cambia metodo, prova ad usare javascript...

codice:
<script>
function SubmitOut()
  {
  var obj=document.MIO_FORM;
  obj.action="tua pagina esterna senza http://";
  obj.submit();
  }
</script>


<form name="MIO_FORM" method="POST" action ="" onsubmit="SubmitOut()">
bla.. bla.. abl..
</form>