Adding Custom Headers

How do I most effortlessly add a custom header "X-List: ${local_part}"
in /usr/local/etc/exim/configure given the following director:

lists:
driver = forwardfile;
file = /usr/local/etc/exim/lists/${local_part},
no_check_local_user,
forbid_pipe,
forbid_file,
errors_to = postmaster@???

Regards,
Sheldon.

-- 
Sheldon Hearn                                   http://axl.iafrica.com/
Training Officer                                        +27-21-658-8515
UUNET Internet Africa                http://iafrica.com/iafrica/
National Dial Support Centre                +27-21-658-8500