Backscatter spam, also called misdirected bounce spam or NDR spam, is a strategy for sending unsolicited email messages that takes advantage of the fact that certain types of mail transfer agent (MTA) programs return the entire message to the sender if the recipient's email address is invalid.
For the strategy to be effective, the spammer must first collect or purchase valid e-mail addresses for real people. The spammer uses these addresses to forge the "from" parts of their spam emails.

Once the spam emails have been assembled, they are sent to imaginary people at real domains. When the domain's web server bounces the emails back to the sender because the recipients don't exist, the spammer succeeds in getting their email delivered to the real person whose address was forged in the "from" part of the email's address.

The goal of a backscatter spam strategy is to circumvent anti-spam filters. Backscatter spam can also be used in a denial of service (DoS) attack to tie up a server's resources by flooding it with non-delivery report (NDR) messages.