Help regarding the mail issue

[Sarfraz Rustam]

Hi to every body; 
I have a problem and that is belong to mail server issues.
I have configured a mail server by using sendmail on RHL 9.
The problem is that the mail send and received out side ( from yahoo
to my domain ) and inside ( form mydomain to yahoo ) is ok from mail
server , but when another system is trying to send the message from
inside ( my doamin ) to yahoo via that mail server, the error comes ,

"
The message could not be sent because one of the recipients was
rejected by the server. The rejected e-mail address was
'sarfrazr at gmail.com'. Subject 'checking', Account: 'Sarfraz's
Account', Server: 'smtp.mydomain.com', Protocol: SMTP, Server
Response: '550 5.7.1 <sarfrazr at gmail.com>... Relaying denied. IP name
possibly forged [202.147.x.y]', Port: 25, Secure(SSL): No, Server
Error: 550, Error Number: 0x800CCC79
"
I did try to send  it via live and via private ips but i failed .

If you have any idea plz send me the solutions.

Regards,

Sarfraz