mail list question

I added an entry to do mailing lists. Without specifying "user" I
suppose the delivery will setuid to the exim user?

What I'd really like is for exim to do setuid to the owner of the list
file. Would that be difficult?

lists:
driver = forwardfile;
file = /usr/local/lib/exim/lists/${local_part},
no_check_local_user,
forbid_pipe,
forbid_file,
errors_to = ${local_part}-request