Hi. I use this tutorial http://www.howtoforge.com/virtual-users-domains-postfix-courier-mysql-squirrelmail-ubuntu8.04 Works great for me, but users haven't SPAM folder in their mailbox. The question is where the SPAM emails go?
Depends on what you set in your amavisd configuration. Spam can be rejected or tagged as spam in the subject line and be put into the normal mailbox.