Should you resend emails to hard bounces, and how are hard and soft bounces defined?
Michael Ko
Co-founder & CEO, Suped
Published 11 Jun 2025
Updated 19 Aug 2025
8 min read
Email bounces are a common occurrence for anyone sending emails, whether for marketing, transactional purposes, or personal communication. They signify that your email could not be delivered to the intended recipient. Understanding why emails bounce and how to categorize these failures is crucial for maintaining good sender reputation and ensuring your messages reach their destination.
The distinction between a hard bounce and a soft bounce is fundamental, as it dictates how you should manage your email list and respond to delivery failures. Mismanaging bounces can lead to significant issues, including diminished email deliverability, increased spam complaints, and even being placed on an email blocklist (or blacklist). It's a key part of email deliverability and managing your sender reputation.
Understanding email bounces: soft vs. hard
To effectively manage your email campaigns and maintain a healthy sending reputation, it is essential to understand the definitions and utility of different email bounce types. A hard bounce indicates a permanent delivery failure. This means the email address is invalid or no longer exists, and subsequent attempts to send to it will also fail. Common reasons include a misspelled email address, a domain name that doesn't exist, or a recipient who has permanently closed their account. When an email hard bounces, the mail server typically returns an SMTP error code in the 5xx series, signaling a permanent failure.
In contrast, a soft bounce signals a temporary delivery issue. The email address itself is valid, but the message couldn't be delivered for a transient reason. This could be due to the recipient's mailbox being full, the email server being temporarily unavailable, the message being too large, or even a temporary block by a spam filter. Soft bounces typically result in SMTP error codes in the 4xx series, indicating that the sender should retry sending the email later. Your Email Service Provider (ESP) will often attempt to resend soft-bounced emails multiple times over a period, hoping the temporary issue resolves itself. More on this is available from Mailchimp’s guide to soft and hard bounces.
While the general rule of thumb categorizes 5xx codes as hard bounces and 4xx codes as soft bounces, the reality can be more complex. Some mail servers might return a 5xx code for what is effectively a temporary issue, or a 4xx code might eventually lead to a permanent failure if the problem persists. It is crucial for ESPs to interpret these codes correctly, as misclassifying a bounce can lead to incorrect list management actions, impacting deliverability.
Bounce type
Definition
SMTP code series
Typical reasons
Hard bounce
Permanent delivery failure, unrecoverable.
5xx
Invalid email address, domain not found, recipient account closed.
Mailbox full, server temporarily down, message too large, temporary filter block.
The critical difference: resending strategy
The general recommendation is to never resend emails to hard bounces. Since a hard bounce indicates a permanent reason for non-delivery, attempting to resend emails to these addresses is futile and harmful. Continuing to send to invalid addresses signals to internet service providers (ISPs) that your sending practices are poor, which can negatively impact your sender reputation. This can lead to your emails being flagged as spam, being blocklisted, or even having your sending privileges revoked by your ESP. Many ESPs automatically suppress hard-bounced addresses to protect your reputation.
For soft bounces, the situation is different. Since these are temporary issues, it's generally acceptable, and often necessary, for your ESP to retry sending the email. Most ESPs have built-in retry mechanisms for soft bounces, which typically involve attempting delivery again after a certain period (e.g., hours or days). If an email continues to soft bounce repeatedly over an extended period, many ESPs will eventually convert it into a hard bounce and suppress the address. This soft bounce tolerance helps prevent continued attempts to undeliverable addresses from harming your reputation.
Hard bounce suppression
Always remove hard-bounced email addresses from your mailing lists immediately. This is not optional if you want to maintain good sender reputation. Continuing to send to these addresses will increase your bounce rate, which is a major red flag for ISPs and can lead to emails landing in spam folders or even your domain being placed on a blocklist.
Even if an email address hard bounces due to a temporary server issue or a strict filter, it's best to err on the side of caution. Attempting to resend to an address that has explicitly returned a permanent error is a risky practice. It's better to clean your list and focus on engaged subscribers, rather than risking your overall deliverability for a potentially non-existent or problematic contact.
Impact on sender reputation and deliverability
Both hard and soft bounces, particularly when they occur frequently, can impact your sender reputation. High bounce rates signal to ISPs that you might be sending to old, purchased, or poorly maintained lists, which are characteristics of spam senders. ISPs use bounce rates, among other metrics, to assess your trustworthiness. A low bounce rate (generally below 2-3%, as indicated by sources like Mailgun) is a strong indicator of a healthy list and good sending practices.
