You need to limit who can send emails to an Exchange 2007/2010 distribution group (for example, a group with an entire site's employees in it which only should receive approved announcements).
You can configure this using the Exchange Management Console.
- Open up the Exchange Management Console and expand Microsoft Exchange > Recipient Configuration > Distribution Group.
- Find the distribution group you want to protect and double click it to open it's properties.
- Switch to the "Mail Flow Settings" tab.
- Double-click the "Message Delivery Restrictions" entry.
- Select "Only sends in the following list" under the "Accept messages from" heading, the add the users you want to be able to send to the list.
- Click OK on both dialogs to get back to the Exchange Management console.
Now anyone who tries to send to that list that you haven't approved will get a bounce saying "Delivery has failed to these recipients or distribution lists" with the error message "#550 5.7.1 RESOLVER.RST.NotAuthorized; not authorized ##"