Function to have WooCommerce send an automatic heads up email to the shop owner for orders that switch to a status of on-hold.

woocommerce-heads-up-email.php

add_action('woocommerce_order_status_changed', 'send_hold_email');

function send_hold_email($order_id) {
	$order_data = new WC_Order($order_id);
	if ($order_data->status=='on-hold') {
		global $woocommerce;
		$mailer = $woocommerce->mailer();

		$message = $mailer->wrap_message(
			__( 'Order placed On-hold', 'woocommerce' ),
			sprintf( __( 'Order %s has been marked as on-hold. Reason: %s', 'woocommerce' ), $order_data->get_order_number(), $posted['reason_code'] )
		);

		$mailer->send( get_option( 'admin_email' ), sprintf( __( 'Order %s placed On-hold', 'woocommerce' ), $order_data->get_order_number() ), $message );

	}
}