It is impossible for us to prevent anyone from sending an e-mail from their account, by changing the 'From' and 'Reply To' fields, to pretend it was sent from your domain.  Whilst many companies are working on ways to prevent this, and it is always possible to prove whether the mail was really sent from your account, or spoofed in this manner, there is currently no procedure in existence which will prevent it in the first instance.

Since the 'sender' does not exist, it comes back to your default address as undeliverable mail.

Most likely, it has come from a cable modem or DSL user's home computer that has been compromised and is sending this out.

There is a possible solution to limit this and that is to use SPF. With SPF, only machines authorized by you will be able to send out emails that come 'from' your domain.

You can create an SPF record here: http://spf.pobox.com/

Once you have done that, email support@lunarpages.com with the created entry and ask them to add it to your zone file.