App Engine provides applications with various services like XMPP and Mail Service. To use these services, you would typically understand the API and use that in your application.
When you deploy your App Engine application that utilises any of these services, you must configure them in the war/WEB-INF/appengine-web.xml file. This is the same file in which you will find your application ID, version and other configuration data.
An example of configuring XMPP and Mail Service is shown below:
<inbound-services> <service>mail</service> <service>xmpp_message</service> </inbound-services>
Often, one forgets including these services or does not configure them correctly and then we end up spending precious time on why the service was not working in the actual instance. One tip is that whenever you update any services in the appengine-web.xml, please ensure that after deployment, these services were indeed made available and configured for your application.
To do that, after deployment, visit the Admin console and visit Administration -> Application Settings.
Scroll down on the right pane and you will see a list of available services for your application. Ensure that the ones that you did setup in the appengine-web.xml are present.
Check out all the App Engine Development Tips.