From the Qmail-Scanner website: "Qmail-Scanner is an addon that
enables a qmail email server to scan all gateway-ed email for certain
characteristics (i.e. a content scanner). It is typically used for its
anti-virus protection functions, in which case it is used in conjunction with
commercial virus scanners, but also enables a site (at a server/site level)
to react to email that contains specific strings in particular headers, or
particular attachment filenames or types (e.g. *.VBS attachments). It also
can be used as an archiving tool for auditing or backup purposes.
Qmail-Scanner is integrated into the mail server at a lower level than some
other Unix-based virus scanners, resulting in better performance. It is
capable of scanning not only locally sent/received email, but also email that
crosses the server in a relay capacity."
