Which SMTP bounce codes should lead to mailing list suppression?
Matthew Whittaker
Co-founder & CTO, Suped
Published 9 Aug 2025
Updated 19 Aug 2025
9 min read
Managing email deliverability effectively requires a deep understanding of what happens after you hit send. One of the most critical aspects is interpreting SMTP bounce codes and knowing when to remove an address from your mailing list. This process, known as suppression, is vital for maintaining a strong sender reputation and ensuring your emails actually reach the inbox, rather than being flagged as spam or landing you on an email blocklist (or blacklist).
Ignoring bounce messages or failing to suppress problematic email addresses can severely damage your standing with internet service providers (ISPs) and email service providers (ESPs). High bounce rates signal to receivers that your list quality is poor or that you might be sending unsolicited mail, which can lead to your IP addresses or domains being blocklisted, significantly impacting your future email campaigns.
I often see confusion around which specific SMTP codes warrant immediate action versus those that indicate a temporary issue. The key is distinguishing between permanent and temporary failures, which the SMTP codes clearly define. Let's delve into these codes and how to manage your mailing lists based on the bounce feedback received.
Understanding SMTP bounce codes
SMTP bounce codes are three-digit numbers issued by receiving mail servers that communicate the status of an email delivery attempt. The first digit indicates the success or failure of the command, while the following two digits provide more specific information. This structure allows for a standardized way to interpret why an email bounced.
Broadly, SMTP codes are categorized into two main types: permanent failures (hard bounces) and temporary failures (soft bounces). Understanding this fundamental distinction is the first step in deciding whether to suppress an email address. A hard bounce indicates a permanent reason why the email cannot be delivered, such as an invalid address. Conversely, a soft bounce signifies a temporary issue that might resolve itself, like a full inbox.
The common SMTP codes usually start with a 4 or a 5, with 2xx codes indicating successful delivery. For deliverability purposes, we're primarily concerned with 4xx and 5xx codes. These codes provide the specific context needed for appropriate mailing list management and to correctly classify and manage SMTP bounce codes.
SMTP code
Meaning
Example
5.X.X
Permanent failure
Hard bounce
4.X.X
Temporary failure
Soft bounce
2.X.X
Success
Delivered
Hard bounces and immediate suppression
Hard bounces, characterized by 5xx SMTP codes, are unequivocal indicators that an email address is permanently unreachable. These codes signify that the email address simply doesn't exist, is invalid, or the domain name is misspelled. When you receive a 5xx bounce, it means there's no chance of future delivery to that address, regardless of how many times you try.
Immediate suppression is critical for hard bounces. Continuing to send emails to these addresses will escalate your bounce rate, which directly harms your sender reputation. ISPs and ESPs use bounce rates as a key metric to evaluate your sending practices. A persistently high bounce rate can lead to your emails being directed to the spam folder, or even result in your domain or IP being added to a blacklist (or blocklist). Most email service providers automatically add hard-bounced addresses to a suppression list to prevent future sends.
Examples of 5xx codes that demand immediate suppression include:
550 mailbox not found: The email address does not exist on the recipient's server.
553 recipient rejected: The recipient address is invalid, or the domain does not accept mail for the user.
554 Transaction failed: Often indicates a permanent failure related to policy issues, such as the sender being blocklisted or the content being identified as spam.
Action required
Any email address that generates a hard bounce should be immediately and permanently removed from your active mailing list. This proactive management of hard bounces helps preserve your sender reputation.
Soft bounces and suppression logic
Soft bounces, identified by 4xx SMTP codes, indicate temporary delivery issues. These are recoverable errors, meaning the email might be successfully delivered if you retry sending it later. Common reasons for soft bounces include a recipient's mailbox being full, server issues, or temporary network problems. Unlike hard bounces, a single soft bounce doesn't necessarily mean you should suppress the address immediately.
The challenge with soft bounces lies in determining when a temporary issue becomes a permanent one. Many ESPs implement a "soft bounce tolerance," where they will attempt to re-send to an address a certain number of times before classifying it as a hard bounce and suppressing it. For instance, a common practice for some ESPs like Mailchimp is to convert soft bounces into hard bounces after several consecutive failures.
My advice is to establish a clear soft bounce tolerance based on your sending volume and the nature of your email content. For most senders, allowing 3-5 consecutive soft bounces within a defined period (e.g., 30 days) before suppression is a reasonable approach. This allows for transient issues to resolve while preventing endless attempts to an effectively dead address. Some examples of 4xx codes are:
421 Service not available: The server is temporarily unavailable or experiencing overload.
450 Mailbox unavailable: The mailbox might be temporarily locked or busy.
451 Local error in processing: A temporary issue on the receiving server prevented delivery.
Handling hard and soft bounces
Hard bounces (5xx codes)
Definition: Permanent delivery failures, often due to an invalid or non-existent email address. These are unrecoverable.
Action: Immediately remove from all mailing lists and add to your suppression list. No further attempts should be made.
Impact on sender reputation: High numbers severely damage your reputation and increase the risk of blocklisting.
