Mit Hilfe von JavaScript kannst du die action in form ändern:
http://de.selfhtml.org/javascript/objekte/forms.htm#action
Dein Beispiel sähe dann so aus:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>formmail</title>
<script language="JavaScript" type="text/javascript">
<!--
function setAction () {
var formObject = document.forms['one'];
var elementObject = formObject.elements['two'];
formObject.action = elementObject.options[elementObject.selectedIndex].value;
}
-->
</script>
</head>
<body>
<form name="one" action="mail_one@domain.net">
<select name="two" onChange="setAction();">
<option value="mail_one@domain.net">mail_one@domain.net</option>
<option value="mail_two@domain.net">mail_two@domain.net</option>
<option value="mail_three@domain.net">mail_three@domain.net</option>
<select>
<input type="submit">
</form>
</body>
</html>