Some people cant send mail to my domain

We use Exchange 2000. Occassionally I have users who tell my that someone is having a problem sending mail to them. The other person will get NDR’s with the following information: “Connected to 64.9.152.139 but sender was rejected.
Remote host said: 554 5.1.0 Sender Denied” Why? I’m not blocking those domains. How can I troublesoot this or is this problem with the other users email system. Where do you start. Thanks.