What is the difference between hard and soft email bounces and how should they be managed?
Michael Ko
Co-founder & CEO, Suped
Published 2 May 2025
Updated 14 Aug 2025
10 min read
Summary
Understanding the difference between hard and soft email bounces is fundamental for any email sender aiming for optimal deliverability and a healthy sender reputation. While both indicate non-delivery, their underlying causes and the actions required to manage them effectively are quite distinct. A hard bounce signifies a permanent delivery failure, whereas a soft bounce points to a temporary issue. Proper classification and management of these bounces are crucial to avoid damaging your sending metrics and ensure your emails reach their intended recipients.
Key findings
Hard bounces: These indicate permanent delivery failures, often due to an invalid email address or a non-existent domain. They are definitive and typically require immediate removal of the address from your mailing list.
Soft bounces: These are temporary delivery issues, such as a full inbox, a temporary server problem, or the email size exceeding limits. They suggest that the email might be delivered successfully on a subsequent attempt.
SMTP codes: SMTP error codes typically categorize bounces, with 4xx indicating soft bounces and 5xx indicating hard bounces.
Misclassification risk: Email Service Providers (ESPs) can sometimes misclassify bounces, leading to unnecessary list cleaning or persistent sending to problematic addresses. Accurate interpretation of bounce codes is crucial.
Impact on deliverability: A high bounce rate, particularly from hard bounces, signals poor list quality to Internet Service Providers (ISPs), which can negatively impact your overall sender reputation and lead to blocklistings.
Key considerations
Immediate suppression for hard bounces: Always remove hard-bounced addresses from your lists immediately to protect your sender reputation and prevent future delivery issues. This is a non-negotiable best practice.
Retry strategy for soft bounces: For soft bounces, implement a strategy of retrying delivery a few times over a period, such as 3 attempts over 15 days. If the email continues to soft bounce, it should eventually be treated as a hard bounce and removed.
Regular list hygiene: Proactive email list validation and maintenance are essential to minimize bounce rates. This includes regularly removing unengaged or invalid subscribers.
Monitor bounce analytics: Utilize analytics provided by your ESP or email deliverability platforms to monitor bounce rates and categorize them correctly. This data informs your list management strategies. As Mailchimp suggests, hard bounced email addresses are generally cleaned from your audience, indicating a permanent reason an email can't be delivered. Mailchimp's guide on soft vs hard bounces highlights the importance of this cleaning process.
What email marketers say
Email marketers face the practical challenge of managing bounces daily, often developing internal heuristics based on their ESP's reporting and operational needs. While the definitions of hard and soft bounces can sometimes feel ambiguous, marketers generally agree on the need for distinct handling. Their experiences often highlight edge cases, such as temporary hard bounces for new corporate email accounts, which require more nuanced approaches than simple automated suppression.
Key opinions
Standard definitions: Marketers generally define hard bounces as permanent failures (address or domain doesn't exist) and soft bounces as temporary issues (inbox full, auto-responders, or other user-defined reasons).
Automated suppression for hard bounces: There's a strong consensus that hard-bounced addresses should be automatically removed from lists, provided the ESP accurately classifies them.
Soft bounce retry policies: Many marketers implement rules to remove addresses after a certain number of consecutive soft bounces over a defined period, turning them into effective 'hard' failures for list management purposes.
Auto-responders are not bounces: Some marketers emphasize that auto-responder messages (e.g., 'out of office') are successful deliveries and should not be classified or treated as bounces, as this misrepresents deliverability.
Google Apps nuances: Certain situations, like initial hard bounces for newly created Google Apps accounts for new employees, pose a challenge, as these might become deliverable later. This suggests a need for specific handling beyond standard bounce rules.
Key considerations
Accurate ESP classification: Relying on an ESP's accurate classification of bounces is crucial to avoid mistakenly removing valid contacts or endlessly sending to invalid ones. Marketers need to trust their platform's bounce handling.
Custom bounce rules: Consider implementing custom rules for soft bounces (e.g., three consecutive bounces over 15 days results in removal) to balance deliverability with list hygiene. This aligns with approaches for managing inactive email subscribers.
Manual overrides for specific cases: Be prepared for situations that require manual intervention or specific overrides, like the Google Apps new employee scenario, where an initial hard bounce might not mean a permanent invalidation.
Review internal processes: If 'fake' bounces occur due to internal processes (e.g., sending invites before accounts are fully provisioned), address these process flaws rather than solely questioning the MX server's bounce reply. Nylas explains that soft bounces are temporary delivery failures, often due to issues like a full inbox or a server problem, while hard bounces are permanent, a crucial distinction for effective management. Their guide on soft vs hard bounce email provides further insights.