Soft bounces (4xx codes)
Definition: Temporary delivery failures, such as a full inbox or a server being temporarily down. These are often recoverable.
Action: Reattempt delivery a limited number of times. If persistent, then suppress the address.
Impact on sender reputation: Moderate impact if managed properly, but persistent soft bounces can accumulate and harm reputation.
While the 5xx/4xx distinction is a good starting point, the specific sub-codes and human-readable messages provide critical context. For example, a 550 bounce with a message like "User unknown" is a clear hard bounce. However, a 550 with a message like "Blocked for policy reasons" might be a temporary issue if you can resolve the underlying policy violation, but it's usually indicative of a more serious problem requiring suppression.
It's also important to note that different ESPs and mail servers might interpret and report these codes with slightly different nuances. A "DNS failure" (code 21) is classified as a soft bounce by some providers, suggesting a temporary network issue, whereas others might treat it more severely if persistent. I recommend consulting resources like the SMTP Field Manual to understand common responses.
Automated bounce processing is essential for large-scale sending. Your email platform should automatically identify hard bounces and add them to a suppression list. For soft bounces, the system should ideally have logic to retry delivery a few times, then suppress the address if the bounces persist. This systematic approach helps ensure optimal email deliverability rates and maintain a clean mailing list.
Identifying specific codes for suppression
Beyond the general 5xx classifications, certain specific SMTP codes consistently indicate a need for immediate suppression. These are codes that clearly state the recipient doesn't exist, the mailbox is disabled, or the message is blocked permanently due to content or sender reputation.
For example, a 550 5.1.1 (Destination mailbox invalid) is a classic hard bounce. Similarly, 554 5.7.1 (Message rejected due to spam content or policy violation) often means your email was outright refused due to reputation issues or flagged content, and retrying won't help without addressing the root cause.
While 4xx codes typically warrant retries, a few specific 4xx codes or repeated occurrences of certain soft bounces can also indicate a need for suppression. For instance, if you consistently receive a 452 (Insufficient system storage) for a particular recipient, and it doesn't resolve over time, it could signify an inactive or abandoned mailbox that should eventually be removed. It is worth checking your blocklist monitoring tools too.
SMTP error messages for immediate suppressiontext
550 5.1.1 <user@example.com>: Recipient address rejected: User unknown
550 5.7.1 Service unavailable; Client host [X.X.X.X] blocked using spamhaus.org
550 5.2.1 <user@example.com>... Mailbox disabled for this recipient
554 5.7.1 Policy rejection on the target address.
Maintaining list hygiene and deliverability
Ultimately, the goal of robust bounce management is to maintain a high-quality mailing list. A clean list improves deliverability, reduces your bounce rate, and helps you avoid being flagged by ISPs and spam filters. By suppressing bad email addresses, you're not just preventing failed deliveries, you're actively safeguarding your sender reputation.
Implementing strong validation processes at the point of email acquisition, such as double opt-in, can significantly reduce hard bounces from the outset. Regular list hygiene, including removing inactive subscribers and those who consistently soft bounce, further contributes to a healthy email ecosystem.
Proactive bounce management is an ongoing effort. It's not a one-time fix but a continuous process that ensures your email program remains effective and your messages consistently reach their intended recipients. Understanding and acting on SMTP bounce codes is a cornerstone of this effort. This also means regularly reviewing your domain reputation.
Views from the trenches
Best practices
Implement a robust bounce processing system that automatically categorizes bounces into hard and soft types for efficient management.
Use email validation tools to clean your lists regularly and prevent sending to invalid addresses from the start.
Monitor your bounce rates closely and investigate any sudden spikes to identify and resolve underlying issues.
Common pitfalls
Failing to differentiate between hard and soft bounces, leading to unnecessary suppression or continued sending to invalid addresses.
Ignoring soft bounces, which can accumulate and eventually lead to a poor sender reputation if not managed with a tolerance threshold.
Not regularly cleaning your email lists, resulting in higher bounce rates and decreased email campaign effectiveness.
Expert tips
Automated bounce management is key. Set up rules to immediately suppress 5xx errors and manage 4xx errors with a retry policy before suppression.
Regularly export and review your bounce logs to identify patterns or recurring issues with specific mail providers or domains.
Consider a double opt-in process for new subscribers to minimize invalid addresses entering your list from the start.
Expert view
Expert from Email Geeks says all 5xx errors are unrecoverable and one bounce is sufficient to remove the address from the list.
2019-10-09 - Email Geeks
Marketer view
Marketer from Email Geeks says if there is an export of bounces, they can help identify any red flags for suppression.
2019-10-09 - Email Geeks
Conclusion: The path to better deliverability
Effective management of SMTP bounce codes is paramount for anyone serious about email deliverability. By understanding the distinction between hard and soft bounces, and applying appropriate suppression logic, you can significantly improve your list hygiene and protect your sender reputation.
Prioritize immediate suppression for hard bounces (5xx codes) and implement a strategic retry policy for soft bounces (4xx codes) before eventual suppression. This disciplined approach ensures your email program remains efficient, avoids blacklists (or blocklists), and maximizes your inbox placement rates.
