$(document).ready(function(){
    //validate this form upon submission and keyup
    $("#contact_form").validate(
        {
            rules: {
                name: "required",
                sender: {
                    required: true,
                    email: true
                },
                message: "required"
            },
            messages: {
                name: "Please enter your name.",
                sender: "Please enter a valid email address.",
                message: "Please enter a message."
            }
        });
});