Marketer view
Email marketer from Email Geeks explains hard vs. soft bounces: A hard bounce occurs when an email cannot be delivered due to a permanent issue, such as an invalid or non-existent recipient address or domain. This type of bounce signals an irreversible problem and requires immediate action. In contrast, a soft bounce signifies a temporary delivery problem, which could be an overflowing inbox, an auto-responder, or another transient user-defined reason. Understanding this distinction is fundamental for effective email list management. For campaigns, this means hard bounces require immediate suppression, while soft bounces can be retried, sometimes with a specific strategy, to maximize deliverability. It's critical for email marketers to differentiate between these two types of bounces to maintain list hygiene. Removing hard-bounced addresses promptly prevents repeated failed attempts, which can negatively impact sender reputation. Soft bounces, on the other hand, often resolve themselves, allowing for re-attempts.
10 Jan 2019 - Email Geeks
Marketer view
Email Platform from Mailchimp defines hard bounces and their management: Mailchimp describes a hard bounce as an indicator of a permanent issue that prevents an email from being delivered to its intended recipient. This permanent failure typically arises from reasons such as an invalid email address, a non-existent domain, or the recipient's server permanently rejecting messages from the sender. This type of bounce signals an irreversible problem. For effective list hygiene and to protect sender reputation, hard bounced email addresses should be immediately and permanently removed (cleaned) from an audience or mailing list. Continuing to send to these addresses can negatively impact deliverability across all recipients. The primary goal of identifying and acting on hard bounces is to maintain a high-quality email list and avoid being flagged as a sender of spam, ensuring better inbox placement for future campaigns.
10 Jan 2024 - Mailchimp.com
What the experts say
For email deliverability experts, the terms 'hard bounce' and 'soft bounce' are often viewed with a critical eye. They acknowledge that while these classifications provide a basic framework, the reality is far more nuanced. Experts emphasize that the specifics of bounce codes and the context of the delivery failure are paramount. They advocate for sophisticated bounce management systems that can interpret these details, allowing for flexible and intelligent retry policies rather than rigid, blanket suppressions. Their insights often touch on the dynamic nature of email addresses and the need for adaptive strategies.
Key opinions
Ill-defined terms: Experts often describe 'hard bounce' and 'soft bounce' as ill-defined terms, whose meanings vary wildly depending on the person and context, leading to potential confusion.
Contextual re-attempts: If a legitimate new sign-up occurs for an address that previously bounced, experts suggest it's reasonable to retry sending. The new consent can override past delivery failures.
Selective retry strategies: It's not unreasonable to retry bounces for a while, perhaps three consecutive bounces over 15 days, especially for specific destination domains or customers where temporary issues are common.
SMTP code interpretation: True understanding of bounces requires interpreting the specific SMTP error codes and diagnostic messages, rather than relying solely on generic 'hard' or 'soft' classifications.
Proactive prevention: Beyond reactive bounce handling, experts emphasize that preventing bounces starts with strong list acquisition practices, regular validation, and monitoring engagement to minimize issues at the source.
Key considerations
Nuanced bounce handling: Implement systems that can handle bounces with more granularity than a simple hard/soft distinction, allowing for selective retries based on specific error codes or domain patterns. This is key to managing complex scenarios like Microsoft 550 5.7.515 access denied bounces.
User intent first: Prioritize user intent. If a user legitimately signs up, a prior bounce shouldn't be an automatic barrier to re-attempting delivery. This ensures you don't lose valuable, engaged subscribers.
Analyze bounce data deeply: Utilize robust tools to analyze bounce feedback, including specific error codes and messages. This data provides actionable intelligence for improving list quality and overall email deliverability. As SendLayer suggests, error codes beginning with a 4 typically indicate a soft bounce, while those with a 5 are a hard bounce, offering a clear guideline for technical teams. SendLayer's article on hard vs. soft bounces details these distinctions.
Expert view
Deliverability Expert from Email Geeks comments on bounce term fluidity: The terms 'soft bounce' and 'hard bounce' are often ill-defined and subject to widely different interpretations across individuals and contexts. This ambiguity can lead to confusion in email deliverability discussions and strategy. Without a universally accepted standard, what one person considers a hard bounce, another might classify differently. This lack of precise definition means that relying solely on these generic terms without understanding the underlying SMTP codes and specific reasons for non-delivery can be misleading. Different Email Service Providers (ESPs) and mailbox providers may also apply their own heuristics, further complicating the landscape. Therefore, effective bounce management necessitates a deeper dive into the specific bounce codes and error messages received, rather than just the generic classification of 'hard' or 'soft.' This detailed understanding allows for more accurate diagnostic and remediation efforts.
