What do different SMTP bounce codes mean for email deliverability and blocks?
Michael Ko
Co-founder & CEO, Suped
Published 22 May 2025
Updated 17 Aug 2025
9 min read
Every email sender faces bounce messages. These are automated notifications from receiving mail servers, indicating that your email could not be delivered to its intended recipient. While they might seem like cryptic codes at first glance, understanding what different SMTP bounce codes mean is critical for maintaining healthy email deliverability and avoiding getting on an email blocklist (or blacklist).
These codes provide invaluable feedback about your email campaigns. Ignoring them can lead to a damaged sender reputation, lower inbox placement rates, and potentially getting your IP address or domain listed on various email blocklists. By delving into the specifics of these codes, we can learn to troubleshoot common delivery issues and improve our email strategy.
Understanding the SMTP code structure
SMTP (Simple Mail Transfer Protocol) bounce codes are three-digit numbers, often followed by a more detailed enhanced status code (like 5.1.1) and a textual explanation. The first digit indicates the status of the delivery attempt, broadly categorizing the outcome. These codes are standardized, though some mail servers may add their own specific messages for more context. Understanding this fundamental structure helps in quickly identifying the nature of the delivery problem.
The initial digit is the most crucial for a quick assessment. A code starting with '2' generally means success, while '4' indicates a temporary failure (a soft bounce), and '5' signals a permanent error (a hard bounce). While the accompanying text can vary widely between mail servers, the numeric code provides a universal language for diagnosing email delivery issues. This consistency allows for automated processing and effective bounce management by email service providers (ESPs).
Mail servers use these codes to communicate with the sending server, detailing why an email could not be delivered. This feedback loop is essential. Without it, senders would be unaware of issues like invalid recipient addresses or temporary server outages, continually sending to problematic addresses and harming their sender reputation. For a deeper dive into these codes, resources like A Guide to Email Bounce Codes can be incredibly helpful.
Code Range
Category
Meaning
Action Required
2xx
Success
Email accepted for delivery
None, delivery successful
4xx
Temporary Failure (Soft Bounce)
Issue is temporary, can be retried later
Usually, your ESP will retry. Monitor for persistence.
5xx
Permanent Failure (Hard Bounce)
Email cannot be delivered, recipient invalid or blocked
Remove recipient from mailing list immediately.
Soft bounces and temporary failures
Soft bounces, typically indicated by 4xx SMTP codes, represent temporary delivery failures. These errors suggest that the email couldn't be delivered at that specific moment, but there's a chance it could go through with a retry. Common reasons include a recipient's mailbox being full, the receiving server being temporarily unavailable, or the message size exceeding limits. ESPs are designed to manage soft bounces by automatically retrying delivery over a certain period. To understand more about this, refer to our guide on how email service providers manage bounces.
While soft bounces are temporary, a high volume of them, especially to the same recipients, can still signal underlying issues. Consistent soft bounces might indicate that an email address is no longer actively monitored, or that there are persistent problems with the recipient's mail server. If an email consistently soft bounces over an extended period, it effectively behaves like a hard bounce, impacting your deliverability and possibly leading to a blacklisting or blocklisting. Keeping a close eye on soft bounce rates is a proactive measure against reputation degradation.
Handling soft bounces
Monitoring: Pay attention to repeated soft bounces from the same email addresses. While ESPs retry, prolonged issues require attention.
Segmentation: Consider segmenting users with persistent soft bounces for re-engagement campaigns, or temporarily suppressing them.
List hygiene: Regularly clean your email list to remove inactive or problematic addresses to maintain a healthy sender reputation. Our guide on the definitions and utility of different email bounces provides more context.
Hard bounces and their impact on reputation
Hard bounces are permanent delivery failures, indicated by 5xx SMTP codes. These emails will never be delivered to the intended recipient. Common reasons for hard bounces include an invalid email address (e.g., a typo), a non-existent domain, or the recipient's mail server permanently blocking your messages due to perceived spam or a blocklist (blacklist) listing. Unlike soft bounces, ESPs typically do not retry sending to hard bounced addresses. It's crucial to remove these addresses from your mailing list immediately to protect your sender reputation. For more details, explore our guide on how hard bounces impact email deliverability.
The impact of hard bounces on email deliverability is significant. A high hard bounce rate signals to Internet Service Providers (ISPs) that your list quality is poor, or that you might be sending to spam traps. This can severely damage your sender reputation, leading to more of your emails being sent to spam folders or even outright rejected. Maintaining a low hard bounce rate is fundamental for successful email campaigns and avoiding punitive measures like IP or domain blacklisting (or blocklisting).
Some specific hard bounce codes carry particular weight. For instance, a 550 5.7.1 code often indicates a general block due to policy issues or content, while 554 can indicate a transaction failure or spam rejection. Understanding these nuanced specific bounce error messages is key to effective troubleshooting. For further reading on SMTP errors, SendLayer offers an article on understanding SMTP errors.
Critical hard bounce codes
550 Invalid Recipient: The email address does not exist. Immediate removal from your list is necessary. This is a common cause of mailbox disabled bounces.
552 Exceeded Storage Allocation: Recipient's mailbox is full, but unlike a 4xx code, this often indicates a permanent state or a deliberate rejection.
553 Relaying Denied/Blocked: Your IP or domain might be on a blocklist (or blacklist). Immediate action is required to investigate blocklists.
