Depending on your version of SquirrelMail and what was installed with it, this link might help: SquirrelMail Anti-Spam Filter Setup. Obviously just ignore the anti-spam portions of the guide and replace 'X-Spam-Level: XXX' with whatever string you want. If you don't have the 'filters' option in your installation, you might have to go with this plugin. I haven't used it, so I'm not positive how / if it will work.
On second thought, why are you using SquirrelMail? :xomunch: