Followup-To
The Usenet header that decided where replies to a post would go, regardless of where the post itself appeared.
A Usenet article could be posted to several groups at once, and left alone every reply would land in all of them. Followup-To:, specified in RFC 1036 in 1987, fixed that: cross-post to five groups, point the header at one, and the discussion gathers there. Newsreaders filled in the right group automatically when you hit reply. The special value poster meant replies should go by email to the author.
Netiquette demanded the header on every cross-post, and ignoring it was one of the more reliable ways to spread a flame war across several hierarchies. The reverse dirty trick existed too: setting followups to an unrelated group, or one that didn't exist at all, so the other side's rebuttals quietly vanished.