NDR - Non-Delivery Receipt or bounce message is a way that spammers often use to implement a spam run.

Most MTA will reply a message to the original sender if it can‘t find a valid recipient. The spammer takes advantage of this reply to implement a spam attack. They send numerous spam to certain MTA, using the spam victim‘s name and email address as the sender. When the target MTA fails to deliver this mail, it will reply a mail to the victim.

 

Using this way, spammer can escape from being listed in blacklist (abuse reputation list) since the actual sending MTA is the target MTA.