Backscatter

What is backscatter?
Backscatter is a type of unsolicited spam / email message that is mistakenly forwarded to an email inbox. They are disguised as bounce messages so that the email server won't filter them as spam.
Backscatter is also known as outscatter, misdirected bounces, blowback, and collateral spam.

Backscatter messages are custom created by spammers to give them a legitimate look. The most common form of backscatter is a bounce message, which is mistaken for a legitimate email message from the mail server.

Typically, the spammer spoofs a person's email address and sends an email (using that person's email address) to an email server / service which categorizes them as spam and bounces the message back. The rejected email, in the form of backscatter, is rejected and returned / returned to the sender.