...
<events>
    <sales_order_shipment_save_after>
    <observers>
        <namespace_module>
        <type>singleton</type>
        <class>Namespace_Module_Model_Observer</class>
        <method>autoEmail</method>
        </namespace_module>
    </observers>
    </sales_order_shipment_save_after>
</events>
....

<?php
    class Namespace_Module_Model_Observer
    {
       public function autoEmail($observer) {
           $shipment = $observer->getEvent()->getShipment();
           $order = $shipment->getOrder();
// your email triggering code here.
       }
    }