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Qmail-Scanner is an add-on that enables a Qmail email server to scan gatewayed email for certain characteristics (i.e. a content scanner). It is typically used for its anti-virus and anti-spam protection functions, in which case it is used in conjunction with external scanners. It also enables a site (at a server/site level) to create "Policy blocks": i.e. react to email that contains specific strings in particular headers, or particular attachment filenames or types (e.g. *.VBS attachments).