[Exim] lookup sending smtp-port on transport time

Hello,
I wonder if it's possible to lookup the sending SMTP
port on transport time. I did some testing and it seems
that it doesn't work at all. Can anyone give me a hint
how to get it running?

environment: Exim v4.14

example:

  remote_smtp:
    driver = smtp
    port = ${extract{port}{${lookup{$domain}lsearch*{/etc/exim-smarthosts}}}}

Thank you in advance!

Best regards Juergen Edner
--
Mail: juergen.edner@???
PGP Key available
---