[exim] Best way to save recipient in outgoing message

Because messages may be forwarded between many machines,
remembering the original recipient makes bounce processing
convenient for large lists.

I'd like to do something like:

begin routers

    # all messages go to their MX defined server
    dnslookup:
       driver    = dnslookup
       self      = send
       transport = remote_smtp

    begin transports

    remote_smtp:
       driver = smtp
       max_rcpt = 1
       add_headers {X-Bounce: ${lc:$recipients}}

This fails though because it appears $recipient is only set for
incoming messages.

Let me know the best way to save the original recipient of an
outgoing messages into a new header field.

Also if someone could point me to where this is covered in the
exim manual I'd appreciate it. I read through the transport and
expansion variables section and could find no indication of when
variables are set (incoming/outgoing SMTP sessions).

Thanks.

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