Where can I find documentation on bounce types and when to suppress?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 Jul 2025
Updated 13 Oct 2025
5 min read
Navigating the complexities of email deliverability often involves a deep dive into understanding why emails don't always reach their intended inboxes. One of the most common hurdles we encounter is the email bounce. These are undeliverable messages that return to the sender, indicating an issue with the recipient's email address or mailbox. Grasping the different categories of bounces and knowing precisely when to suppress a recipient is fundamental to maintaining a healthy sender reputation and achieving optimal email performance.
Properly managing bounces isn't just about cleaning your lists, it's about protecting your domain's credibility with Internet Service Providers (ISPs). Overlooking bounce rates can lead to your emails being flagged as spam, reduced inbox placement, or even worse, your sending IP address ending up on an email blacklist (or blocklist).
Understanding email bounce types
Email bounces are broadly categorized into two main types: hard bounces and soft bounces. Each type signals a different kind of delivery failure and requires a distinct approach for management.
A hard bounce signifies a permanent delivery failure. This means the email address is invalid, doesn't exist, or the domain name is incorrect. Examples include invalid email addresses like typos, or recipients who have permanently closed their accounts. When an email hard bounces, it means future attempts to send to that address will also fail. To understand this in more detail, you can read our guide on what hard and soft email bounces are.
Conversely, a soft bounce indicates a temporary delivery issue. This could be due to a full mailbox, a server being temporarily down, or the message size exceeding limits. Soft bounces are often recoverable, meaning subsequent attempts to send to the same address might succeed. However, repeated soft bounces can also signal an underlying problem that needs attention.
Understanding hard vs. soft bounces
Knowing the difference between hard and soft email bounces is critical for effective list management. Hard bounces require immediate removal, while soft bounces warrant monitoring and a threshold-based suppression strategy.
When to suppress email addresses
The golden rule of bounce management is simple: always suppress hard bounces immediately. Continuing to send to these addresses harms your sender reputation, wastes resources, and increases the likelihood of your IP address or domain being flagged by ISPs. Most Email Service Providers (ESPs) automatically handle this by suppressing hard-bounced addresses.
For soft bounces, the decision to suppress is more nuanced. Since they are temporary, you generally want to retry sending. However, if an address consistently soft bounces over multiple attempts within a specified timeframe, it's a strong indicator that the issue is persistent, and suppression becomes necessary. The specific threshold, such as how many soft bounces before suppressing, can vary, but a common practice is three to five consecutive soft bounces over a short period. This systematic approach is key to managing email bounce effectively.
Hard bounce handling
Immediate suppression: Remove addresses from your mailing list permanently. No further attempts should be made.
Prevent future sends: Mark the contact as invalid to prevent accidental re-adding. This helps maintain list hygiene.
Impact: Significantly damages sender reputation and deliverability if ignored.
Soft bounce handling
Retry sending: Most ESPs automatically retry soft bounced emails. This is usually over a short period.
Threshold suppression: Suppress after a specific number of consecutive soft bounces, typically 3-5. Our guide on recommended soft bounce logic can assist here.
The details of bounce management, including specific error codes and retry logic, are often handled by your email service provider. However, understanding the underlying principles allows you to effectively audit and optimize your email campaigns. We have a detailed guide on how email service providers manage bounces.
Common bounce codes and their meanings
Bounce messages often include SMTP (Simple Mail Transfer Protocol) codes that provide specific details about why an email failed to deliver. These codes, along with their accompanying descriptions, are crucial for diagnosing deliverability issues. While there are hundreds of possible codes, some are more common than others and indicate specific actions that should be taken.
For instance, a 550 code often signals a hard bounce, such as an invalid recipient, demanding immediate suppression. On the other hand, a 4xx series code, like 421, might indicate a temporary issue like a server being busy. For a comprehensive overview, explore our resource on common email bounce messages and their meaning.
Code series
Meaning
Action required
2xx
Success, delivered
No action, email delivered
4xx
Temporary failure (soft bounce)
Monitor, retry sending. Suppress if persistent (e.g., mailbox full).
High bounce rates, especially hard bounces, can severely damage your sender reputation. ISPs and mailbox providers monitor bounce rates closely. A consistent pattern of sending to invalid addresses suggests poor list hygiene or even spamming behavior, leading to lower inbox placement or even blacklisting of your sending IP or domain. This impact can be significant, as detailed in our guide on how bounces impact email deliverability.
To minimize bounces, proactive list management is essential. Regularly clean your email lists, use double opt-in for new subscribers, and implement real-time email verification where possible. This prevents invalid addresses from entering your list in the first place.
Best practices for minimizing bounces
Validate email addresses: Use real-time validation tools at the point of signup.
Double opt-in: Confirm subscriber intent before adding them to your list. This ensures engagement.
Regular list cleaning: Remove inactive subscribers and consistently bouncing addresses. This impacts your email deliverability rate.
Views from the trenches
Best practices
Implement a tiered suppression policy for soft bounces, progressively increasing suppression strictness based on bounce frequency and type.
Segment your audience based on engagement levels to prioritize validation and cleaning efforts on less active subscriber segments.
Regularly monitor your bounce reports and understand specific bounce codes to identify systemic issues rather than just individual address problems.
Use email validation services before sending to new lists or reactivating old contacts to reduce initial bounce rates.
Common pitfalls
Ignoring soft bounces, thinking they will resolve themselves, which can lead to reputation damage over time.
Not having a clear policy for when to remove soft-bounced addresses, resulting in bloated and underperforming lists.
Failing to differentiate between bounce types, leading to premature removal of temporarily unavailable addresses or prolonged sending to invalid ones.
Sending to purchased or old lists without prior validation, which instantly inflates bounce rates and triggers spam filters.
Expert tips
Consider that what might be a 'soft' bounce for one ISP (e.g., mailbox full) could be treated as a 'hard' bounce by another if it persists.
Regularly review bounce classifications from your ESP, as they can sometimes generalize specific SMTP codes.
Pay close attention to 'DNS failure' or 'message too large' soft bounce reasons, as they might indicate configuration issues on your side or the recipient's.
Some providers will consider a high volume of soft bounces from your domain a sign of poor sending practices and may block (blacklist) future emails.
Marketer view
Marketer from Email Geeks says they recalled a document from Return Path that listed bounce types and suppression thresholds, like 3 hard bounces or 5 mailbox full errors, but couldn't locate it.
2020-05-15 - Email Geeks
Marketer view
Marketer from Email Geeks mentioned they downloaded an SFMC bounce flow image that they found useful for understanding bounce processes.
2020-05-15 - Email Geeks
Maintaining a healthy sender reputation
Effectively managing email bounces is a cornerstone of successful email marketing and deliverability. By understanding the distinction between hard and soft bounces, implementing appropriate suppression strategies, and actively monitoring bounce codes, you can significantly improve your sender reputation and ensure your messages reach their intended audience. Proactive list hygiene and adherence to best practices are key to keeping your bounce rates low and your inbox placement high.
For advanced insights and tools to help you manage your email deliverability, explore Suped's features for comprehensive DMARC reporting and monitoring.