Hard bounces are particularly damaging because they are immediate and permanent. Each hard bounce is a definitive sign of an unreachable address, and continuing to send to such addresses can quickly lead to your IP or domain being added to a DNS-based blocklist (DNSBL). Once on a blocklist, your emails are likely to be rejected by many mail servers, severely impacting your overall deliverability. This is why immediate suppression of hard bounces is non-negotiable.
Hard bounce impact
Reputation: Significantly damages sender reputation, indicating poor list hygiene or acquisition practices.
Deliverability: Can lead to email rejections, higher spam scores, and being placed on blocklists (or blacklists).
Action: Requires immediate removal of the address from your list.
Soft bounce impact
Reputation: Less damaging individually, but repeated soft bounces to the same address can eventually hurt sender reputation.
Deliverability: Leads to delayed delivery or eventual failure if the temporary issue persists.
Cost: Can consume resources if ESP retries are excessive without eventual success.
Action: ESPs typically retry; monitor for conversion to hard bounce and then suppress.
Soft bounces, while less severe, can still accumulate and negatively affect your reputation. If an email address consistently soft bounces over multiple campaigns, it's often a sign that it's no longer a viable contact, or that there's a persistent issue with the receiving server. Many ESPs have policies to convert persistently soft-bouncing addresses into hard bounces after a certain number of attempts or a period of time, automatically suppressing them to protect your sending reputation. It's a key part of knowing what an acceptable bounce rate is.
Best practices for managing bounces
Effective bounce management is crucial for the long-term health of your email program. The primary best practice is stringent list hygiene. Regularly cleaning your email list by removing invalid or unengaged addresses is paramount. This includes promptly suppressing hard bounces and monitoring soft bounces for addresses that consistently fail to deliver.
Utilize double opt-in for new subscribers. This ensures that new contacts explicitly confirm their email address, significantly reducing the chances of invalid addresses or spam traps making it onto your list. Also, monitor your email analytics closely. Keep an eye on your bounce rates, complaint rates, and open rates. Spikes in bounces can indicate underlying issues with your list or sending practices that need immediate attention.
Implementing strong email authentication protocols like SPF, DKIM, and DMARC also contributes to better deliverability by proving your legitimacy and reducing the likelihood of your emails being flagged as suspicious. DMARC reports, in particular, provide invaluable feedback on your email authentication results, including bounce data that can help you identify deliverability issues. Reviewing your email deliverability test results can also help you prevent issues from arising.
Maintain meticulous email list hygiene by promptly removing hard bounces and unengaged subscribers.
Implement double opt-in to verify subscriber addresses and reduce invalid sign-ups.
Regularly monitor your bounce rates and DMARC reports for early detection of deliverability issues.
Ensure proper SPF, DKIM, and DMARC configuration to authenticate your emails and build trust with ISPs.
Common pitfalls
Resending emails to hard-bounced addresses, which severely damages sender reputation.
Ignoring consistently soft-bouncing addresses, as they can eventually be treated as hard bounces.
Not segmenting lists or sending irrelevant content, leading to higher soft bounce rates and complaints.
Failing to review bounce codes, missing critical insights into delivery problems.
Expert tips
Some 5xx SMTP codes, while technically hard bounces, might indicate temporary issues, requiring nuanced interpretation.
Mailbox providers' responses can vary and may not always strictly adhere to RFC standards for bounce codes.
Prioritize monitoring your overall bounce rate and deliverability metrics over rigid adherence to specific SMTP code interpretations.
Understand that what constitutes a 'hard' or 'soft' bounce can sometimes be blurry in practice due to non-standard server responses.
Marketer view
Marketer from Email Geeks says they found it to be self-evidently a bad idea to resend to hard bounces, as it damages sender reputation.
2019-01-11 - Email Geeks
Expert view
Expert from Email Geeks says that there can be technical issues that result in a hard bounce, indicating an inactive account, even if the account is not truly inactive. They also noted that non-standard usage of SMTP codes means a 5.2.1 'Mailbox disabled, not accepting messages' does not always signify a truly inactive account, but rather that the email address will not accept that specific email from the sender.
2019-01-11 - Email Geeks
Maintaining a healthy email list
Understanding the difference between hard and soft bounces is not merely an academic exercise; it's a critical component of effective email deliverability. While hard bounces demand immediate suppression to protect your sender reputation, soft bounces allow for retries, giving temporary issues a chance to resolve.
Ultimately, a proactive approach to list management and adherence to best practices will minimize bounces, improve your deliverability, and ensure your emails consistently reach the inboxes of your intended audience. Prioritizing list hygiene and interpreting bounce feedback correctly are key to a successful email program.
