add-custom-field-in-order-mail.php

/**
 * Add a custom field (in an order) to the emails.
 */
function wb_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;
}
add_filter( 'woocommerce_email_order_meta_fields', 'wb_woocommerce_email_order_meta_fields', 10, 3 );