You need to allow another machine to relay through your Exchange 2003 server without authenticating.
This is done via Exchange System Manager.
- Open Exchange System Manager and expand Administrative Groups > SITE > servers > Server Name > Protocols > SMTP.
- Right click on the “Default SMTP Virtual Server” and select Properties.
- Switch to the “Access ” tab.
- Click the “Relay” button.
- Click the “Add” button. Enter the IP of the machine that needs to be able to send email and click OK on each dialog.
- Close the Exchange System Manager and test sending email from the other server. It should be able to relay now.