Problems with sending emails in Outlook express

I get the following error when i try and send a message

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

I am running Server 2003, I have a Mail server(pop3, SMPT) configured. I am not behind a firewall, I DO HAVE "MY SERVER REQUIRES AUTHENTICATION" CHECKED.

My Domain:  mydomain.com

I can send emails to my own account(localy, bob@mydomain.com), I cant send emails to other accounts(bob@otheraccount.com).

BUT, i sent one successful email to bob@otheraccount.com

I dont know how and i dont know why.

Please HELP!!!!