Control message (Usenet)
Usenet articles meant to be obeyed rather than read: they told servers to create groups, remove groups or delete posts.
An article with a Control: header was treated by the news server as an instruction. newgroup created a group, rmgroup removed one, checkgroups sent a full list of what a hierarchy ought to contain, and cancel withdrew a single article. sendsys asked every server to reply with its configuration — handy for mapping the network, and equally handy for flooding someone's mailbox.
Originally servers trusted all of it. Forged rmgroup messages for popular groups became a recurring prank, and many admins stopped acting on them automatically. From the mid-1990s control messages for the major hierarchies were signed with PGP, and servers such as INN could verify them.