Bounce back email is not just an indicator of failed emails; it directly affects your email deliverability, sender reputation, customer communication, and ultimately your revenue. Whether you are sending a newsletter, a transactional email, or a promotional campaign, bounce backs can affect your engagement.
When too many emails bounce, mailbox providers such as Google, Yahoo, etc, start to distrust your domain, causing more of your emails to land in spam, even if the recipients are valid. Hence, understanding email bounces, how they occur, and what bounce back email means is essential for protecting your deliverability and sender reputation.
This guide breaks down email bounce types, causes, and prevention strategies, and best practices to keep your emails landing exactly where they should.
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Email bounce vs bounce back emails: Differences
Most people treat email bounces and bounce back emails as the same thing, but in reality, they refer to two different parts of the email delivery process or email marketing campaign. Here’s a simple analogy for easy understanding.
- Email bounce = The failure event (the email was not delivered)
- Bounce back email = The notification telling you that the failure happened
Email bounce:
An email bounce is the actual failure that occurs when your email cannot be delivered to the recipient’s inbox. In short, an email bounce is a failed delivery event. This failure is recorded by ISPs (Gmail, Yahoo, Outlook) and email platforms and is an important deliverability metric.
For example, you send an email to “[email protected].” Since the domain has a typo, the email cannot be delivered, resulting in an email bounce. Many other reasons can be attributed to your email getting bounced.
Bounce back email:
A bounce-back email is the automated message that you receive when an email bounces. It is also known as a
- Non-delivery report (NDR)
- Delivery status notification (DSN)
- Returned email message
- Mail delivery failure notice
When the email you send bounces, a bounce back email is generated and sent to you. This bounce back email is a report containing the reason why the email failed, an SMTP error code (like 550, 421, 552), and a few other details.
Also Read: Email Automation: An Ultimate Guide for Beginners 2025
Types of email bounce
An email bounce can either be temporary or permanent. Based on this, email bounce can be categorized into two types, and they are,
- Soft bounce – Temporary Delivery Failure
- Hard bounce – Permanent Delivery Failure
Soft bounce (Temporary delivery failure)
A soft bounce occurs when an email reaches the recipient’s mail server but cannot be delivered at the moment. This issue is typically temporary, meaning the email might succeed if retired. Common causes of soft bunces are,
- Recipient’s mailbox is full – The inbox has reached its storage limit.
- Recipient’s email server is down – Maintenance, outages, or temporary connectivity issues.
- Email size is too large – Attachments are pushing the message beyond allowed limits.
- Temporary email filtering – The server requires verification of the sender before accepting additional messages.
- Greylisting – Some servers intentionally reject unknown senders on the first attempt.
Hard bounce (Permanent delivery failure)
A hard bounce in email indicates a permanent problem that prevents an email from ever being delivered. Unlike soft bounces, retries will not resolve the issue. Hard bounces are taken very seriously by ISPs and can severely damage your sender reputation. Causes include,
- Invalid or non-existent email addresses – Typos, fake names, abandoned inboxes.
- Recipient’s domain does not exist
- Server permanently rejects your email – Due to spam filters, blocklists, or authentication issues.
- Blocked sender – The recipient has blocked your domain or IP.
- Spam trap or invalid list source – Often caused by purchased or scraped email lists.
How to read a bounce back email?
Every bounce back email follows a pattern, and learning how to read it helps you quickly identify whether the email bounce is temporary, permanent, or caused by reputation, content, or infrastructure issues. Here are the steps to read a bounce back email.
Step 1: Identify the bounce type
The first thing to look for is whether the bounce is:
- Soft bounce -> Temporary problem
- Hard bounce -> Permanent problem
Bounce back emails often include lines like,
‘This is a soft bounce’, ‘Permanent failure’, ‘User unknown.’
Or they would have reference codes that you can easily interpret. This tells you whether to retry the email or remove the address.
Step 2: Look for the SMTP error code
This is the most important part in reading a bounce back email. If you look at it, every email includes a numerical code that tells you exactly why the email failed. Every SMTP code is usually in the format: X.X.X or XYZ
Soft bounce (temporary error). SMTP error codes are 4xx series. Some of the soft bounce error codes are,
- 4.0.0 indicates a Generic temporary failure
- 421 indicates Service not available/server busy
- 450 indicates Mailbox unavailable
Hard bounce (permanent error) SMTP error codes are 5xx series, and some of them include,
- 5.0.0 indicates permanent failure (generic)
- 551 indicates the user doesn’t exist
- 552 indicates the user exceeded storage allocation
The table below shows some of the most commonly occurring SMTP error codes.
| SMTP Code | Bounce Type | Meaning | Action |
| 421 | Soft | Server busy | Retry |
| 450 | Soft | Mailbox unavailable | Retry |
| 451 | Soft | Temporary local issue | Retry |
| 452 | Soft | Storage full | Retry |
| 4.2.2 | Soft | Mailbox full | Retry |
| 550 5.1.1 | Hard | User does not exist | Remove |
| 551 | Hard | Invalid recipient | Remove |
| 552 | Hard | Storage exceeded (long-term) | Remove |
| 553 | Hard | Invalid mailbox format | Remove |
| 554 | Hard | Rejected (policy/spam/blacklist) | Investigate reputation |
| 5.7.x | Hard | Blocked due to authentication/spam | Fix SPF/DKIM/DMARC |
Step 3: Read the server’s explanation
Right after the SMTP code, you’ll find a short text explanation, such as:
- “Mailbox full”
- “User does not exist.”
