In Exchange Server 2010 (download), moderation enables you to control messages sent to groups and individuals based on the human element: a moderator-not who sent it. Moderation isn't limited to groups. Like delivery restrictions, you can also moderate mail sent to individual mailboxes or mail contacts, in the same way as mail sent to distribution groups. If you wanted to moderate mail from a mailbox/contact, you would have to set up a transport rule, where moderation is available as an action. For the rest of this post, I'll be talking about moderated groups/lists.
Full Article: Exchange Team