Unable to relay to the external server due to authentication issue

Hi,

I have setup an ms exchange server 2003 and I am having trouble to send email out to an external mailing account such as hotmail and gmail from a PC outside the exchange server domain. However, I am able to do so while I am logged on to the server using the admin account.

The error message I’ve been getting is “550 5.7.1 Unable to relay for xxx@hotmail.com”.

Can anyone help?
BTW, I am sending mail using a program written in vb.net.