Followup: N. On USENET, A {posting} Generated In Response To
Another Posting (as Opposed To A {reply}, Which Goes By Email
Rather Than Being Broadcast).
:followup: n. On USENET, a {posting} generated in response to
another posting (as opposed to a {reply}, which goes by email
rather than being broadcast). Followups include the ID of the
{parent message} in their headers; smart news-readers can use
this information to present USENET news in `conversation' sequence
rather than order-of-arrival. See {thread}.
-- The AI Hackers Dictionary