mail server problem!

pls help:
"The message could not be sent because one of the recipients was
rejected by the server. The rejected e-mail address was
'xxxxxx@yahoo.com'. Subject 'SCRISOARE', Account: 'xxx.xx.xxx.xx',
Server: 'xxx.xx.xx.xx', Protocol: SMTP, Server Response: '550 Unable
to relay ', Port: 25, Secure(SSL): No, Server Error: 550, Error Number:
0x800CCC79"

when i change the ip class on the computer, everything works fine.
but i need it to keep that ip.
how can i allow that ip class on mail server - rh9.
many tkx.

Edit /etc/access file
and verify:
localhost.localdomain RELAY
localhost RELAY
127.0.0.1 RELAY
xxx.xxx.xxx.xxx RELAY

and check /etc/hosts too.

tkx myx9!