[Exim] question on relaying / spam

would you take a look at at the below situation and let me know what you
think and how it can be dealt with?

Thanks.

mail server for domain "mydomain.com"
someone telnets to the server and sends the following info:

Return-path: <josephmobutu700@???>
From: joseph Mobutu Sese-Seko <josephmobutu700@???>
To: 3e5c23c4.8040501@???
Reply-To: jmobutu12730@???

exim determins "3e5c23c4.8040501@???: Unrouteable address" and
attempts to return the email to the "return path" thus sending a spam to
josephmobutu700@???

Thanks very much.