From http://wiki.apache.org/spamassassin/VBounceRuleset
The “Virus Bounce Ruleset” is a SpamAssassin ruleset to catch “backscatter”.
Backscatter is mail you didn’t ask to receive, generated by legitimate, non-spam-sending systems in response to spam.
Continue reading original article at http://technotes.trostfamily.org/?p=54