remember, these are inbound relay controls. I wouldn't put my own domain name in the first and third field. If I try to relay a message over your server, and use a firstname.lastname@example.org as sender, the server probably will relay it for me even though I'm not really in your domain.
If you have other internal SMTP servers, you can put their IP addresses in the third field, enclosed in square brackets: [10.0.140.50]. And you definitely need a * in the fourth field!
There's another possibility: someone is sending out spam with a email@example.com as sender address. That would cause all the non deliverable messages to be routed to your server, and may cause a lot of dead mail in your mail.boxes. That may give the impression that your server is used to route spam, but in fact you're only getting NDR's.