550 relaying mail to () is not allowed

I have Outlook 2002 and am have set up my ISP (Cox.net)email. I can see email from recipients, but when I try to send I recieve the “550 relaying mail to (recipient’s name) is not allowed” I have provided login information and even tried checking the “log on to incoming server before sending mail” box. I can send mail to my own account (cox.net) and see it come through and can send and recieve test messages, but whenever I go to send to another recipient, I get this message. I have to go to Cox’ Webmail client to send toanyone else. Please help.

Thanks