[Exim] untrusted_set_sender and address lists in exim4

Hi exim experts,

I have a problem. When I used exim 3 the untrusted_set_sender directive
took only a boolean, but in exim 4 it needs an 'address list'. What an
address list? I need to be able to allow my www-data user to send email
with a different sender line so that errors don't get sent back to
www-data.

Does this 'address list' specify users which are allowed to send with
modified Sender lines, or does it specify the addresses users are
allowed to set the Sender line to? Everything I try fails to work, and
I'm not even sure I have the format correct!

I'm stumped, I have no clue at all - any help will be appreciated.

Kind regards,
Eli