Stop incoming mail from using my own domain name

[Dan Bullock]

I will never have incoming mail coming from the outside which uses my 
domain name.  I'd like to create a rule which prevents it.

The problem is that I received one of the new Bagel virus emails with 
the password-protected ZIP payload.  It has a FROM of my domain.  It 
even faked the message-id for my domain.

How can I give a high SA score to such an email?
Thanks,
Dan