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. Here are some examples, courtesy of Al Iverson: Misdirected “undeliverable email” bounce messages from spam runs, from mail servers who “accept then bounce” instead of [...]