Not being a Postfix person, I found all of the other suggested examples lacking something, mostly they assumed you understood Postfix configuration. Since I don't want or need to know how to use Postfix above and beyond using it on a dedicated DSPAM box, I submit a simpler configuration is in order.
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Postfix as a Simple Relay - Yet another (hopefully simpler) way to use DSPAM and Postfix together by JohnPeacock.