[Exim] need help with exim error mails

Hello,

I have a problem and need your help!

I have a PHP mail script that receives mail and places them in a mysql
database.
When a e-mail doesn't match a user account it sends a error e-mail back to
the sender. The problem now is that many senders have bogus addresses and
this generates error e-mails from the exim server.

My question is, how can I tell exim to send failed messages to another
e-mail address other than the default "account_name@hostname" or to
/dev/null?

Thanks