function sendAjax() {
    var xhr = new XMLHttpRequest()
    xhr.open('POST', siteUrl + 'ajax/ajax-mailchimp.html')
    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8')

    xhr.onload = function() {
      if(xhr.status === 200) {
        console.log(xhr.responseText)
      }
    }
    xhr.send(encodeURI('action=/craftMailchimped/submittal/receiveEmail&email=fakeemailaddress@yahoo.com'))
  }

<?php
namespace Craft;

class CraftMailchimped_SubmittalController extends BaseController {

    public function actionReceiveEmail() {
        echo 'Wuzup?';
    }
}