PHP Curl Request To IIS Results In Request Format Is Invalid?

I am trying to use curl to access 3rd party webservice, I used the following code which works well if I try it on my own linux server, the data is being sent ok, but the IIS on the 3rd party server returns an error.

$longdata is a long string of data, maybe over 1000 characters long the 3rd party has many working clients with various implementations so the problem is on my side. what do I need to add to the request in order to get this through ?

<?php
$c = curl_init();
// curl_setopt($c, CURLOPT_HTTPHEADER, array('Expect:'));
curl_setopt($c, CURLOPT_URL, 'http://XXX.com/test/index.asmx');
curl_setopt($c, CURLOPT_POST, 1);
curl_setopt($c, CURLOPT_HEADER, 1);
curl_setopt($c, CURLOPT_RETURNTRANSFER, 1);
$post = array('param1' => 'XXXX', "param2" => "Y", "Param3" => $long_data);
curl_setopt($c, CURLOPT_POSTFIELDS, $post);
$response = curl_exec($c);
echo $response;
/*

Response:

HTTP/1.1 100 Continue
HTTP/1.1 500 Internal Server Error
Date: Tue, 05 Apr 2011 14:11:51 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Cache-Control: private
Content-Type: text/plain; charset=utf-8
Content-Length: 100
Request format is invalid: multipart/form-data; boundary=----------------------------5d738237d9e0.
*/
?>