NDRs

Non-delivery reports (NDRs) are automatic notifications sent when an email cannot be delivered to the intended recipient. In most systems, an NDR is a bounce message generated by the recipient mail server and returned to the sender as part of a delivery status notification (DSN). The content explains that delivery failed and provides diagnostic information intended to help identify why.

NDRs are generally categorized as hard bounces or soft bounces. A hard bounce indicates a permanent failure, such as an invalid recipient address or a non-existent domain. A soft bounce signals a temporary problem, such as a full mailbox, a server being unavailable, or message size limits being exceeded. Some systems also classify transient failures with varying urgency.

Common causes include misspelled or deactivated addresses, closed mailboxes, DNS or MX record issues, blacklisting, attachment size, or restrictive anti-spam policies. NDRs typically include a status code according to RFC 3463, a diagnostic code with a human-readable explanation, and the original recipient and sender information. Some implementations redact parts of the original message to protect privacy.

Handling and mitigation: senders should verify addresses, correct typos, and avoid sending to hard-bounced addresses; administrators should monitor bounce rates, maintain clean mailing lists, and implement proper authentication (SPF, DKIM, DMARC) to improve deliverability. Privacy considerations include limiting exposure of message content in NDRs and following data-retention policies.