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.