Where can I find documentation on bounce types and when to suppress?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 Jul 2025
Updated 19 Aug 2025
8 min read
Understanding why your emails bounce and how to manage those bounces is fundamental to successful email deliverability. Every email marketer and sender eventually encounters them, but the sheer volume and variety of bounce messages can be overwhelming. It often feels like you're searching for a single, comprehensive guide that neatly categorizes every bounce type and provides clear rules for when to suppress an address. However, the reality is more nuanced than a simple lookup table.
Email service providers (ESPs) and mailbox providers each have their own ways of classifying and handling bounces, making a universal documentation on bounce types elusive. While a definitive, central resource detailing every possible bounce code and a precise suppression threshold (e.g., 3 hard bounces, 5 mailbox full) might not exist in one tidy package, general principles and best practices guide us. The key lies in understanding the core types of bounces, interpreting their underlying causes, and knowing how to respond to maintain a healthy sender reputation.
I'll guide you through the common classifications of email bounces and explain the rationale behind suppressing certain addresses. We'll also explore where to look for relevant information and how to apply these insights to your email program.
Understanding hard versus soft bounces
Broadly speaking, email bounces fall into two main categories: hard bounces and soft bounces. Each signifies a different kind of delivery failure and requires a distinct approach to management. Understanding what defines a hard versus a soft bounce is the first step in effective bounce handling.
Hard bounces
A hard bounce indicates a permanent delivery failure. This means the email address is invalid and will never successfully receive mail. Common reasons for hard bounces include a non-existent email address, an invalid domain, or a recipient server blocking mail permanently. When a hard bounce occurs, the sender immediately receives a notification, and the address should be removed from your list without delay. Continued attempts to send to a hard-bounced address signal poor list hygiene to mailbox providers and can severely damage your sender reputation.
Soft bounces
In contrast, a soft bounce signifies a temporary delivery failure. This means the email reached the recipient's server but bounced back for a transient reason. Common causes include a full mailbox, the recipient's server being temporarily unavailable, the message being too large, or exceeding the recipient's server rate limits. Unlike hard bounces, a soft bounce doesn't necessarily mean the address is bad forever. However, repeated soft bounces to the same address suggest an ongoing issue and should eventually lead to suppression.
Hard bounces
Definition: Permanent failure, email address does not exist or is invalid.
Cause: Invalid recipient, domain non-existent, or permanent block.
Action: Immediate suppression from all future sends.
Soft bounces
Definition: Temporary failure, often due to transient issues.
Cause: Mailbox full, server temporary down, message too large.
Action: Monitor, then suppress after multiple consecutive bounces.
Interpreting bounce codes and categories
Beyond the hard and soft classifications, bounce messages often include specific SMTP (Simple Mail Transfer Protocol) error codes and descriptive text. These codes, such as those detailed in this SparkPost article on bounce classification, provide much more granular insight into why an email failed to deliver. For instance, a 550 code usually indicates a permanent error, like a non-existent user, whereas a 421 might mean a temporary service unavailability.
Mailbox providers often classify these raw SMTP responses into more user-friendly categories. For example, Salesforce Marketing Cloud categorizes bounces into types like undeliverable, blocked, or content-related. Similarly, Microsoft Dynamics 365 offers categorized bounce errors. While the exact labels may differ, the underlying intention is to help senders understand the nature of the delivery issue.
For a deeper dive into these technical nuances, exploring what different SMTP bounce codes mean can be incredibly insightful. It provides a clearer picture of whether a bounce is temporary or permanent, and what specific action, if any, you might need to take beyond simple suppression.
SMTP Code
Common Category
Description
Suppression Action
550
Permanent failure (Hard Bounce)
Mailbox not found, user unknown, host unknown, rejected for policy reasons (e.g., on a blacklist or blocklist).
Immediately suppress.
554
Permanent failure (Hard Bounce)
Transaction failed due to policy violation or general error.
Immediately suppress.
421
Temporary failure (Soft Bounce)
Service not available, connection error, or too many connections.
Retry. Suppress after multiple attempts.
451
Temporary failure (Soft Bounce)
Requested action aborted (e.g., local error in processing).
Retry. Suppress after multiple attempts.
552
Permanent failure (Hard Bounce)
Requested mail action aborted: exceeded storage allocation.
Often treated as hard bounce for suppression.
