title: How to avoid the "This message may not have been sent by" warning when sending email using Google App Engine?
body: <p>I have a python GAE app that sends emails like in the <a href="http://code.google.com/appengine/docs/python/mail/sendingmail.html" rel="noreferrer">example</a> using the address of a registered administrator for the application as the "sender" address. When an email arrives from such an API call, here's a pic of <a href="https://i.imgur.com/ovAAQ.png" rel="noreferrer">the attached warning</a>.</p>
  
  <blockquote>
    <p>This message may not have been sent by: EmergencyButtonApp@gmail.com 
    <a href="http://mail.google.com/support/bin/answer.py?hl=en&amp;ctx=mail&amp;answer=185812" rel="noreferrer">Learn more</a>  Report phishing</p>
  </blockquote>
  
  <p>Am I doing something wrong? None of the 3 options mentioned in the "Learn More" link are relevant I believe: the sender is a gmail address so the "SPF and DKIM authentication" solution isn't relevant, I'm not sending to a mailing list and I'm not using SMTP (I think, though maybe GAE is...).</p>