how to dispose a incoming email and then send some words back using google-app-engine

I read the doc:

from google.appengine.api import mail
mail.send_mail(sender="support@example.com",
                  to="Albert Johnson <Albert.Johnson@example.com>",
                  subject="Your account has been approved",
                  body="""
    Dear Albert:

Your example.com account has been approved.  You can now visit
http://www.example.com/ and sign in using your Google Account to
access new features.

Please let us know if you have any questions.

The example.com Team
""")

I know how to send an email using GAE, but how 开发者_如何学编程to check an incoming email and then do something?

Thanks

---

Here's the page in the docs that deals with how to receive email.