Some customers want to block all mail sent from inside the company to a specific email address outside the company. To do this, create a contact in the Active Directory and give it the email address to which you want to block mail. Then, on the “Exchange General“ tab of the contact's properties*, you can configure who is allowed to send to the contact and who isn't. If your intent is to keep everyone from being able to send to this contact, set a message restriction that allows only a dummy user to send mail to the contact (this is much more efficient than creating a huge list of users and setting the contact to reject mail from users in that list).
* You may have to wait for the Recipient Update Service to process the contact before you can see it.