What is the best way to collect SPAM from users?

[Yashodhan Barve]

Hi

I have a MailScanner+Postfix box in front of a MS Exchange server. Very 
little SPAM is getting thru that.

I want to collect that and feed it to sa-learn. Forwarding the mails 
breaks the headers so that is not an option.

I was thinking of having a common mailbox in exchange to which users 
would move SPAM & HAM and then I would POP it and feed it to sa-learn.

So what is a good approach that works? and how do I automate it?

thanks..
yashodhan