[Help with IIS 6.0 SMTP server]

I have set up the server correctly to best i can tell.

it is Pop3 service with smtp on IIS 6.0 on server 2003 small business ed.

I am useing authentication useing windows built in user groups.

Here is my SMTP Virtual Server properties:

Authentication: all are check (anonymous acess,Basic,integrated Windows Authentication)

If i uncheck Anonymous I no longer can log into the server to send emails,

With it checked i still require a password and username..... dont understand this.

under connection i have checked all except the list below,

and under relay I have check only the list below, then i have checkes Allow all computers which successfully authenticate to relay, regardless of list above.

Then I have all my users added to my security list.

other then that all settings are default to the quick wizard.

The problem i am haveing is :

I can send and recieve email from inside the domain , and i can recive email from out side the domain. I cannot send email to some people out side of the domain. Example

I can send email to runbox.com and yahoo.com hot mail ect.

But not to any other email sites.

I get a error message:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'somesite@somewhere.com'. Subject 'can't tell yah', Account: 'mail.mydomain.com', Server: 'mail.mydomain.com', Protocol: SMTP, Server Response: '550 5.7.1 Unable to relay for somesite@somewhere.com', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

can any body help?