I send tons of email to customers who get their messages from us. because some of my systems use an email addresse for actually sending, and also use a second address as the "sender", Exchange considers this a relay. As a result I have to leave Anonymous, Basic, and WIndows Integrated authentication turned on. I have a spmmer doing relay off my server now. He is sending email with addresses such as this one:
How can I block these when I encounter one, without getting in the way of the rest of my outgoing email?