Sector 5 Policy - Googlemail

I've searched Googlemail and the web, but, can't find an answer to this.

Some spam submitted to SpamCop via my Googlemail account generates a mail 
delivery Failure Notification stating the reason as "PERM_FAILURE: Message 
rejected for Sector 5 policy reasons."  As I couldn't find any info 
anywhere what this means I mailed Google, but, it doesn't look as if I'll 
get a reply.  Anyone have any idea what is generating this bounce, what a 
Sector 5 Policy is?  It only happens once or twice a week so it's not too 
big an issue, but, if I can prevent it all the better.

-- 
Rob

http://www.flickr.com/photos/canopus_archives/