Using Kmail?

1. Select Create from the Folder menu.
2. Enter a folder name (for example, spam and click OK.
3. Select Configure Filters... from the Settings menu.
4. Click on the New Filter button (left corner).
5. Click on the Rename button and type spam.
6. Type X-Spam-Flag in the left text box under Filter Criteria.
7. Type YES in the right text box (full line: X-Spam-Flag contains YES.
8. Under Filter Actions, select move to folder and spam from the pull-down menus.
9. Click Apply and close the window.