Email communication plays a crucial role in modern business and personal interactions. However, there are instances when emails fail to reach their intended recipients, resulting in bounce messages or non-delivery reports (NDRs). In this comprehensive guide, we will explore the concept of email bounce messages and NDRs, their causes, and how to effectively address them. By understanding these concepts, you can optimize your email deliverability, minimize bounce rates, and ensure your messages reach the right recipients.

What are Email Bounce Messages and NDRs?

Email bounce messages and NDRs are notifications generated by mail servers when an email fails to be delivered to the intended recipient. These notifications inform the sender about the delivery failure and often provide additional information regarding the reason for the failure.

Causes of Email Bounce Messages and NDRs

There are several common causes for email bounce messages and NDRs:

Invalid or Non-existent Email Address: If the recipient's email address is invalid or no longer exists, the email cannot be delivered, resulting in a bounce message or NDR.

Mailbox Full: If the recipient's mailbox is full and unable to receive new messages, the email may bounce back to the sender with a notification.

Server Issues: Temporary server issues, such as a server being down or experiencing high traffic, can result in the failure to deliver an email and trigger a bounce message or NDR.

Spam Filtering: Emails that are flagged as spam by the recipient's email server or spam filter may be rejected and result in a bounce message or NDR.

Message Size Limit Exceeded: If the email exceeds the maximum size limit set by the recipient's email server, it may be rejected, and a bounce message or NDR will be generated.

Types of Email Bounce Messages and NDRs

Email bounce messages and NDRs can be categorized into different types based on the severity and nature of the delivery failure. Some common types include:

Transient Failures: Transient failures are temporary delivery failures that can be resolved by reattempting the email delivery. Examples include a full mailbox, a temporarily unavailable server, or a connection timeout. In these cases, the email server will often make multiple delivery attempts before generating a bounce message or NDR.

Persistent Failures: Persistent failures are permanent delivery failures that cannot be resolved through further delivery attempts. Examples include an invalid email address, a non-existent domain, or a blocked email account. In these cases, the bounce message or NDR indicates that the email cannot be delivered.

Interpreting Email Bounce Messages and NDRs

When you receive a bounce message or NDR, it's essential to interpret the information provided accurately. Here are some key elements to consider:

Error Codes: Bounce messages and NDRs often include error codes that provide specific information about the reason for the delivery failure. These codes can help diagnose the issue and determine the appropriate action to resolve it.

Error Descriptions: In addition to error codes, the bounce message or NDR may include a description or explanation of the error. This information can provide further context and assist in troubleshooting the delivery failure.

Diagnostic Information: Advanced bounce messages and NDRs may include diagnostic information, such as SMTP logs or headers, that can be useful for identifying the root cause of the delivery failure.

Resolving Email Bounce Messages and NDRs

Resolving email bounce messages and NDRs depends on the specific cause of the delivery failure. Here are some general steps to address these issues:

Verify Recipient Information: Double-check the recipient's email address for accuracy. Ensure that the address is valid and active.

Review Bounce Message or NDR: Carefully examine the bounce message or NDR to understand the reason for the delivery failure. Look for error codes, descriptions, and diagnostic information.

Take Appropriate Action: Based on the information provided, take the necessary steps to resolve the issue. This may include contacting the recipient to verify the email address, freeing up mailbox space, or addressing any server-related problems.

Modify Email Content: If the email is being flagged as spam, review and modify the email content to comply with best practices and avoid triggering spam filters.

Consider Email Delivery Services: If you consistently encounter email delivery issues, consider using a reliable email delivery service or email service provider (ESP) that specializes in optimizing deliverability and handling bounce messages and NDRs.


Q: Can bounce messages and NDRs be prevented entirely?

A: While it is not possible to prevent bounce messages and NDRs entirely, following best practices can help minimize their occurrence. Maintaining a clean email list, regularly updating recipient information, and adhering to email sending guidelines can reduce the likelihood of delivery failures.

Q: What should I do if my emails consistently bounce or result in NDRs?

A: If you experience consistent bounce messages or NDRs, it is essential to investigate and address the root cause. Review your sending practices, email content, and recipient information. Consider seeking professional assistance from email deliverability experts or using reputable email delivery services.

Q: Are there tools available to help diagnose bounce messages and NDRs?

A: Yes, there are tools and services available that can assist in diagnosing bounce messages and NDRs. These tools can analyze bounce codes, provide detailed reports, and offer recommendations for improving email deliverability.

Q: What is the impact of bounce messages and NDRs on email deliverability?

A: High bounce rates and NDRs can negatively impact email deliverability. Email service providers and internet service providers monitor these metrics to assess sender reputation. Consistently high bounce rates may result in emails being marked as spam or blocked by recipient servers.


Email bounce messages and NDRs are important indicators of email delivery failures. By understanding their causes, interpreting the information provided in bounce messages and NDRs, and taking appropriate action, you can improve your email deliverability, maintain sender reputation, and ensure effective communication with your recipients. Remember to regularly review and update your email list, follow best practices, and consider utilizing professional email delivery services to optimize your email sending and reduce bounce rates.