This seems like a smart approach: Stop spam at the SMTP-level with Whitelists and Bayesian filtering using the Anti-Spam-SMTP-Proxy (ASSP):

“The Anti-Spam SMTP Proxy (ASSP) Server project aims to create an open source platform-independent SMTP Proxy server which implements whitelists and Bayesian filtering to rid the planet of the blight of unsolicited email (UCE). UCE must be stopped at the SMTP server. Anti-spam tools must be adaptive to new spam and customized for each site’s mail patterns. This free, easy-to-use tool works with any mail transport and achieves these goals requiring no operator intervention after the initial setup phase.”

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