Issues with Open Relay in Exchange 2010

We have successfully migrated to Exchange 2010. We have a new web server that stays in the DMZ. We developed the website in which if user fill an online form with their email address they will receive email confirmation. So the process behind that is from the web server it then sends an email to our CAS Array server then it expects our CAS Array servers will relay this email to outside. In our CAS server we specify two receive connector. One for internal and one for external. The internal one have Ip address of the web server specified to allow to send to. The second connector allow Messagelabs Ip addresses as we are using them to filter email and smtp routing outside. In the Send Connector we just specify all Smtp traffic is going to messagelabs.

Now user from outside once submit the form complain that they have not received email confirmation. From the webserver if Iam using SMTP command line to try to send email to external email such as gmail then I got issue: 550 5.7.1 Unable to relay. Sending between internal email address is ok.

Searching through google and they say it is a common behavior of Exchange 2010 which has Open Relay disabled by default. I believe that is a good practice and do not want to turn that on as spam email issue.

Just wonder is there any method to overcome that. It is working fine under exchange 2003 but i do not know why it does not work under exchange 2010.