Deploying Exchange 2000 server

Hello! I am deploying exchange server 2000, and i have a big problem. With my configuration, i am sending mails but i can’t received ansewrs, even new mails. I have an internet domain name and a permanent connexion to internet. In fact my exchangeis include in the small business server 2000 and i have all ready deploy isa server. Can any body bring me support? Thanks