Sender bounces happen when an email is rejected because of sender-related signals or the receiving server's delivery policies, rather than because the recipient's email address does not exist.
Understanding the difference between a lead bounce and a sender bounce is important because the troubleshooting steps are very different.
Example | What went wrong | Reason |
Lead bounce | The recipient address is invalid or unavailable |
|
Sender bounce | The receiving server rejected the message based on the sender, message, authentication, or its security policies |
|
With a lead bounce, the recipient address itself is generally the issue.
With a sender bounce, the recipient address may still be valid. The receiving server decided not to accept the email based on factors such as your sending domain, authentication, message content, infrastructure, or the recipient organization's security policies.
For example, the same recipient could potentially accept an email sent from a different sending domain or infrastructure.
Sender bounces can happen for several reasons. The most common are:
Mailbox providers evaluate signals associated with your sending domain and infrastructure when deciding whether to accept an email.
Rejections can increase when:
A domain is relatively new
Sending volume is increased too quickly
Sending patterns change suddenly
Previous sending has generated negative engagement signals
The receiving provider does not trust the sending infrastructure
You may see responses such as:
550 5.7.1
However, the exact meaning depends on the full SMTP response returned by the receiving server.
Receiving servers use authentication mechanisms such as SPF, DKIM, and DMARC to verify the sender.
Sender bounces may occur when:
SPF is missing or incorrectly configured
DKIM signing fails
DMARC alignment fails
DNS changes have not propagated correctly
The sending domain does not match the expected authentication configuration
A common response associated with authentication failures is:
550 5.7.26
If you see authentication-related bounce messages, check the DNS configuration for the affected sending domain.
The receiving provider may reject an email after analyzing its content.
Certain combinations of message content, links, formatting, domains, and sending patterns can trigger filtering.
This does not necessarily mean that one specific word caused the bounce. Email providers evaluate multiple signals together.
Things worth reviewing include:
Excessive links
Tracking links
URL shorteners
Image-heavy emails
HTML-heavy formatting
Attachments
Highly promotional language
Repetitive templates sent at high volume
If content appears to be the common factor, test a simpler version of your email before increasing sending volume again.
Companies often use additional email security providers such as:
Mimecast
Proofpoint
Microsoft Defender
Barracuda
These systems may apply stricter security and delivery policies than consumer mailbox providers.
For example, an email sent to a valid corporate recipient may be rejected by Mimecast because of the organization's security policy.
In Smartlead, these types of rejections may be classified as sender bounces because the recipient itself is not necessarily invalid.
This is especially common when sending to enterprise organizations.
The SMTP response is one of the most useful pieces of information when investigating sender bounces.
Here are some commonly seen examples:
SMTP code | Typical meaning | Recommended action |
| Recipient does not exist | Verify or remove the lead |
| Recipient address not found | Verify or remove the lead |
| Message rejected due to policy | Review the complete SMTP response, sending reputation, content, and authentication |
| Authentication failure | Check SPF, DKIM, and DMARC |
| Security or policy-related rejection | Review the full bounce message to identify the receiving provider's reason |
| Temporary delivery failure | Allow Smartlead to retry where applicable and monitor whether the issue continues |
Important: Do not diagnose a bounce using only the first few digits of the SMTP code.
Two providers may use similar SMTP codes for different reasons. Always review the complete bounce response returned by the receiving mail server.
What is considered a high sender bounce rate?
There is no single percentage that applies to every provider or sending setup, but consistently increasing sender bounces should be investigated early.
As a general guideline:
Sender bounce rate | Recommendation |
Below 2% | Continue monitoring |
2–5% | Investigate the bounce reasons and affected mailboxes/domains |
Above 5% | Consider pausing or reducing sending while you investigate |
A sudden increase is often more important than the percentage alone.
For example, if a mailbox normally has almost no sender bounces and suddenly begins receiving policy rejections from multiple providers, investigate before continuing to scale sending.
If sender bounces suddenly increase, avoid continuing at the same volume while investigating.
Continuing to send emails that are repeatedly rejected can negatively affect future deliverability.
Pause the affected mailbox or reduce sending volume until you understand the cause.
Review the SMTP responses for the affected emails.
Look for patterns such as:
The same SMTP code
The same receiving provider
The same sending domain
The same mailbox
The same campaign
Similar email content
For example, if most sender bounces are coming from recipients protected by Mimecast, the issue may be specific to that security gateway rather than all of your sending.
The full SMTP response is much more useful than looking only at the bounce percentage.
Verify the DNS configuration for each affected sending domain.
Check that:
SPF is configured correctly
DKIM is enabled and signing emails correctly
DMARC is published and properly aligned
Your sending mailbox is configured correctly
Authentication issues should be resolved before resuming higher sending volumes.
Determine whether the sender bounces are happening:
From one mailbox
Across multiple mailboxes
On one domain
Across several domains
Only with one receiving provider
Across Gmail, Outlook, and enterprise recipients
This helps narrow down the problem.
For example:
Only one mailbox affected:
Check that mailbox and its configuration.
All mailboxes on one domain affected:
Investigate the domain and DNS configuration.
Mostly Mimecast or Proofpoint recipients affected:
The receiving organization's security policies may be contributing to the rejection.
Multiple providers rejecting the same sending domain:
Review the domain, authentication, content, and sending patterns more closely.
Try simplifying your message.
A good troubleshooting version should:
Use mostly plain text
Avoid unnecessary images
Reduce the number of links
Avoid URL shorteners
Avoid attachments
Avoid excessive formatting
Use natural, relevant messaging
Keep the first email concise
If possible, test the revised copy on a smaller volume before scaling again.
Large or sudden changes in sending volume can contribute to delivery problems.
When using newer mailboxes or domains:
Start with conservative sending volumes
Increase volume gradually
Avoid sudden spikes
Spread sending across the configured campaign schedule
Monitor bounce and reply performance while scaling
Avoid immediately pushing a new mailbox or domain to its maximum sending limit.
Warm-up can help establish normal sending and receiving activity for mailboxes, especially before campaign sending begins.
For newer infrastructure:
Enable warm-up before starting campaigns
Allow the mailbox to build sending history
Ramp campaign volume gradually
Continue monitoring mailbox health
Warm-up can support healthy sending patterns, but it should not be treated as a guaranteed fix for authentication, content, reputation, or recipient-policy issues.
Imagine you email:
The server responds:
550 5.1.1 User unknown
The recipient does not exist.
Result: Lead bounce.
The server responds with a security-policy rejection from Mimecast.
John's address may still be completely valid, but the receiving security system decided not to accept your message.
Result: Sender bounce.
That distinction is important because removing John from your lead list would not necessarily solve the underlying issue.
Before returning to your normal sending volume, check the following:
Review the most common SMTP bounce responses
Confirm SPF is configured correctly
Confirm DKIM is working
Confirm DMARC is published
Check whether one mailbox or domain is responsible for most bounces
Check whether bounces are concentrated around one receiving provider
Review and simplify your email copy
Reduce links, images, and unnecessary formatting
Confirm sending volume has not increased too quickly
Restart at a lower volume and monitor performance
A sender bounce does not automatically mean that the lead's email address is invalid.
It means the receiving server did not accept the message based on factors associated with the sender, the message, authentication, infrastructure, or the receiving organization's security policies.
When sender bounces increase:
Check the SMTP response → identify the pattern → fix the underlying cause → resume sending gradually.