10 Jan 2019 - Email Geeks
Expert view
Email Deliverability Expert from Spamresource.com discusses the importance of accurate bounce interpretation: Steve Atkins, an expert writing on Spamresource.com, frequently highlights that understanding bounce messages goes beyond simply categorizing them as 'hard' or 'soft'. He emphasizes the need to interpret the specific SMTP error codes and diagnostic messages provided by the receiving mail servers. These details reveal the true nature of the delivery failure, whether it's a permanent invalid address or a transient network issue. Misinterpreting these codes can lead to incorrect list management decisions, such as prematurely suppressing a valid email address due to a temporary server glitch or persistently sending to a definitively invalid one. The granularity of bounce feedback is crucial for sophisticated senders to fine-tune their mailing strategies. Therefore, deliverability professionals must equip themselves with the knowledge to decode these messages accurately. This capability allows for more precise list hygiene, ensuring that valuable subscribers are retained while truly problematic addresses are promptly removed, thereby preserving sender reputation.
18 Mar 2024 - Spamresource.com
What the documentation says
Technical documentation and industry standards provide the foundational definitions for hard and soft bounces, often correlating them with specific SMTP response codes. This structured approach helps email systems to programmatically differentiate between permanent and temporary delivery issues. The emphasis is on precise categorization to ensure compliance with email protocols and effective automated bounce handling, which is crucial for maintaining server health and message routing efficiency.
Key findings
Permanent vs. temporary: Documentation consistently defines hard bounces as permanent delivery failures and soft bounces as temporary ones, emphasizing the lasting nature of the problem as the core differentiator.
SMTP error codes: Technical specifications link bounce types to SMTP reply codes. Typically, 5xx codes signify permanent errors (hard bounces), while 4xx codes indicate temporary issues (soft bounces).
Common soft bounce reasons: Documentation often lists specific reasons for soft bounces, such as a full mailbox, a temporarily unavailable server, or an excessively large message size.
Automated handling: The purpose of these classifications is to enable automated systems (ESPs, mail servers) to process bounces correctly, implementing immediate suppression for hard bounces and retry logic for soft bounces.
Key considerations
Adherence to protocols: Email senders and ESPs should adhere to the established SMTP protocols and error code definitions for accurate bounce processing. This helps ensure interoperability and consistent deliverability across the internet.
Robust bounce processing: Implement or utilize systems that are capable of parsing and correctly interpreting bounce messages and their associated SMTP codes to apply appropriate actions. This is often integrated into systems that manage DMARC, DKIM, and SPF responses.
Impact on sender reputation: Documentation implicitly supports the idea that failing to manage bounces according to their permanent or temporary nature can degrade sender reputation. Proper handling maintains the trust of receiving mail servers. Twilio notes that a hard bounce indicates that an email has been permanently rejected because the recipient's email address is invalid or does not exist. Twilio's blog on email bounce management provides further insight into these rejections.
Technical article
Official Documentation from Mailchimp defines hard bounces: Mailchimp's official guidelines state that a hard bounce signifies a permanent inability to deliver an email. This is typically due to persistent issues like invalid or non-existent recipient addresses. These types of bounces communicate a definitive failure, indicating that the email address is no longer viable for communication and should be treated as such. The documentation emphasizes the critical action required: hard bounced email addresses must be cleaned (removed) from an audience or mailing list. This immediate suppression is a standard practice to maintain list quality and prevent negative impacts on sender reputation, which can arise from continually attempting to send to invalid addresses. Adhering to this principle is fundamental for maintaining positive standing with internet service providers and ensuring that future email campaigns achieve optimal deliverability rates by targeting only valid and reachable recipients.
10 Jan 2024 - Mailchimp.com
Technical article
Technical Guide from SendLayer explains SMTP error codes for bounces: SendLayer's technical guide to email bounces outlines that email error codes provide a standardized way to differentiate between bounce types. Specifically, most SMTP error codes beginning with a '4xx' digit typically indicate a soft bounce, pointing to a temporary delivery issue that might resolve on its own. These include errors like a transient server problem or an inbox that is temporarily full. Conversely, error codes starting with a '5xx' digit are generally associated with a hard bounce. These codes signify a permanent delivery failure, often due to an invalid recipient address or a non-existent domain. Understanding these distinctions based on SMTP codes is crucial for automated bounce processing. This standardized numerical classification allows email systems to automatically categorize bounces and apply appropriate handling rules, such as retrying for '4xx' codes and immediate suppression for '5xx' codes, ensuring efficient and protocol-compliant email delivery management.
