NDR
A non-delivery report: the message a mail system sends back to the sender to say a message could not be delivered, with the reason and usually an error code.
What it means
NDR is Microsoft's usual term; others say bounce message, and the standards speak of delivery status notifications. Microsoft describes the NDR as the most common type of DSN, telling the sender a message was not delivered, and its NDRs in Exchange Online include the error code (the enhanced status code), an explanation, possible fixes and technical details for administrators.
An NDR goes to the envelope sender, not to the From address, and is itself sent with a null envelope sender so that a failed NDR cannot bounce again. Because spammers forge sender addresses, NDRs for messages a person never sent, called backscatter, are a familiar nuisance.
Related terms
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Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes