Browsers traditionally, are a little particular about who you POST to.. For instance, using AJAX to post to a server from another domain can be blocked if the right headers are not included in the response. Some call this Cross Domain AJAX.
Simply put, you need to implement Cross-Origin Resource Sharing. In most browsers, that means including the following header:
You can also whitelist ...
Access-Control-Allow-Origin: http://yourdomain.com, http://yourotherdomain.com
For versions of internet explorer, you may need to use the XDomainRequest object. Some browsers simply dont support it and you need to use alternatives to support them, however that comes with its own set of security considerations.
Good article here: http://www.nczonline.net/blog/
For the propellerheads: http://www.w3.org/TR/cors/