The way the Exim mail system works on Lunarpages, the email is accepted from the sender BEFORE the filters get at it, so the SMTP server is not going to return the sender with a "no such address" error no matter what you set up.

You can put a brief message after :fail: which is returned above the returned email. That's about as good as anything if you have to bounce by sending an email.

However, since most spam emails have forged headers, and most email viruses have forged sender addresses, your bounce is mostly likely to be undelivered and second most likely to be sent to the wrong person.

So given an almost zero chance of reaching the sender with your bounce, I think the better thing is to send nothing.