How to properly reply on the email received?

Question

Once email message is received, I send some reply to the user with GAE usage:

class EmailReplyHandler(webapp2.RequestHandler): # parse email details manually
    def post(self):
        msg = email.message_from_string(self.request.body)
        from_realname, from_emailaddr = email.utils.parseaddr(msg['from'])
        ...
        email = mail.EmailMessage()
        email.to = from_emailaddr
        ...
        email.send()

...
app = webapp2.WSGIApplication([('/_ah/mail/report@myappid\.appspotmail\.com', EmailReplyHandler),

What should I indicate in the reply that user's mail software will recognize that as reply? Should I keep the same subject and add RE: as prefix? What else? Is there any solution to use different subject?

Answer 1

Actually, it depends on the email client.

You can try by parsing Message-ID header of the inbount mail and adding it to In-Reply-To and References headers of the outbout (your reply) mail.

There are other headers you could use, see Sending Mail with Headers section.