I'm looking for a way to prevent people to send emails to anyone but a whitelist of trusted contacts / trusted domains.
Currently we're using an outlook macro to achieve that, but i'm wondering if there's a way to do this within Exchange ?
WE're urrently using Exchange 2003, but are planning on upgrading to exchange 2010. Maybe this feature already exists in Exchange 2010 ?
You can certainly restrict them to sending only to particular domains. You can set up these domains on the Address Space tab of the SMTP connector properties.
You can restrict external message delivery to specific domains; this feature is available both on Exchange 2003 and Exchange 2010.
About internal messages and restricting to specific recipients, with Exchange 2003 your options are somewhat limited; instead, with Exchange 2010's transport rules, you can do pretty much anything you want to message delivery: http://technet.microsoft.com/en-us/library/dd638183.aspx.
I don't know where it would be located in exchange 2003, but I'm using exchange 2007 and you can use the "Recipient Filtering" feature which is located in the Exchange Management Console under the "Anti-Spam" tab located inside of the "Hub Transport" page which is located under the "Organization Configuration" area of the management console.
This should work for you if 2003 has a similar setup to 2007.