The following are a serie on "Virtual Users And Domains With Postfix, Courier And MySQL (CentOS 5.1)" in howtoforge:

Install Spamassassin, ClamAV, and Amavisd-new

You can just run emerge on these packages. Note that as of today, the mail-filter/spamassassin in Portage is still version 3.2.1, which is a bit old. If you want the latest, you can find the latest dev version from Gentoo mail-filter category packages page. To install the masked mail-filter/spamassassin-3.2.5, put the following line in /etc/portage/package.keywords: