NNTP host computers use control messages to communicate with each other. Control messages are commands that are used to create and remove newsgroups or cancel messages that have already been posted. For example, if a new newsgroup has been created, the host providing the newsfeed sends a control message to hosts receiving the newsfeed, indicating that a new newsgroup has been created. The Internet News Service then uses this information to determine whether a new newsgroup public folder should be added to the public folder hierarchy.
Microsoft Exchange Server uses three types of control messages.
|NEWGROUP||Broadcasts that a new group has been created.|
|RMGROUP||Broadcasts that a group has been removed.|
|Cancel||Broadcasts a request to delete a single item posted to a newsgroup.|
When Microsoft Exchange Server receives NEWGROUP and RMGROUP control messages, it queues them until you decide whether to accept the messages. If you don't trust a particular USENET host, you can delete the control messages from the host without accepting the changes. If you accept a NEWGROUP control message, the Internet News Service creates a public folder for the new newsgroups. If you accept an RMGROUP control message, the Internet News Service deletes the public folder for the deleted newsgroup.
Cancel control messages are processed transparently when an item in a newsgroup is deleted. The Internet News Service accepts Cancel control messages only after it has verified that the user deleting the item is in the organization's global address list and that the control message originated from that organization. Microsoft Exchange Server also generates its own Cancel control messages when an item is deleted from a newsgroup public folder.