Active Spam Killer and MS Exchange

I have a setup with a mail forwarded to a public mail server 
(spamassassin+qmail-scanner) that filters out about 90% of the spam before 
forwarding (split-horizon DNS)the mail to an internal MS Exchange Server. 
I've been told 90% isn't cutting it. Has anyone setup ASK in this type of 
environment where ASK pre-filters for another mail server?


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