[Exim] send copy of bounce message to postmaster

Hello,
I want to send a copy of every bounced message to the postmaster.
Therefor I added the following router and transport to the
configuration. The result looks fine and everything seems to work
without a problem.
But normally a bounce message contains a "X-Failed-Recipients:"
head line. If I try to add it to the bounce message by adding
"header_add = "X-Failed-Recipients: $local_part@$domain" to the
transport it will be added to the original message and not the
newly generated bounce message. How can I solve thos problem?

### router ###

    unknown_user:
      driver = accept
      transport = unknown_user_reply

### transport ###

    unknown_user_reply:
      driver = autoreply
      to = $sender_address
      bcc = postmaster
      from = "\"Mail Delivery System\" <Mailer-Daemon>"
      subject = "Mail Delivery failure"
      text = "The following address(es) failed:\n\n$local_part@$domain"
      return_message

Any ideas?

Juergen Edner
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