A spam method that relies on mail servers returning messages to the sender. The spammer forges a valid email address in the "From" field of the spam message and makes the "To" address a phony email recipient, but to a real company. Since there is no email account with the phony name, the receiving company's mail server bounces back the message before sending it to the spam filter. As a result, the spam winds up in the inbox of the intended recipient as undelivered mail. Also called "bounce spam" and "reverse non-delivery report spam" (reverse NDR spam). See
NDR spam.