Assuming your mail server is Exchange, because you are using Outlook...

Default policy in Exchange 2003 is to allow authenticated users to relay and Outlook authenticates, so from Outlook you can send to external mail domains. But default policy is to refuse to relay for unauthenticated connections.

If this is the case, you can:

1. Modify your script so it authenticates with the mail server.
2. Change the server to allow the system your script runs on to relay without authentication.

http://support.microsoft.com/kb/823019 tells you how to configure security and relaying restrictions on Exchange 2003, in case it is not familiar to your IT guy. If you are running some other version or server, google for allow smtp relay and the name and version of your software. It is a common issue, so it will be easy to find informaiton.