[Exim] how to disable or change Return-path?

How can I stop exim adding Return-path: <www-data@???> ?

www-data is already a trusted user, so the "sender" isn't set anymore. But
if a delivery fails (i.e. user unknown) exim returns the mail to www-data.
How do I prevent this? I'd like the mail to return to the "From:" address.

tia
mk2