[Exim] Copy of someone's email

Greetings,

I am trying to setup a system filter as in the
FAQ.http://www.exim.org/exim-html-4.20/doc/html/FAQ.html#TOC328

How to get a copy of someone's mail.

in my configure.main:
    system_filter = /usr/exim/mail.filter
    #system_filter_file_transport = /tmp/test

This is the content of my File: exim.filter
    if $local_part is "pboucher"
    then
    unseen deliver someone@???

I know I have to configure a transport function. But I don't understand how.
What is missing from this configuration for my filter to work?.

Thank you for your time.

Patrick