how to apply spam filter to forwarded emails on postfix?

[sluggish_homer]

Hi,

I have built postfix mail server on fedora. I have Spamassassin and Procmail works well to filter emails coming to my domains but the problem is all the emails which will be forwarded from my mail server to other accounts such as gmail.com or yahoo.com will not be filtered but will be delivered directly without being filtered first. I tried using Usermin's module Filter and Mail Forwarding but it won't work or maybe i just configured it wrongly.

Please help.

Thanks.