Using Mozilla?

1. Select New and then Folder... from the File menu.
2. Enter a folder name (for example, spam).
3. Select Message Filters... from the Tools menu.
4. Click the New... button.
5. Type spam in the Filter name: text field.
6. Pull down the menu which reads Subject and select Customize....
7. Type X-Spam-Flag, and then click Add and then OK.
8. Pull down the menu again and select the new X-Spam-Flag option.
9. Type YES is the text field (full line: X-Spam-Flag Contains YES).
10. Select spam from the pull-down menu to the right of Move to folder.
11. Click OK and close the Message Filters window.