What spam are you talking about? Aside from the regular incoming spam generated outside, I don't get anything coming from inside DreamHost.
Check the full headers to see where it's really coming from.
I don't get any spam at all on DreamHost hosted email. There's plenty at Google hosted, but their filters usually toss it in the spam folder automatically for the most part. (I look in there sometimes to catch up on the latest viagra deals).
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As I run the spam through Spamcop.net, the complaint emails are being sent to abuse.dreamhost.net. Before hosting our email here, I never saw dreamhost as one of the hosts we send abuse notices to.
Did you follow Scott's suggestion and check the headers to see where the spam is originating from?