Steve,
After reading the FAQ, it seems not to support QMail. Please see:
http://www.sng.ecs.soton.ac.uk/mailscanner/faq.shtml#1615. Qmail / Postfix support
MailScanner does not directly support Qmail or Postfix. I wish it did, but there are only 24 hours in a day and I have a lot of other things to do except write MailScanner. I also have 1500 real users to support, as well as 4000 MailScanner users. So sorry, it doesn't. And it won't very soon either.
Now, I do not know what "not directly" means, so if you get it working, please report back here and let us know. As I would very much be interested in this type of product.
Thanks,
G