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A non-delivery report (NDR) is a report that is automatically generated by a mail server to inform the sender that their email message was not successfully delivered. The NDR comes in the form of an email from the sender's mail server and contains information on why the delivery was unsuccessful. Most of the time this is due to a wrong destination email address placed on the "To" field or the receiver's inbox being full and no longer able to receive new messages.
Non-delivery reports are also known as non-delivery receipts, bounced messages, non-delivery notifications (NDN) or delivery status notifications (DSN).
Non-delivery reports are generated by mail servers based on a standard SMTP 500 error coding system. Specifically, it is generated by the sender's mail server based on a SMTP 500 error code that is given by the receiver's mail server. But errors that bar the message from being delivered can occur at many stages during mail delivery, such as the sender's mail server being unable to find the receiver mail server for any reason, the mail having been received but the server storage suddenly becoming full so the mailer daemon cannot replicate the mail, or the receiver has configured the email server to forward emails to another mail server which has failed for some reason. All of these scenarios are valid delivery failure scenarios and have their own SMTP 500 code that informs the sender why the email has bounced.
Sample error codes for NDRs include: