Searching DNS records

I’ve used the function below before to check the validity of an email address. However, the DNS check is now always returning false regardless – even if the email address is valid. I call the function like: if (IsEmailAddrValid($HTTP_POST_VARS["emailaddress"], true)) What I don’t understand is what DNS server is PHP using to find this information? I’m not running a local DNS server and I’m sitting behind a proxy firewall, is this what’s causing the DNS check to fail? If so is there a work around to manually provide an IP address for the DNS server to use, etc? Thanks James function IsEmailAddrValid($email, $check_dns = false) { if( (preg_match('/(@.*@)|(\.\.)|(@\.)|(\.@)|(^\.)/', $email)) || (preg_match('/^.+\@(\[?)[a-zA-Z0-9\-\.]+\.([a-zA-Z]{2,3}|[0-9]{1,3})(\]?)$/', $email)) ) { if($check_dns) { $host = explode('@', $email); // Check for MX record if( checkdnsrr($host[1], 'MX') ) return true; // Check for A record if( checkdnsrr($host[1], 'A') ) return true; // Check for CNAME record if( checkdnsrr($host[1], 'CNAME') ) return true; } else { return true; } } return false; }