Thanks for responding Joerg.
My Domino server is already set up to reject emails sent TO a person in my company that does not exist in the Domino Directory. What I'm looking for is a way to block messages that are supposedly "from" people in my company but in fact, are not. I think this is known as spoofing.
Currently, I put my own domain (i.e. acme.com) into the From field of an Allow rule in kSpam. However, that means any spam that looks like it is from my own domain gets through.
So for example, let's say "Bob" is a real id in my comany and "Sally" is not. If an email comes in to Bob@acme.com from Sally@acme.com, kSpam let's it through because of my allow rule. However, "Sally" is not in my Directory.