Transparent Image not being downloaded by Hotmail (PHP)

Question

I'm trying to create a mini tracking web beacon using SLIM framework (php) but it seems only hotmail is refusing it - It's working fine on another VPS of mine with classic php.

Code :

$app->get(
"/test/:token",
 function ($token) use ($app) {
    $hostname=gethostbyaddr($_SERVER['REMOTE_ADDR']);
    $QUERY_STRING = preg_replace("%[^/a-zA-Z0-9@,_]%", '', $_SERVER['QUERY_STRING']); 
    //saves ip address and timestamp
    $str=date("D dS M,Y h:i a") . ",". $_SERVER['REMOTE_ADDR'] .",".$QUERY_STRING.",".$hostname.",".$_SERVER['HTTP_USER_AGENT'].","."\n";

    file_put_contents("tracker.txt", $str, FILE_APPEND);

    //header("content-type: image/gif");
    //$app->contentType("image/jpg");
    $app->response()->header('Content-Type', 'content-type: image/gif');

    //43byte 1x1 transparent pixel gif
    echo base64_decode("R0lGODlhAQABAIAAAAAAAAAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==");

 });

This works fine on GMAIL or any other email.

Answer 1

OK - it appears hotmail has NO problem with the image , but rather with the server due to me using it's ip address - When using a domain name it works with no problem. Security Measures I guess