Notes/Domino 4 and 5 Forum

I have one Domino server that serves as SMTP server.When setup the SMTP server,I didn't choose "Relay Host for messages leaving the local Internet domain".Under this condition ,I can't send a mail to some Internet mail-address,ex 1111@yahoo.com. With "Relay Host for messages leaving the local Internet domain"enabled ,I can send mail to all mail-address but any external messages can't delivered to my Domino server.The relay host is a linux server.What's happened and how can I send,recive mail with or without relay host?Please please help me,thanks a lot.