I've written a guide explaining how to setup spam and virus filtering with Postfix, Amavisd-new, Spamassassin and helper programs. Perhaps the Guide should be renamed to something like Gentoo Linux spam and virus filtering server guide. Source version also available by replacing .html with .xml Comments and proposals welcome.
Committed as http://www.gentoo.org/doc/en/mailfilter-guide.xml thankyouverymuch