From Lunarpages Web Hosting Wiki
Helping you in the fight against spam e-mails is one of our top priorities here a Lunarpages. With Plesk in hand, you have the ability to filter out most spam before it gets to you.
How to Setup Spam Filters in Plesk
The first thing you must do is login to your Plesk control panel. Once you have made it to the front page, click on the domain name related to the e-mail address you wish to filter. Next, go to "Mail" and then click on the e-mail address you want to edit.
Now click on the icon labeled "Spam Filter" and in the first check mark box you see, check it to "Enable spam filtering". Hit the "OK" button on the bottom of the screen and spam filtering is now enabled for this e-mail account.