Bounced email on Google App Engine

Hope that helps Yes, you can change the sender. But, that email must be listed as an administrator (developer). Your app can have more than one administrator.

this will help Just direct your mail server for domain.com to forward string@domain.com to string@your-appid.appspotmail.com or your-appid@appspot.com. The inbound email processor doesn't care what the original "to" address is -- it will process it regardless. You have access to the headers programmatically, so you can decide to treat it differently depending on what email it came in through, if you want.

this one helps. I know that this doesn't answer the question directly but the normal practice here isn't to attempt to parse the bounce message at all.
Instead when you send your emails out you set a different Return-Path header for each email that you send. You can then uniquely identify which user this message is from.

it helps some times mail.send() is only passing the message to the SMTP server, the response simply states that the server successfully received/accepted the message for further processing. To check this disconnect your computer from the net or intentionally break the mail config and try again - mail.send() should fail since it can't connect to the SMTP server to hand it the message.
The actual message processing (including destination address check) happens later on the SMTP server and other servers on the mail delivery path. Processing failures typically cause error emails to be sent back (bouncing) - this is a different level in the email exchange protocol stack than the one at which mail.send() operates.