more accurate data for list health management.
Cons:Requires more sophisticated bounce processing capabilities from the ESP. May require manual intervention or deeper analysis for ambiguous cases.
Considerations: Not all ESPs offer this level of control over bounce handling logic.
Building a resilient email program
Beyond immediate troubleshooting, building a robust email program means focusing on long-term strategies that minimize bounces, both true and false. Proactive list hygiene is paramount. Regularly cleaning your email lists of inactive or invalid addresses through regular validation can significantly reduce the number of hard bounces you encounter.
Strong email authentication protocols, including SPF, DKIM, and DMARC, are also vital. Proper authentication assures mailbox providers that your emails are legitimate, reducing the likelihood of them being flagged as spam or rejected due to trust issues. DMARC monitoring can provide valuable insights into authentication failures that might contribute to unexpected bounces.
Finally, continuous monitoring of your sender reputation and IP/domain blocklists allows you to detect issues early. If you see a sudden spike in hard bounces, especially from specific domains or smaller providers, it could indicate a temporary block (or blacklist) or a system outage that may resolve itself. Proactive monitoring helps you differentiate between a permanent issue and a transient one, allowing for more appropriate action.
Views from the trenches
Best practices
Always review the raw bounce messages, not just your ESP's classification, to understand the true reason for non-delivery.
Segment your email lists and consider a re-engagement strategy for contacts who hard bounce but you suspect are still valid.
Proactively clean your email lists using a reliable validation service to minimize sending to truly invalid addresses.
Common pitfalls
Blindly trusting ESP hard bounce classifications without investigating the underlying SMTP error codes from the recipient mailbox.
Aggressively removing users from your list after just one hard bounce, potentially cutting off deliverable subscribers.
Failing to monitor your sender reputation and blocklist status, which can lead to unexpected rejections.
Expert tips
Establish a clear communication channel with your ESP's support team to discuss their bounce handling policies and potentially request adjustments for specific cases.
Understand that smaller ISPs and mailbox providers might have less stable infrastructure, leading to temporary hard bounces that resolve on their own.
Implement robust email authentication (SPF, DKIM, DMARC) to build trust with mailbox providers and reduce the chance of emails being blocked.
Expert view
Expert from Email Geeks says that spam blocking is a common reason for temporary hard bounces, and mailbox providers like Yahoo and Gmail can also experience glitches that lead to such issues.
2024-10-28 - Email Geeks
Expert view
Expert from Email Geeks says that you need to examine the rejection message from your ESP to determine the exact cause of a hard bounce, as ESPs define hard bounces differently.
2024-10-28 - Email Geeks
Navigating false hard bounces
The phenomenon of deliverable users receiving hard bounces and being marked as undeliverable is a complex issue that requires more than a superficial understanding of bounce classifications. It challenges the conventional wisdom that a hard bounce is always a permanent, irreversible failure.
By delving into the actual SMTP rejection messages, understanding the nuances of ESP bounce handling, and implementing robust email practices, you can often re-engage these seemingly 'undeliverable' contacts. This proactive approach not only improves your email deliverability and sender reputation but also ensures that valuable communications reach their intended recipients.
Mailchimp) sends back a specific SMTP error code that indicates a delivery failure. Your ESP then translates this code into a 'hard' or 'soft' bounce based on their internal logic and algorithms. Most of the time, this classification is accurate, but it's not foolproof.
Gmail or 
Yahoo Mail) even if the mailbox itself is valid and will accept mail later.
Outlook. They might experience more frequent outages or simply have different, sometimes stricter, bounce classification rules. This can lead to a 'user not found' error even for an active mailbox if their system is experiencing a momentary glitch, or if your IP or domain is on a shared blacklist (or blocklist) they consult.
