Step 1 - Login to Usermin

Step 2 - Select Email Filters on the left

Step 3 - Complete the form like the below. 

Be sure to correctly specify the Starts With as 

![SPAM]!

Step 4 - Click "Create"

Now when spam classified email arrives, it will automatically move to your Spam folder. You can of course put this in any folder of your choosing, but "Spam" is the default.