configure the mail

hi ,
i am using details are HP-UX B.11.11 U 9000/800 .

My scenerio as follows, i am using mailx and send mail command to send a mail . it's working fine for my client email id say abc@xyz.com. but i cannot send mail to any othere domains. i think that someone already cofigured xyz domain . My questios are,
1 . How to know the already congifured mail id's.
2. How can i configure the new domain like cde.com,abc.com etc,
3. is it possible to configure free mail domain's like yahoo.com,hotmail.com etc.,if yes how can i achieve this .

some time i getting mail to my unix user name . i eed to for ward this to my mail id . i have tried to fine the .forward . but i could't find . can anyone help me to know how to achieve this task.

Thanks in advance.