Zen and the Art of the Internet - Brendan P. Kehoe

The Newsgroups: line isn't limited to just one group—-an article can be posted in a list of groups. For instance, the line

Newsgroups: sci.space,comp.simulation

posts the article to both the groups sci.space and comp.simulation. It's usually safe to crosspost to up to three or four groups. To list more than that is considered "excessive noise."

It's also suggested that if an article is crossposted a Followup-To: header be included. It should name the group to which all additional discussion should be directed to. For the above example a possible Followup-To: would be

Followup-To: sci.space

which would make all followups automatically be posted to just sci.space, rather than both sci.space and comp.simulation. If every response made with a newsreader's "followup" command should go to the person posting the article no matter what, there's also a mechanism worked in to accommodate. The Followup-To: header should contain the single word poster:

Followup-To: poster

Certain newsreaders will use this to sense that a reply should never be posted back onto The Net. This is often used with questions that will yield a summary of information later, a vote, or an advertisement.