Windows Server 2003 SMPT Error

Ok I have setup my pop3 on win2k3 server enterprise. The system can recieve email from anybody on the internet and display no problem in outlook express. Now from Outlook Express if I send a message to another account from my domain, or to myself, email works ok. As soon as attempt to send to somebody outside, like my aol email, it fails with error

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'SDiabloXXX@XX.com'. Subject 'Re: test', Account: 'XXXXXXXXXXXXX', Server: 'XXXXXXXXXXXXXXXXXX', Protocol: SMTP, Server Response: '550 5.7.1 Unable to relay for URL', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79


my address is emazingautos.com


I have tried everything everyway and still have no luck, any help would be greatly appreciated- Thanks