outgoing and incoming mail problem

Our organization wide mail servers (front and back end) works fine. We have an isa firewall and our mail server can be accessed from outside the firewall. When we tried to e-mail outside the organization, the mail client shows that it has been sent. But no where. Is there any particular setup that we have to perform on the isa inorder to make it work for outbound and inbound mails? Can anyone help?