title: How to verify sender of incoming email address in Google App Engine?
body: <p>I'm trying to write a heavily email-based application in the Python SDK of Google App Engine. I've noticed that Google <a href="http://code.google.com/appengine/docs/python/mail/receivingmail.html" rel="nofollow noreferrer">allows you to receive email via its API</a>, and that it easily gives you access to the standard fields like From, To, Body, etc. However, if I'm trying to verify that an email address came from who it said it came from (kind of in the way that Posterous does it for you), how can I? I don't have access to any of the email headers, so I can't check the MX record of the sending server's IP address or anything fancy like that. </p>
  
  <p>Any ideas? </p>