Summary
Email deliverability experts and platform documentation consistently define hard bounces as permanent delivery failures, primarily due to invalid or non-existent email addresses, often signaled by 5xx SMTP error codes. Conversely, soft bounces are temporary delivery issues, such as a full inbox or server problems, typically indicated by 4xx codes. The overwhelming consensus is to never resend emails to addresses that have hard bounced, advocating for their immediate removal or suppression to safeguard sender reputation. For soft bounces, however, the system may automatically retry sending, as these issues are often transient and may resolve over time. A notable nuance from experienced marketers highlights that despite standard SMTP definitions, real-world ISP behavior can sometimes blur the lines, where some 'hard' bounce codes may not imply permanent unrecoverability, requiring ESPs to interpret actual bounce meanings beyond strict code adherence.
Key findings
- Hard Bounce Definition: A hard bounce signifies a permanent delivery failure, typically because the email address is invalid, non-existent, or permanently blocked. These are generally indicated by 5xx SMTP error codes, such as 550.
- No Resend to Hard Bounces: There is a strong consensus among email marketing experts and ESPs: never resend emails to addresses that have hard bounced. Doing so significantly harms sender reputation and deliverability.
- Immediate Suppression: Hard-bounced addresses should be immediately removed or suppressed from mailing lists. This practice is vital for maintaining list hygiene and protecting a sender's reputation with Internet Service Providers (ISPs).
- Soft Bounce Definition: A soft bounce indicates a temporary email delivery issue. Common causes include a full inbox, a recipient's server being temporarily unavailable, or a server timeout. These are typically associated with 4xx SMTP error codes, such as 450.
- Soft Bounce Retries: Soft bounces often resolve on their own. Email service providers typically automatically retry sending messages to soft-bounced addresses for a period, making resending potentially viable for the system.
Key considerations
- Beyond SMTP Codes: While SMTP 5xx codes generally signify permanent hard bounces and 4xx codes temporary soft bounces, real-world ISP behavior and technical issues can blur these definitions. ISPs do not always strictly comply with RFC standards, meaning a 5xx code might not always indicate a truly unrecoverable or inactive account.
- ESPs' Interpretation: Email Service Providers (ESPs) often need to interpret the true meaning of bounce codes, as some 'hard' bounces may, in practice, behave more like 'soft' ones. This requires sophisticated systems to prevent immediate, unnecessary suppression of potentially recoverable addresses, despite the initial bounce classification.
- Evolving Best Practice: The understanding of hard bounces is evolving; while immediate suppression of permanent failures remains crucial for sender reputation, not every 5xx bounce necessarily warrants immediate, irreversible deactivation, depending on an ESP's ability to discern nuance.
What email marketers say
13 marketer opinions
When managing email lists, a critical aspect of maintaining deliverability involves understanding and correctly handling email bounces. Industry experts widely agree that hard bounces, signifying permanent delivery failures often due to invalid addresses or permanent blocks, necessitate immediate removal from mailing lists to preserve sender reputation. Conversely, soft bounces denote temporary issues, such as a full inbox or server unavailability, which email service providers (ESPs) typically attempt to retry. While standard SMTP codes define 5xx as hard and 4xx as soft, seasoned marketers caution that Internet Service Providers (ISPs) do not always strictly adhere to these standards, meaning some 'hard' bounce codes may not represent truly unrecoverable addresses in practice. This complexity underscores the importance of ESPs' sophisticated interpretation systems to differentiate genuine permanent failures from temporary anomalies.
Key opinions
- Hard Bounces: Permanent Failure: A hard bounce represents a permanent email delivery failure, typically due to an invalid email address, a non-existent recipient, or the domain being permanently blocked. These are commonly signaled by 5xx SMTP error codes.
- Strict No-Resend Rule for Hard Bounces: Email marketing experts unequivocally advise against resending emails to addresses that have hard bounced. Attempting to do so severely damages sender reputation and negatively impacts overall email deliverability.
- Prompt Removal of Hard Bounces: Contacts associated with hard bounces must be immediately removed or suppressed from mailing lists. This is a fundamental practice for maintaining list hygiene and safeguarding a sender's standing with Internet Service Providers (ISPs).
- Soft Bounces: Temporary Issues: Soft bounces indicate temporary delivery problems. Common causes include a recipient's inbox being full, the server being temporarily unavailable, or a message exceeding size limits. These are generally associated with 4xx SMTP error codes.
- Automated Retries for Soft Bounces: Soft bounces are often transient, and email service providers (ESPs) typically handle these by automatically retrying the send for a certain period. This means manual resending by the marketer is usually unnecessary for soft bounce scenarios.
Key considerations
- ISP Discrepancies in Bounce Coding: While SMTP 5xx codes typically indicate permanent failures and 4xx codes temporary issues, Internet Service Providers (ISPs) often do not strictly adhere to RFC standards for non-delivery reports. This means some 5xx codes might not signify a truly inactive or unrecoverable account.
- The Role of ESPs in Bounce Interpretation: Given the non-standard usage of SMTP codes by some receiving servers, Email Service Providers (ESPs) play a crucial role in interpreting the actual meaning of bounce messages. Their systems must discern if a 'hard' bounce is genuinely unrecoverable or if it behaves more like a temporary issue.
- Nuanced Approach to 'Hard' Bounces: The understanding of what constitutes an unrecoverable hard bounce is evolving. Although immediate suppression is critical for sender reputation, not every 5xx bounce necessarily warrants irreversible deactivation, especially if an ESP can identify it as a transient or miscategorized issue.
