Exchange server configuration Problem…

Just now i’ve installed exchange server5.5.
Created users and tried to connect it thru Outlook express But unble to send mail to other users it says the following error

“The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was ‘kgsenthil@kgcampus.kgisl.com’. Subject ‘test’, Account: ‘saevai’, Server: ‘saevai’, Protocol: SMTP, Server Response: ‘550 Unable to relay for kgsenthil@kgcampus.kgisl.com’, Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79″ ”

I did an straight forward installtion.Is there any configuration has to be done to send mails to users.