app.yaml

- url: /_ah/mail/.+
  script: handle_incoming_email.app
  login: admin
...
inbound_services:
- mail

handle_incoming_email.app

import logging
import webapp2
from google.appengine.ext.webapp.mail_handlers import InboundMailHandler

class ReceiveEmail(InboundMailHandler):
    def receive(self,message):
        logging.info("Received email from %s" % message.sender)
        plaintext = message.bodies(content_type='text/plain')
        for text in plaintext:
            txtmsg = ""
            txtmsg = text[1].decode()
            logging.info("Body is %s" % txtmsg)
            self.response.out.write(txtmsg)

app = webapp2.WSGIApplication([
  ReceiveEmail.mapping()
], debug=True)