[Exim] headers_add in remote_smtp transport does nothing

Hello,

I'd like to add a "X-Resent-To" header when mails are forwarded.
Mail routing is done by ldap lookups. As multiple forwarding addresses
would cause headers like

X-Resent-To: address1@???, address2@???

when adding this in the director, I add a "X-Resent-By: my.server.name"
instead and would like to add the "X-Resent-To" header later in the
transport:

remote_smtp:
  driver = smtp
  headers_add = "${if match{$h_x-resent-by:}{my.server.name} \
                 {X-Resent-To: $local_part@$domain}fail}"

But this is not working. Unfortunately even debug level 9 doesn't
say anything about this condition, so I cannot find the problem :-(
I tried it in the router, too, but all the same.

Is there an error in the config?

Thanks,
Marc