Adding Comment Meta to a new comment notification email

The solution was to add a priority number to the comment_post action that was saving my rating value, as shown below:

function save_comment_meta_data( $comment_id ) {
    if ( ( isset( $_POST['rating'] ) ) && ( $_POST['rating'] != '') ) {
        $rating = wp_filter_nohtml_kses($_POST['rating']);

        add_comment_meta( $comment_id, 'rating', $rating );
    }
}
add_action( 'comment_post', 'save_comment_meta_data', 5 ); // Adding a priority number of 5 fixed the problem!