 <p>Google has actually since added <a href="https://developers.google.com/appengine/docs/python/mail/bounce" rel="nofollow">a method for receiving bounced messages</a> via an HTTP Request. It requires adding to your app.yaml:</p> <pre><code>inbound_services: - mail_bounce </code></pre> <p>Which will cause a request to hit <code>/_ah/bounce</code> each time a bounce is received. You can then handle the bounce by adding a handler for it. See the section <a href="https://developers.google.com/appengine/docs/python/mail/bounce" rel="nofollow">there</a> on Handling Bounce Notifications for more details on how to glean the additional information from those requests.</p>