gistfile1.txt

add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );

function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
    $fields['meta_key'] = array(
        'label' => __( 'Label' ),
        'value' => get_post_meta( $order->id, 'meta_key', true ),
    );
    return $fields;
}

This code is wc_doing_it_wrong as of version 3.0, and should be $order->get_id() instead of $order->id