Marketer view
Email marketer from Email Geeks explains that he disagrees with resending to addresses that have hard bounced, and notes that many 5XX SMTP codes are effectively "soft" in practice, as receivers often don't adhere strictly to the hard bounce definition, requiring ESPs to interpret actual bounce meanings.
13 Jun 2023 - Email Geeks
Marketer view
Email marketer from Email Geeks shares his evolving understanding of hard bounces, initially believing resending is harmful but then acknowledging that some "hard" bounces may not be unrecoverable. He suggests it's not always best practice to immediately render hard bounces inactive, despite SMTP codes typically defining 5xx as hard and 4xx as soft.
26 Feb 2025 - Email Geeks
What the experts say
2 expert opinions
Understanding the distinction between hard and soft bounces is fundamental for effective email list management and maintaining strong sender reputation. Hard bounces signify a permanent inability to deliver an email, typically due to an invalid or non-existent address, and demand immediate removal from mailing lists to protect deliverability. In contrast, soft bounces indicate a temporary delivery issue, such as a full inbox or a server being temporarily offline, which often leads to automated retry attempts by the sending platform. Therefore, emails should never be resent to addresses that hard bounce, while soft bounces are handled differently due to their transient nature.
Key opinions
- Hard Bounce Nature: A hard bounce indicates a permanent email delivery failure, resulting from issues like an invalid email address or a non-existent domain.
- Action for Hard Bounces: Addresses that hard bounce must be immediately removed from mailing lists, and emails should never be resent to them to prevent harm to sender reputation.
- Soft Bounce Nature: A soft bounce signals a temporary problem preventing email delivery, such as a recipient's inbox being full or the mail server being temporarily unavailable.
- Handling Soft Bounces: Unlike hard bounces, soft bounces often prompt email service providers to automatically retry sending, as these issues are typically transient.
Key considerations
- Preserving Sender Reputation: Consistently attempting to send emails to hard-bounced addresses significantly damages sender reputation, leading to lower deliverability rates for legitimate emails across your entire list.
- Maintaining List Hygiene: Regular and immediate removal of hard-bounced addresses is paramount for maintaining a clean, engaged mailing list, which is a cornerstone of good email deliverability.
Expert view
Expert from Spam Resource explains that a hard bounce signifies a permanent delivery failure, such as an invalid email address or non-existent domain, and these addresses should be immediately removed from the mailing list, meaning you should not resend to them. A soft bounce, however, indicates a temporary issue, like a full inbox or server unavailability.
11 Mar 2022 - Spam Resource
Expert view
Expert from Word to the Wise clarifies that a hard bounce signifies a permanent delivery failure, indicating an invalid or non-existent email address, and should lead to immediate removal from the mailing list to prevent future resends. A soft bounce, conversely, denotes a temporary issue like a full mailbox or server problem, which typically results in retry attempts.
22 Oct 2021 - Word to the Wise
What the documentation says
5 technical articles
The consensus among leading email platforms and technical standards is clear: never attempt to resend emails to addresses that result in a hard bounce. A hard bounce signifies a permanent delivery failure, usually due to an invalid or non-existent email address, and is often indicated by a 5xx SMTP error code. Conversely, a soft bounce signals a temporary delivery issue, such as a full inbox or a server being momentarily unavailable, typically associated with 4xx SMTP errors. While hard bounces necessitate immediate suppression to safeguard sender reputation, soft bounces are generally handled by automated retries from email service providers, as these issues are often transient.
Key findings
- Hard Bounce Definition: Across various platforms, a hard bounce is defined as a permanent email delivery failure, most often caused by an invalid or non-existent recipient address, as confirmed by Mailchimp, SendGrid, HubSpot, and Salesforce.
- Soft Bounce Definition: A soft bounce indicates a temporary email delivery problem, such as a recipient's inbox being full or the mail server being temporarily unavailable, a consistent explanation from Mailchimp, SendGrid, HubSpot, and Salesforce.
- Mandatory Hard Bounce Suppression: All surveyed sources, including Mailchimp, SendGrid, HubSpot, and Salesforce, strongly advise against resending to hard-bounced addresses, emphasizing their immediate removal or suppression to protect sender reputation.
- Soft Bounce Retry Mechanism: For soft bounces, email service providers typically handle retries automatically, as these issues are temporary and may resolve on their own, a practice noted by Mailchimp, HubSpot, and Salesforce.
- SMTP Error Code Guidance: The IETF's SMTP protocol specifies that '5xx' error codes denote permanent failures (hard bounces), while '4xx' error codes signify temporary issues (soft bounces) that might be successful on retry.
Key considerations
- Impact on Sender Reputation: Consistently sending emails to hard-bounced addresses, as highlighted by HubSpot and Salesforce, significantly degrades a sender's reputation, leading to lower deliverability for future campaigns.
- Automated Bounce Management: The automatic suppression of hard bounces and the retrial of soft bounces by ESPs like Salesforce and HubSpot streamline list hygiene, reducing the manual burden on marketers while preserving deliverability.
Technical article
Documentation from Mailchimp explains that a hard bounce is a permanent delivery failure because the email address is invalid or non-existent, and you should never resend emails to them. A soft bounce is a temporary issue, such as a full inbox or server problem, which may resolve itself, making resending potentially viable.
21 May 2024 - Mailchimp Knowledge Base
Technical article
Documentation from SendGrid explains that hard bounces are permanent email delivery failures, often due to an invalid recipient address, and should be immediately suppressed. Soft bounces are temporary issues, such as a recipient's inbox being full or the server being temporarily unavailable, which may eventually resolve.
11 Jan 2025 - SendGrid Documentation
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