Just installed a new Exchange 2007 server (x64 Server 2003 OS) into our existing Exchange 2003 environment.  Exchange has been up and running well for over a month.  I am now trying to better our spam protection.  I am using the built in Exchange 2007 Anti-Spam features as well as Trend Micro's messaging security agent.  Our company is fairly small so we only have 1 box with all needed roles installed on it (Mailbox, Client Access, Hub).
All Anti-spam features under Organization configuration -> Hub transport are Enabled.  I have added the following IP Block List Providers into Exchange 2007:
1) SpamHAUS
2) SORBS
3) SpamCOP
Although spam has gone down since the new email server, my users are still receiving annoying viagra spam messages.  The thing that I am confused about is that I go into the messaging options of the spam email and find the originating IP address and then run the test-ipblocklist provider against the IP address.  It comes back as true, meaning the IP address was indeed listed on the block list but then why did my Exchange not block the message in the first place??    
Is there something else I can check out or any other info. would be much appreciated?

Let me know if you'd like more information to help troubleshoot.
Thanks!