Alright, the only thing that i could find in the plugin source are these lines of code:
$query = $db->getQuery(true);
$query->select('o.order_id, u.user_email, o.order_status, o.order_full_price, o.order_number')
->from('#__hikashop_order o')
->leftJoin('#__hikashop_user u ON u.user_id = o.order_user_id')
->where('o.order_id = '.$order_id);
$db->setQuery($query);
but the #__hikashop_user.user_email is filled in in the database. SO that would be the problem.
Upon furter investigation i came acros that the db colum #__hikashop_user.user_partner_email isn't filled in. And that is the problem i think....