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Non-delivery reports (NDRs) are system messages that report the delivery status of a message to the sender. The messages are a subclass of a general message information structure that is known as delivery status notifications. Delivery status notifications describe three kinds of situations:
NDRs are generated when a message cannot be delivered. If the computer can detect the reason for the failed delivery, it maps the reason onto a status code, and a corresponding error message is printed. For NDRs, most numeric error codes are reported in the form of "5.X.X" and are described as permanent failures. However, certain transient conditions cause "4.X.X" codes.