[Exim] forcing bounced mail + directing unknown mail to post…

I want a copy of any incoming emails addressed to unknown users to go
to postmaster. I added this to the DIRECTORS section of my exim.conf:

defaultuser:
driver = smartuser
new_address = postmaster@???
no_verify_recipient
no_verify_sender

This works fine, but with one side-effect. I STILL want the sender to
receive the bounced email, so that they know about the error. With this
setup, they don't receive any bounced errors.

So, as a silly hack, I just set it to this:
new_address = postmaster@???,usernotfound@???

This does generate a bounce since usernotfound does not exist, but the
bounce states that "usernotfound" is not found. I'd rather have it
state the original email address in the error message.

What's the easiest way to do this? Thanks.

-Mike