Definition - What does Backscatter mean?
Backscatter is a type of unsolicited spam/email message that is mistakenly directed to an email inbox. They are disguised as bounce messages, so that they are not filtered as spam by the email server.
Backscatter is also known as outscatter, misdirected bounces, blowback and collateral spam.
Techopedia explains Backscatter
Backscatter messages are custom created by spammers to give them a legitimate appearance. The most common form of backscatter is a bounce message that is mistaken for a legitimate email message by the mail server.
Typically, the spammer spoofs the email address of an individual and sends an email (using that individual's email) to an email server/service that categorizes it as spam and bounces the message back. The bounced email, in the form of backscatter, is rejected and bounced/returned back to the sender.