To nobody in particular:
IMO, it is a Bad Idea to try to embed one address in another, and an
especially Bad Idea to extend SMTP to try to facilitate that. If you
want to know whose mail bounced, sure, add a small token to the return
address when sending the mail. Associate that small token with the
original recipient address in a database, so when you get the NDN you
can look up that token and see whose mail bounced. But don't try to
make it invertible. You're only making the service less reliable by
trying to do so, and also tempting people to try to decode information
which should be opaque to them.
Keith
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