hi....good morning

Could not send mails to external domains like yahoo,gmail from windows 2003 mail server

 I am trying to programmatically send an email to end users based on certain bahaviours (such as forgotton password, etc). But, I am getting the error: 

Mailbox unavailable. The server response was: 5.7.1 Unable to relay for (email address).

I have done some reading about this and it seems that most people are able to fix this error by doing the following:

1) Right click on Default SMTP Virtual Server and select properties 
2) Select the access tab 
3) Click the Relay button 
4) Click the Add button 
5) Type in your ip address

I have tried this but am still getting the same error. . 
does anybody have any idea what else i might be doing wrong?? 

Thanks in Advance.