HTML Form POST on other domain

Hi,
I created a html form code to subscribe users to my mailing list system.
I’ve my wordpress on http://www.domaina.it and my mailing list system on http://www.domainb.it

Code is very simple.

<form id="formmail" name="formmail" method="post" action="http://www.domainb.it?activationpage">
  <label for="nome">Nome</label>
  <input type="text" name="nome" id="nome" />
  <label for="email"><br />
  E-Mail</label>
  <input type="text" name="email" id="email" />
  <input name="idgroup" type="hidden" id="idgroup" value="73" />
  <input name="mailing" type="hidden" id="mailing" value="10" />
  <input name="delete" type="hidden" id="delete" value="0" />
  <input name="myurlok" type="hidden" id="myurlok" value="http://www.erbamil.it/contattaci/?nl=ok" />
  <input name="myurlerror" type="hidden" id="myurlerror" value="http://www.erbamil.it/contattaci/?nl=ko" />
  <input name="myurlcheck" type="hidden" id="myurlcheck" value="http://www.erbamil.it/contattaci/?nl=gia" />
  <input name="iscriviti alla newsletter" type="submit" value="Invia" />
</form>

The code display correctly my form on http://www.domaina.it, but when I click on submit button browser redirect to http://www.domainb.it?activationpage without make subscription.

How I can use this custom html form in my wordpress?

thanks