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Short for Non-Delivery Receipt, NDR is an e-mail related acronym. When an e-mail message is composed and sent to an invalid e-mail address the receiving, or in some cases the sending, e-mail server attempts to send an NDR to the sender of the e-mail message, to inform them that the e-mail address used is invalid.

See also hard bounce.
Also see Why E-Mails Bounce in the Did You Know section of Webopedia.

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