Im reciving mail, but when i try to send mail i get this error message:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'ircd@10.0.0.89'. Subject 'Test', Account: '10.0.0.1', Server: '10.0.0.1', Protocol: SMTP, Server Response: '550 Relaying is prohibited', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

Any Clue as to why?
Thanks