Whitelist redirect allowed hosts - http://wp-doin.com

whitelist_hook.php

<?php

add_filter( 'allowed_redirect_hosts', 'wp_doin_allowed_redirect_hosts', 10 );

/**
 * Make sure that external host is allowed
 * @hook allowed_redirect_hosts
 */
function wp_doin_allowed_redirect_hosts($content) {
	$content[] = 'http://example.com';
	return $content;
}