PS : I do find Subject prefixes ugly too... Bu I hadn't found a more handy solution.
Don't use squirrelmail, or better yet - fix it. I suggest you replace squirrel with Zimbra, it's much better. I'm just a happy user.
Zimbra is commercial groupware suite. SquirrelMail is free webmail application. You are suggesting to replace whole user's email system with some proprietary locked product.