How to find out why a NDR from my mail server

I have a Exchange 2003, one user tried to send anyone an email within our domain and they get a NDR saying my server “refused” their email.

How do I find out excatly why my server refused their email?

Any advice would be great.

Again this is only one user sending from a yahoo account..they ended up sending from their gmail account with no problem. I test the server with my yahoo account and no problem getting through.