This table illustrates that while soft bounces are temporary, certain types (like a persistent 552 for mailbox full) might lead to eventual suppression. The key is to understand the nuances of which SMTP bounce codes indicate suppression.
When to suppress and why it matters
The decision to suppress an email address is critical for maintaining a high sender reputation and avoiding email blacklists (or blocklists). As a general rule, all hard bounces should be suppressed immediately. Sending to invalid addresses signals to mailbox providers that your list quality is poor, which can lead to your emails being flagged as spam or outright rejected. This proactive approach helps prevent your emails from going to spam.
For soft bounces, the strategy is slightly different. Since these are temporary, you generally retry sending to the address a few times over a set period. However, if an address consistently generates soft bounces, it's a strong indicator that the issue is not truly temporary or that the recipient is no longer active. Many ESPs automatically handle this by applying a soft bounce suppression logic, where an address is added to a suppression list after a certain number of consecutive soft bounces.
Automating this suppression process is crucial. Regularly scrubbing your list of bounced addresses prevents wasted sending efforts and, more importantly, protects your sender reputation from being degraded by high bounce rates. Remember, a high bounce rate is a red flag to mailbox providers and can lead to lower inbox placement or even having your sending IP or domain placed on a blocklist.
Best practices for bounce suppression
Hard Bounces: Always suppress immediately. They indicate permanent errors.
Soft Bounces: Implement a threshold for suppression (e.g., 3-5 consecutive soft bounces over a short period). Most ESPs do this automatically.
Regular Cleaning: Periodically review your suppression lists to ensure accuracy and remove any problematic addresses.
Finding documentation on bounce types
Unfortunately, there isn't one single, universally recognized documentation on all bounce types and when to suppress. The closest you might come are historical guides from industry veterans or documentation provided by major email sending platforms. The email ecosystem is highly dynamic, and bounce behaviors can evolve.
Most comprehensive documentation on specific bounce types, including categorizations and handling rules, resides within the help centers and developer documentation of various email service providers or specific email security vendors. These resources often provide lists of common bounce messages and their meanings, tailored to how their systems process them. However, they typically focus on how ESPs manage bounces internally, rather than giving you a universal playbook.
Your best approach is to consult the documentation provided by your specific email sending platform. They will outline how they classify bounces (hard, soft, transient, block, etc.) and what their automated suppression rules are. Additionally, familiarizing yourself with generic SMTP status codes (RFC 3463 and RFC 5321) will give you a foundational understanding that applies across all email systems. You can also research lists of bounce and block codes for more detailed explanations.
Key takeaways for bounce management
Effectively managing email bounces, including both hard bounces and soft bounces, is a cornerstone of maintaining strong email deliverability. By promptly suppressing invalid addresses and strategically handling temporary failures, you reduce your overall bounce rate, protect your sender reputation, and ultimately ensure your legitimate emails reach the inbox. While a single, all-encompassing document detailing every bounce type and universal suppression rule doesn't exist, a strong understanding of the core principles and diligent monitoring will serve you well.
Focus on the categories of bounces provided by your specific sending platform and pay attention to common SMTP codes. Automate your suppression processes where possible, especially for hard bounces. This proactive approach not only keeps your lists clean but also safeguards your email program's long-term health.
Views from the trenches
Best practices
Maintain a clean email list by promptly removing hard bounces and consistently soft-bounced addresses.
Utilize your ESP's bounce reporting and automatic suppression features to manage bounced emails.
Understand the difference between hard and soft bounces and respond appropriately for each type.
Common pitfalls
Ignoring soft bounces can lead to deliverability issues and wasted sending resources over time.
Not having an automated suppression process for hard bounces can severely harm your sender reputation.
Manually attempting to re-send to hard-bounced addresses, as this is counterproductive.
Expert tips
For soft bounces, a common best practice is to suppress an address after 3-5 consecutive soft bounces over a short period. This threshold balances retries with list hygiene.
While a universal list of 'when to suppress' for every specific bounce code is rare, focus on the 'actionable' codes that clearly indicate a permanent failure.
Leverage DMARC reports to get insights into delivery failures and identify sources of bounces across different mailbox providers.
Marketer view
Marketer from Email Geeks says they were trying to recall a document that listed specific bounce types and the number of bounces after which an email should be suppressed, like three hard bounces or five mailbox full errors.
2020-05-15 - Email Geeks
Marketer view
Marketer from Email Geeks says they thought the document might have been something that Return Path used to provide years ago.
