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That's another attack, I've been up close and personal with. It almost mimics a DDoS attack.

That company had Symantec as well. What helped them was that I upgraded to the newest version of Mail Security for Exchange. It has a pretty good spam filter on it. My next move would have been as you suggested and use a spam-filtering service.

The only problem is that I mainly deal with small companies, so they are very concerned about costs. Especially now.