How should different bounce types be classified and handled by ESPs?
Matthew Whittaker
Co-founder & CTO, Suped
Published 24 May 2025
Updated 16 Aug 2025
9 min read
Email bounces are an inevitable part of sending emails, but how an Email Service Provider (ESP) classifies and handles them significantly impacts your email deliverability. Poor bounce management can lead to degraded sender reputation, higher blocklist (or blacklist) rates, and ultimately, a much lower inbox placement. Understanding the nuances of different bounce types and the appropriate handling strategies is crucial for maintaining a healthy email program.
The traditional approach categorizes bounces into two broad types: hard bounces and soft bounces. While this simple division provides a basic framework, it often oversimplifies the complex reasons behind email delivery failures. A more granular classification is necessary for ESPs to provide meaningful insights and for senders to take effective action.
Effective bounce handling isn't just about removing invalid addresses. It's about interpreting specific error codes, understanding the context of the bounce, and implementing intelligent suppression and retry logic. This proactive approach helps preserve your sender reputation and ensures your legitimate emails reach the inbox, rather than being caught in spam filters or blocked outright.
Basic bounce categories
Email bounces are fundamentally categorized into two main types: hard bounces and soft bounces. A hard bounce signifies a permanent delivery failure, meaning the email will never be delivered to that address. Common reasons include non-existent mailboxes (mailbox not found), invalid domain names, or accounts that have been permanently disabled. These bounces are critical because they indicate an irreversible issue with the recipient address. ESPs should immediately and permanently remove these addresses from mailing lists to protect sender reputation. You can learn more about the definitions and utility of block, soft, and hard email bounces.
Conversely, a soft bounce indicates a temporary delivery issue. This could be due to a full mailbox, the recipient's server being temporarily unavailable, or the message size exceeding limits. Unlike hard bounces, soft bounces suggest that the email might be deliverable if retried later. ESPs typically implement a retry schedule for soft bounces, attempting to resend the email over a period of time. However, if a soft bounce persists after several retries, it should eventually be converted into a hard bounce and the address suppressed. The distinction is crucial, as noted by Braze, Hard bounce vs soft bounce, where they highlight that hard bounces are permanent failures while soft bounces are temporary.
The way ESPs classify these primary bounce types sets the foundation for their deliverability management. Proper classification ensures that valid emails aren't unnecessarily suppressed, while invalid ones are removed swiftly, preserving the sender's positive sending reputation. This careful balance is vital for email campaign success and overall email deliverability.
Beyond hard and soft: granular bounce types
While hard and soft bounces are standard, a sophisticated ESP goes deeper into SMTP response codes to provide more actionable insights. These detailed codes offer specific reasons for delivery failures, allowing for more precise handling and better reporting. For instance, a 550 response often indicates a non-existent user, while a 4xx code usually points to a temporary issue, like a server being busy or an administrative limit being exceeded. Understanding what different SMTP bounce codes mean is key.
Example SMTP bounce messages from Yahoo
554 30 Sorry your message to XXXXXXX cannot be delivered. This mailbox is disabled (554.30).
552 1 Requested mail action aborted mailbox not found
Beyond the general categories, ESPs should classify bounces further based on the specific type of rejection, such as:
Reputation-based blocks: These occur when the recipient's mail server detects suspicious sending patterns, such as high spam complaints or unusual sending volume from your IP or domain. Mailbox providers like Yahoo often issue 5.2.1 errors for these. Such blocks, or blacklists (sometimes referred to as blocklists), indicate a serious threat to deliverability and require immediate attention to understand why your email is blacklisted.
Content blocks: If an email's content triggers spam filters, it may be blocked. This means the content is perceived as spammy, either due to specific keywords, suspicious links, or poor formatting. These often manifest as soft bounces with specific error messages.
Authentication blocks: Failures in email authentication protocols like SPF, DKIM, or DMARC can lead to blocks. These indicate that the receiving server cannot verify the sender's legitimacy. ESPs should provide detailed reports on these failures to allow senders to correct their DNS records. This is critical for preventing your emails from going to spam and for ensuring proper DMARC, SPF, and DKIM alignment.
By dissecting bounce messages into these granular categories, ESPs empower their clients with the specific information needed to diagnose and resolve underlying deliverability issues, rather than just knowing an email 'bounced'.
Handling strategies for ESPs
A robust bounce management system within an ESP involves more than just categorizing bounces. It requires specific handling protocols for each type to optimize deliverability and sender reputation. For hard bounces, immediate and permanent suppression of the recipient address is non-negotiable. Continuing to send to these addresses will harm your sender reputation and increase the likelihood of future blocks and blacklistings.
For soft bounces, the strategy is more nuanced. ESPs should implement a well-defined retry schedule. This involves attempting to resend the email at increasing intervals over a set period. If, after a specified number of retries or a certain duration, the email still soft bounces, the address should then be treated as a hard bounce and suppressed. This prevents indefinite retries to temporarily unavailable mailboxes, which can unnecessarily consume resources and potentially trigger spam traps. Effective soft bounce suppression logic is critical for good deliverability.
Recommended ESP actions
Automated Suppression: Immediately remove hard bounced addresses from all active mailing lists.
