[exim] detainting a sender address in a router/transport

Hello all,
I've now spent several hours to read through all kind of Exim
documentation and mailing list postings, to find-out how to fix my
specific router/transport configuration to get rid of tainted data,
without success.
I understood that a tainted value needs to be converted to an untained
one by running a lookup function. Unfortunately, I haven't found a way
to untained the $sender_address yet so that it is available in
$sender_address_data. I hope you will be able to point me to a working
solution.

I'm using a router/transport to send email to a print by parsing it to a
script. Email should be accepted from local domains and in addition from
a list of external addresses, which are stored in a separate file.

Here is my configuration which worked in the past without any issues:

print_route:
   driver     = manualroute
   local_part_prefix = print/
   route_list = domain.de
   senders    = *@+local_domains : lsearch*@;/var/mail2print/senderaddresses
   transport  = print_transport

print_transport:
   driver  = pipe
   command = /usr/bin/mail2print -sender "$sender_address" -queue 
"$local_part"
   home_directory = /tmp
   user    = exim

I have expected that the following router should work, because the
senders verification in the router succeeds and that
$sender_address_data then contains the untainted address, but the
variable always stays empty.

print_transport:
   driver  = pipe
   command = /usr/bin/mail2print -sender "$sender_address_data" -queue 
"$local_part_data"
   home_directory = /tmp
   user    = exim

Thank you in advance for your help!

Regards
Juergen

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