Backscatter happens when a system accepts a message and later sends a non-delivery message to an address that was forged as the sender.
The innocent forged address receives a bounce for mail it never sent.
Prefer rejection during SMTP
When possible, reject clearly unacceptable recipients or messages during the SMTP transaction. The connecting sender then owns notification behavior.
After accepting responsibility for a message, delivery failures have different semantics, which is why careful acceptance policy matters.