Granular Reporting: Provide detailed bounce classifications in client dashboards, including specific SMTP codes and descriptive reasons. SendGrid and Mailgun are examples of providers that offer detailed bounce classifications to help users understand delivery issues more clearly.
Feedback Loop Integration: Automatically process spam complaints from ISPs to remove those recipients from lists, further protecting sender reputation.
Customizable Retry Logic: Allow senders to fine-tune soft bounce retry attempts based on campaign type or urgency.
Transparent reporting of bounce data is also vital. ESPs should provide clients with clear, accessible data that breaks down bounces by type, reason, and volume. This enables senders to identify trends, pinpoint issues with their list hygiene or content, and adjust their sending strategies accordingly. Without this detailed visibility, senders are left in the dark about the true health of their email program.
The challenge of classification and standardization
Despite the critical role of bounce classification, the industry faces a significant challenge: a lack of standardized bounce categorization. As highlighted by Word to the Wise, many ESPs don't make granular rejection messages clearly visible, instead grouping them into vague categories like “soft bounce.” This ambiguity can prevent senders from accurately diagnosing and addressing the root causes of their deliverability issues. Without consistent standards, each ESP defines hard and soft bounces internally, sometimes even introducing other arbitrary categories like spam bounces. This inconsistency hinders effective email deliverability management across different platforms.
The core of the problem lies in the disconnect between the detailed SMTP error codes provided by receiving mail servers and the simplified classifications presented to senders. For example, a 554 30 for a disabled mailbox or a 552 1 for a mailbox not found, as seen in Yahoo responses, are clearly permanent failures. Yet, some ESPs may not consistently mark these as explicit hard bounces or provide enough detail to the sender to understand the definitive nature of the problem. This can lead to senders inadvertently continuing to try and reach invalid addresses, damaging their reputation. This is why disabled mailbox email bounces should always be classified as hard bounces and acted on immediately.
Current ESP approach
Many ESPs group complex SMTP responses into simple "hard" or "soft" categories, often without exposing the underlying diagnostic codes. This can be problematic because a single soft bounce classification might cover various temporary issues, some of which are more serious than others. This broad-brush approach means clients miss out on granular data that could inform better sending practices.
Ideal ESP approach
ESPs should provide transparent access to all raw SMTP bounce codes and classify them into specific, actionable categories like "reputation block," "content filter," or "mailbox full." This allows senders to accurately identify and resolve problems. They should also offer clear guidance on expectations for sharing bounce data with clients.
The industry would benefit from a more unified approach to bounce classification, allowing senders to move between ESPs without re-learning proprietary bounce categories. This standardization would improve overall deliverability practices and empower senders to make more informed decisions about their email lists and campaigns. The current fragmentation leaves much room for improvement, ultimately impacting email marketers' ability to improve email deliverability.
The importance of precise bounce management
Effective bounce classification and handling are cornerstones of good email deliverability. For ESPs, this means moving beyond simple hard/soft categorizations to provide granular insights derived from SMTP response codes. This allows for precise suppression and retry logic, minimizing negative impacts on sender reputation and ensuring that email campaigns reach their intended recipients.
For senders, understanding these classifications and the logic behind them is paramount. It empowers you to refine your mailing lists, adjust content strategies, and proactively manage your sender reputation. By working with an ESP that offers transparent and detailed bounce reporting, you can turn delivery failures into actionable intelligence.
Ultimately, the goal is to maximize inbox placement and campaign effectiveness. A sophisticated approach to bounce management is not just a technical requirement, but a strategic advantage in maintaining a healthy and successful email program. Staying informed about your bounce types and proactively addressing them is a continuous process for optimal email deliverability issues.
Views from the trenches
Best practices
Always suppress addresses that result in hard bounces immediately and permanently.
Prioritize detailed SMTP error codes for bounce classification, not just generic categories.
Implement a consistent retry schedule for soft bounces before converting them to hard bounces.
Provide transparent and granular bounce reports to clients for actionable insights.
Common pitfalls
Over-simplifying bounce classifications into just "hard" or "soft" without underlying detail.
Failing to automatically suppress hard bounced addresses, leading to repeated attempts.
Not converting persistent soft bounces into hard bounces, wasting resources and hurting reputation.
Lacking clear, standardized bounce explanations for clients, causing confusion.
Expert tips
Use enhanced status codes (e.g., 5.x.x) for more precise bounce reasons.
Monitor FBLs (Feedback Loops) to catch and suppress spam complaints quickly.
Analyze bounce trends over time to identify underlying deliverability issues.
Consider a tiered approach for soft bounce retries based on the specific error type.
Expert view
Expert from Email Geeks says that bounce messages like "mailbox is disabled" or "mailbox not found" from Yahoo should definitely be classified as hard bounces. This means the ESP should stop trying to deliver messages to these addresses and advise customers to suppress them from future mailings immediately.
2023-04-26 - Email Geeks
Expert view
Expert from Email Geeks says that most 5xx SMTP codes generally indicate a permanent failure and should be treated as hard bounces, with only very rare exceptions.
Yahoo often issue 5.2.1 errors for these. Such blocks, or blacklists (sometimes referred to as blocklists), indicate a serious threat to deliverability and require immediate attention to understand why your email is blacklisted.
SendGrid and 
Mailgun are examples of providers that offer detailed bounce classifications to help users understand delivery issues more clearly. 
