The term "backscatter" is also used to describe a side-effect of email spam, viruses and worms. In this context, an alternate, more distinguishing term ("outscatter") is also used, since the traffic isn't directed to the original destination, but to a third party instead. Since a 2002 Klez variant, a large proportion of malignant email is sent with a forged sender address, but some mail servers do not take this into account. They generate bounce messages for spam or viruses - which of course go to an innocent party.
Since these messages were not solicited by the recipients, are substantially similar to each other, and are delivered in bulk quantities, they themselves can qualify as unsolicited bulk email or spam. As such, systems that generate e-mail backscatter can end up being listed on various DNSBLs and be in violation of ISPs Terms-of-Service for being abusive.
Due to controversial aspects of its design, the stock (unpatched) qmail mailserver is more likely than most to produce such bounces. For instance, qmail's "wildcard" delivery mechanism and security design prevents it from rejecting messages during SMTP transactions. When email addressed to nonexistent recipients can't be rejected at the SMTP connection, the only alternative is to auto-reply to the sender address, which causes email backscatter if the sender address is valid and forged.