NDR, or Non-Delivery Report, is a notification sent to the sender of an email or message when the intended recipient cannot be reached. This can occur for various reasons, such as an incorrect email address, a full inbox, or the recipient's server being down. NDRs help users understand why their communication failed to reach its destination. In the context of email systems, NDRs are often generated automatically by mail servers. They typically include details about the failure, such as the reason for non-delivery and any relevant error codes. Understanding NDRs can assist users in troubleshooting communication issues effectively.