- “Message rejected”
- “IP Blocked”
- “Your domain failed authentication.”
And this tells you what exactly went wrong.
Step 4: Identify the issue
Now, you need to identify whether the issue is on your side or the recipient’s side. Bounce bounce-back email will indirectly tell you where the failure occurred.
Recipient-side issues:
- Mailbox full
- Inbox doesn’t exist
- Domain not found
- Recipient server down
Sender-side issues:
- SPF fail
- DKIM fail
- DMARC reject
- Blocklisted IP
- Message flagged as spam
These steps help you know who needs to fix the problem.
Step 5: Look for authentication failures
Scan for terms like:
- “SPF fail”
- “DKIM signature invalid.”
- “DMARC reject”
If any appear, the bounce likely occurred because your domain wasn’t authenticated correctly – meaning your sender reputation is at risk.
Step 6: Check for policy or security reactions
Bounce back emails often include lines like:
- “Message content rejected.”
- “Policy restriction”
- “Too many emails sent.”
- “Your IP is blocked.”
These include deliverability or reputation issues, not technical ones.
Step 7: Review message headers
Some bounce emails include part of your original message header.
Check for:
- Correct sending domain
- Correct return-path
- IP mismatch
- Wrong routing server
Headers can confirm whether the bounce is from misconfiguration or routing problems.
Step 8: Determine final action
Based on all the above:
If it’s a soft bounce (4xx codes):
- Retry sending
- Monitor if it repeats
- If repeated → mark the address as “temporarily suppressed.”
If it’s a hard bounce (5xx codes):
- Remove the address immediately
- Do NOT retry
- Validate your list acquisition method
- Check for signs of fake or mistyped emails
Hard bounce = permanent failure → sending again harms your reputation.
How does bounce back email affect reputation?
Bounce back emails play a major role in shaping your sender reputation. When the email bounce rate increases, it signals poor list quality and can push your emails toward spam. Some of how these bounce back emails affect your reputation are listed below.
- High bounce rates signal poor list quality.
Providers assume you’re emailing invalid or fake addresses, reducing trust and lowering reputation. - Hard bounces do the most damage.
They indicate permanent issues like nonexistent inboxes and reflect poor list hygiene. - Bounce spikes trigger spam filters.
Sudden or repeated failures resemble spammer behavior, causing throttling or blocking. - Authentication errors inside bounce reports hurt reputation.
SPF, DKIM, or DMARC failures show misconfiguration or insecurity, reducing sender trust. - Frequent soft bounces accumulate over time.
Even temporary issues weaken your reputation when they occur repeatedly. - Bounce-related delivery issues reduce engagement signals.
Fewer real recipients mean lower opens and clicks, further harming the sender’s score. - Consistent bounces lead to throttling or blocking.
Mailbox providers slow or stop your emails if too many bounce regularly. - Bounce patterns feed into sender reputation algorithms.
High or recurring bounce rates negatively impact domain/IP scoring. - Bad bounce behavior affects your entire domain.
Marketing, transactional, and support emails may all start landing in spam. - Ignoring bounce messages accelerates reputation decline.
Continuing to send to bad addresses signals negligence, worsening your score.
How to prevent email bounce
Email bounce can significantly affect your sender reputation and deliverability. This can be minimized with the right practices, tools, and sending habits. Here’s a small checklist on how to significantly reduce email bounces.
1. Keep your email list clean and updated
A clean email list is the most important defense against hard bounces. Some practices include,
- Regularly removing inactive or outdated emails
- Immediately deleting addresses that hard bounce
- Avoiding purchased or scrapped lists
- Running a periodic list hygiene (every 3-6 months).
This ensures high deliverability and engagement.
Also Read:How to Grow your Email List Fast: 18 Proven Strategies
2. Validate emails before sending
Use real-time or bulk verification tools to catch invalid addresses early. Examples include,
Zero bounce, Never bounce, Clearout, Emailable
Verification instantly flags:
- Misspelled domains
- Fake emails
- Disposable email addresses
- Inactive inboxes
This prevents unnecessary hard bounces.
