There is no need to make a query string. Just put your values in an object and jQuery will take care of the rest for you.
var data = {
name: $("#form_name").val(),
email: $("#form_email").val(),
message: $("#msg_text").val()
};
$.ajax({
type: "POST",
url: "email.php",
data: data,
success: function(){
$('.success').fadeIn(1000);
}
});