We added an X-Envelope-From header composed out of "bounce-$user_id-$hashed_message_id-$package_id@$server_url".
We then added a linux user "bounce", configured the postfix seperator "-" and changed the postfix maildir format so that mails will be stored in seperate files (instead of mbox format). This puts all incoming mail beginning with "bounce-*" in a dedicated maildir, i.e. /home/bounce/Maildir. A scheduled proc will then process every newly bounced mail, determine the user_id and the package_id from the bounce adress and call the package-specific service-contract, if one exists.
The same process (scanning for incoming mails) could be used to stuff email-replies to bboard-postings in the appropriate forum...
