Requirements:

  qmail has had the QMAILQUEUE patch applied

  qmail-qfilter
    http://www.untroubled.org/qmail-qfilter/

  Perl
    http://www.perl.org

  PersistentPerl recommended
    http://daemoninc.com/PersistentPerl/

Installation:

  Create qmail-qfilter working directory hierarchy as follows

  su -
  mkdir -p /var/qmail/qfilter/{temp,scan,hold,log}
  chown qmaild:clamav /var/qmail/qfilter
  (assuming clamd runs as user clamav)
  chown qmaild:nofiles /var/qmail/qfilter/{temp,hold,log}
  chown qmaild:clamav /var/qmail/qfilter/scan
  chmod -R 750 /var/qmail/qfilter
  chmod +t /var/qmail/qfilter/scan

Before compiling qmail-qfilter

  Change TMPDIR in qmail-qfilter.c to:
  #define TMPDIR "/var/qmail/qfilter/temp"

Create qmail-qfilter.sh wrapper

  #!/bin/sh
  exec /path/to/qmail-qfilter /path/to/qmail-qfilter-clamdscan

Example of modified /etc/tcprules/smtp

  127.:allow,RELAYCLIENT=""
  :allow,QMAILQUEUE="/path/to/qfilter-wrapper.sh",DEBUG=""

Don't forget to edit /etc/ld.so.conf and run ldconfig

  /usr/local/bglibs/lib