Backscatter spam is a side-effect of spam sent from a forged domain which is then bounced back to the forged domain.
A spammer sends an email with the ‘From’ address forged as domain 1 to domain 2. If the email server of domain 2 accepts the email but then later rejects it, the bounce-back is sent to domain 1. The email server of this domain had nothing to do with the initial submission of the email but has to deal with the bounce-back email.
The problem lies with the email server of domain 2 accepting the email and then later deciding that the email should be bounced-back to domain 1. If the recipient email server had rejected the email while the spammer had initially connected in, the spammer would have got the rejection and no third party would be involved. However as the email is accepted, the connection to the spammer is broken and when the email is later rejected, the innocent third-party will receive a bounce-back message.
Backscatter can be reduced by rejecting email at SMTP connect time