I don't think you'll have much SPAM-blocking success with MMDF. It was designed before SPAM became a problem, and to my knowledge, it hasn't been updated in a long time. It's hard enough just to keep it from being used as an OpenRelay.
Sendmail has much more support, but I don't know if it alone can cut SPAM.
We went to an outside service which cut our SPAM intake by about 90%. Best $4/month (per person) we spend!!