Mac OS X.4 Server - Spam filter not working

The Apple discussion board appears to be temporarily down so I thought I might ask my question here.

I've turned on the built in spam filter
Set up 2 accounts (junkmail and notjunkmail)
And I've also done some manual training using sa-learn so the it now has analysed a total of 800 messages (mixed junk and notjunk but mostly junk)
I've turned the hit counter all the way down to 1

No emails are begin tagged as spam

 

True spam

I've temp switched back to my own private spam filter which has so far been 100% and I've got all email it tags as spam going to junkmail@

Whenever I switch this off I get hundreds of spam emails coming through.