NDR (Non-Delivery Report)

What is an NDR (Non-Delivery Report) in Email?

An NDR, or Non-Delivery Report, is an automated message sent by a mail server to notify the sender that an email could not be delivered to its intended recipient. Also known as a bounce message, an NDR explains why the delivery failed, providing diagnostic information such as error codes, timestamps, and mail server responses. Understanding NDRs helps administrators identify delivery issues and maintain healthy email systems.

NDRs are a normal part of the email lifecycle and are generated whenever a message cannot be routed, accepted, or processed by the recipient’s server. Common causes include invalid addresses, full mailboxes, spam rejections, or authentication failures. Each NDR includes technical details that indicate whether the failure is temporary or permanent.

How Non-Delivery Reports Work

When an email is sent, the sending mail server transfers the message to the recipient’s mail server using SMTP. If the recipient’s server rejects the message for any reason, it returns a reply with a specific error code. The sending system then generates a Non-Delivery Report to inform the sender of the problem.

NDRs typically contain the following information:

• The recipient address that failed
• The SMTP error code and textual reason for the failure
• The original message headers or content
• Time and date of the failed delivery attempt
• The mail server that issued the response

Example of an SMTP error in an NDR:

In this case, the 550 status code indicates a permanent failure caused by an invalid recipient address. Temporary errors (4xx codes) often mean the mail could not be delivered immediately but might succeed on retry.

Types and Causes of NDRs

NDRs are classified into two main categories based on the SMTP response code:

• Soft Bounce (4xx): Temporary delivery failure due to issues like full inboxes, greylisting, or server unavailability.
• Hard Bounce (5xx): Permanent delivery failure caused by invalid addresses, domain resolution problems, or blocked senders.

Common causes of Non-Delivery Reports include:

• Typographical errors in recipient addresses
• Deactivated or deleted mailboxes
• Mail rejected by spam or security filters
• Domain or DNS misconfigurations
• SPF, DKIM, or DMARC authentication failures

Analyzing NDRs is a critical part of email administration. Persistent hard bounces may indicate outdated mailing lists or sender reputation problems that require corrective action.

NDRs and DMARCeye

DMARCeye helps organizations reduce Non-Delivery Reports caused by authentication errors and misconfigured DNS records. By continuously monitoring SPF, DKIM, and DMARC results, DMARCeye detects when messages are rejected due to policy enforcement and identifies which senders or domains are responsible.

The platform’s visibility into delivery patterns allows administrators to distinguish between legitimate bounces and issues triggered by failed authentication. With these insights, teams can correct misaligned mail streams, maintain sender reputation, and improve overall deliverability performance.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.