How to configure win 2003 mail server?

Hello,
Myself Imran Shaikh working in software development company. I have windows server 2003 with service pack 2 this server is my primary domain, i have configured email services from add/remove components. I created one test mail id & configured a email client for it (outlook express). Now the problem is i can send & recive mail only from these mail id but m not able to recive mails from other domain like xyz@yahoo.com etc etc. can any help me out? its urgent…………….imransh21@gmail.com

Thanks in advance