Bounce codes vs. actual blocks and blocklists
There's a common misconception among senders that any email not landing in the inbox is a 'block.' While a true block or blocklist (blacklist) listing is a form of non-delivery, it's essential to differentiate it from other types of bounces. For instance, a temporary deferral (a 4xx code) is not the same as a permanent block due to a blocklist (blacklist) entry. Mailbox providers (MBPs) might temporarily defer mail for various reasons, but this doesn't automatically mean your sender IP or domain is blocked.
Temporary deferrals (soft bounce)
SMTP code: 421, 451, or 452.
Reason: Recipient server busy, full mailbox, or temporary policy issues. For instance, 421 means the service is unavailable, often due to too many connections.
Deliverability impact: Generally low impact if isolated. ESPs will retry. However, consistent 4xx bounces can signal issues with recipient engagement.
Permanent blocks (hard bounce/blocklist)
SMTP code: 550, 554, etc., often with specific text.
Reason: Invalid recipient, content violation, or IP/domain blocklisted (blacklisted). Sometimes, a 550 User unknown could be an issue with a corporate gateway's database, as some experts have observed.
Deliverability impact: High impact. Can severely damage sender reputation, leading to significant inbox placement issues or total message rejection. Requires immediate action, including list suppression and potentially blocklist monitoring.
Mailbox providers often use more nuanced signals than just explicit blocklist (blacklist) entries. A message might be shunted to the spam folder, or simply not count as delivered, without a clear 5xx bounce code. This can make troubleshooting challenging, as the explicit feedback isn't always present. This is why a comprehensive approach to managing block bounces is essential.
Example of a specific blocklist bounce message
421 4.7.1 Service unavailable, client host [192.0.2.1] blocked using XYZ. Check your reputation on Example Blocklist.
Actionable steps to improve deliverability
Effectively managing SMTP bounce codes is vital for maintaining high email deliverability. The primary goal is to minimize hard bounces and address the root causes of soft bounces. By doing so, you protect your sender reputation and ensure your legitimate emails reach the inbox. It's an ongoing process that requires diligent monitoring and proactive list management.
One of the most immediate actions you can take is to implement strict list hygiene. Any email address that generates a hard bounce should be immediately removed from your mailing list. Repeatedly sending to invalid addresses signals poor sending practices to ISPs, which can lead to your IP or domain being added to a blocklist (or blacklist). For instance, specific codes should lead to mailing list suppression.
For soft bounces, analyze the patterns. If you see consistent soft bounces from a particular domain, it might indicate a specific issue with that recipient's mail server or a policy you're inadvertently violating. Adjusting your sending patterns, content, or authentication might be necessary. Getting SMTP bounce logs from an ESP is the first step to understanding what is happening.
Clean your list regularly: Implement processes to automatically remove hard bounces and suppress persistent soft bounces. This improves overall email deliverability.
Monitor your sender reputation: Keep an eye on your sender score and check if your domain or IP is listed on any blocklists (blacklists). Use a blocklist checker regularly.
Implement email authentication: Properly configure SPF, DKIM, and DMARC to prove your legitimacy and reduce the likelihood of being marked as spam. For more on this, consult our simple guide to email authentication.
Warm up new IPs/domains: Gradually increase sending volume on new IPs or domains to build a positive sending history with ISPs.
Review content: Ensure your email content is not triggering spam filters. Avoid spammy keywords, excessive images, or broken links.
Views from the trenches
Best practices
Actively monitor SMTP bounce codes and categorise them as soft or hard bounces.
Immediately suppress email addresses that generate hard bounces to protect your sender reputation.
Analyze patterns in soft bounces to identify potential underlying issues with recipient mailboxes or content.
Implement robust email authentication (SPF, DKIM, DMARC) to improve email deliverability and trust with ISPs.
Regularly clean your email lists to remove inactive or invalid addresses, preventing future bounces.
Common pitfalls
Ignoring soft bounces, assuming they are always temporary and will resolve themselves.
Failing to suppress hard bounced email addresses, leading to continuous sending to invalid contacts.
Misinterpreting a temporary deferral as a permanent block, leading to unnecessary panic or incorrect actions.
Not having a system to automatically process bounce logs, requiring manual review which is time-consuming and error-prone.
Focusing solely on numerical codes and overlooking the detailed text message that often provides crucial context.
Expert tips
Some corporate gateways use non-standard SMTP codes, which require careful interpretation, sometimes looking at text messages.
While
4xx
codes typically mean temporary deferrals, an expert noted that a
550 User unknown
Marketer view
Marketer from Email Geeks says that anything other than inboxing, such as emails going to the spam folder, deferrals, or invalid recipients, can often be perceived as a 'block' when escalated to mailbox providers.
2023-10-26 - Email Geeks
Marketer view
Marketer from Email Geeks says that delivery without an open can also be considered a block by some internal metrics.
2023-10-26 - Email Geeks
Conclusion
SMTP bounce codes are more than just error messages; they are crucial diagnostic tools for anyone involved in email marketing or operations. By understanding the nuances between soft bounces and hard bounces, and recognizing when a temporary issue might escalate into a permanent block (or blacklist), senders can proactively safeguard their email deliverability and sender reputation.
Regular monitoring, immediate action on hard bounces, and continuous list hygiene are fundamental practices. Embracing these insights ensures that your emails consistently reach their intended recipients, fostering better engagement and preventing your valuable messages from falling into the digital void or landing you on an unwanted blocklist.
