ho risolto in modo meno complesso...

<form action="http://wpop4.libero.it/email.php" method="post">
<input type=hidden value=tuo_nick name="LOGIN">
<input type=hidden value=tua_pw name="PASSWD">
<input value="libero.it" name="dominio" type="hidden">
<input type=hidden value="libero" name="choice">
<input type=submit value="vai all'email" class="submit" >
</form>