I have a very simple HTML form that uses POST and its action calls a PHP script on my web server.
Here is the kicker... the html that contains the form isn\'t host
If you're only experiencing the issue in IE, their XSS filter may be to blame. This article provides details for disabling it.
To avoid this problem entirely, try posting your form to a PHP script on your server, and in that script, create a cURL session that posts the form to the other script. The XSS transaction occurs independently of the client's web browser, averting these browser-based security restrictions in the process.