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You said a server level spam filter. Can I assume it's something similar to Barracuda? If so, just ignore the rest of this post. Otherwise, you might want to look at that solution.
We use the Spam Firewall 400. It blocks close to 1.5 million spam attempts per day. We recently had to upgrade from the 200. Our mail server will bog down to 8+ hour queue times if the Barracuda goes offline.
Depending on you requirements, the Spam Firewall 100 starts at just under $700, and the annual subscription for updates (An essential element) runs about $400. This is obviously an enterprise product, not a consumer one.
I have no stock or financial interest in this company, but I am a very satisfied customer.