Also available are some other links that should help:http://www.akadia.com/services/postfix_spamassassin.html.
There is some broad information about SpamAssassin, dspam and ClamAV at:http://searchenterpriselinux.techtarget.com/tip/1,289483,sid39_gci1106893,00.html#.
And, of course, Google is always your friend. A quick search for Postfix, SpamAssassin and dspam will turn up a number of results.
This was first published in July 2008