3. Use double opt-in for new subscribers
Double opt-in requires users to confirm their subscription, which:
- Eliminates fake or mistyped email addresses
- Ensures high-intent subscribers
- Reduces spam complaints and bounce rates
This simple step can reduce bounces by 30-50%
4. Authenticate your domain
Mailbox providers expect authenticated senders. Make sure you have:
- SPF – proves your server is authorized to send
- DKIM – verifies your message wasn’t altered
- DMARC – defines how receivers handle failures
Without proper authentication, emails may bounce or get rejected
5. Use a reliable SMTP/Email sending service
Do not rely on basic hosting email or PHP email. Use a trusted SMTP provider such as:
- Amazon SES
- SendGrid
- Mailgun
- Brevo
- Postmark (best for WooCommerce transactional emails)
These systems ensure proper routing, reputation management, and lower bounce rates.
6. Monitor bounce rates and take action quickly
Track soft and hard bounces across campaigns. Fix issues when you see:
- Sudden bounce spikes
- Repeated failures in specific domains (example includes Gmail, Yahoo)
- Authentication failures
- Temporary blocks
Respond early before reputation declines.
Also Read: How to Reduce Email Bounce Rates
7. Re-engage or remove inactive subscribers
Inactive users lead to low engagement, which indirectly increases bounce risk.
Implement:
- Re-engagement campaigns
- Sunset policies (removing inactive users after X months)
- Engagement-based segmentation (send less to unengaged)
Also Read:Email Segmentation: Complete Guide for 70% More Revenue
This helps preserve list quality and sending reputation.
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8. Avoid sending attachments and large files
Large emails (especially with heavy attachments):
- Trigger soft bounces
- Increase spam scoring
- Cause server size restrictions
Use cloud links instead (Google Drive, Dropbox).
9. Warm up your sending domain or IP (If new)
New domains or IPs must build trust gradually. Start with,
- Low-volume sends
- Slowly increasing daily limits
- Sending to your most engaged subscribers first
A warmed-up domain reduces both bounces and spam placement.
10. Avoid spam triggers in content
Spammy content can cause policy-based bounces. Avoid,
- Excessive sales language
- All-caps subject lines
- Overuse of images
- Too many links
- URL shorteners
- Misleading subject lines
Cleaner content = fewer behavioral blocks.
11. Use proper list acquisition methods
To prevent bounces:
- Do not buy or rent email lists
- Avoid mass imports without validation
- Capture emails directly from genuine users (store checkout, forms, newsletter signup)
High-quality sources produce low bounce rates.
12. Check DNS, MX, and sending configuration regularly
Technical misconfiguration can cause:
- Routing failures
- Authentication errors
- Domain-based rejections
- Temporary blocks
Ensure DNS records and email settings are always up to date.
13. Implement throttling for large campaigns
Sending too many emails too quickly causes:
- Rate limiting
- Temporary server rejections
- Soft bounces (4xx codes)
Throttle your sending volume for large lists.
14. Avoid repeated sends to the same bounced addresses
Sending too many emails too quickly causes:
- Rate limiting
- Temporary server rejections
- Soft bounces (4xx codes)
Throttle your sending volume for large lists.
15. Segment your audience by engagement and recency
Segmenting allows you to protect your reputation by:
- Sending more frequently to active users
- Reducing sends to unengaged
- Eliminating risky addresses that produce bounces.
This keeps your deliverability strong.
Segment your audience and target specific segments and increase your email engagement rates using Retainful
Conclusion
Bounce back emails are more than just an inconvenience—they’re a clear signal that something is wrong with your email practices. Whether it’s a poor sender reputation, technical configuration issues, or a lack of proper list hygiene, ignoring bounce back emails can lead to long-term deliverability problems, increased costs, and a damaged brand reputation.
By taking proactive steps, such as maintaining a clean list, authenticating your domain, and monitoring bounce rates, you can significantly reduce bounces and ensure that your emails reach their intended recipients. Tools like Retainful can help streamline this process for WooCommerce store owners by enabling automatic cart abandonment recovery, improving email deliverability, and tracking engagement metrics to avoid bounces altogether.
Ultimately, managing bounces is about maintaining a healthy and trusted relationship with your email recipients and mailbox providers, which is essential for long-term email marketing success. With the right tools and practices, you can reduce bounce rates and see your email campaigns achieve greater success.
Related Read:
- 12 Most Important Email Marketing Metrics & KPIs to Track
- What is an Email Suppression List (Benefits + How to Manage)
- 7 Best Double Opt-in Email Examples + Best Practices
Frequently Asked Questions
An example of a bounce back email message might look like this:
“550 5.1.1 – User unknown. The email address you attempted to reach does not exist.”
This message indicates that the recipient’s email address is invalid or non-existent.
You don’t initiate a bounce back email yourself. It occurs automatically when an email cannot be delivered due to issues like incorrect recipient addresses, full mailboxes, or server errors. The mail server sends the bounce back message.
Bounced emails are undelivered due to problems like invalid addresses or server issues. Blocked emails are actively rejected by the recipient’s server, often due to spam filters, block listing, or sender reputation issues. Both prevent delivery but for different reasons.
A bounce back is an automatic response from the recipient’s mail server indicating that your email couldn’t be delivered. It provides details such as the reason for the failure, whether it’s a full mailbox, invalid address, or server rejection.
