In some scenarios an organization will want to prevent email users from sending messages to certain external domain names. This can be achieved with Exchange Server 2007 using a Transport Rule.
Open the Exchange Management Console and navigate to Organization Config/Hub Transport.
Start a New Transport Rule.
Give the rule an appropriate name and description.
Choose conditions of “From users inside the organization” and “When a message header contains specific words“. Click on “message header” and change it to “To” (without the quotes), and then click on “specific words” and add the domain name, for example “@fabrikam” (again without quotes).
Click Next and choose actions of “Send bounce message to sender…” and “Silently drop the message“.
You can modify the bounce message that is sent to the sender so that they or the IT admins can easily tell why the email was rejected.
Complete the Transport Rule wizard and test the new rule by trying to send an email to that domain name.
You should now receive a bounce message from the Exchange server with the text that you configured.