The same origin policy is applicable only for browser side programming languages. So if you try to post to a different server than the origin server using JavaScript, then the same origin policy comes into play but if you post directly from the form i.e. the action points to a different server like:
<form action="http://someotherserver.com">
and there is no javascript involved in posting the form, then the same origin policy is not applicable.
See wikipedia for more information