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 to check an incoming email and then do something?

Thanks

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