The Newsgroups line specifies which newsgroup or newsgroups the article belongs in. Multiple newsgroups may be specified, separated by a comma. Newsgroups specified must all be the names of existing newsgroups, as no new newsgroups will be created by simply posting to them.
Wildcards (e.g., the word "all") are never allowed in a Newsgroups line. For example, a newsgroup "net.all" is illegal, although a newsgroup name "net.sport.football" is permitted.
If an article is received with a Newsgroups line listing some valid newsgroups and some invalid newsgroups, a site should not remove invalid newsgroups from the list. Instead, the invalid newsgroups should be ignored. For example, suppose site A subscribes to the classes "btl.all" and "net.all", and exchanges news articles with site B, which subscribes to "net.all" but not "btl.all". Suppose A receives an article with "Newsgroups: net.micro,btl.general". This article is passed on to B because B receives net.micro, but B does not receive btl.general. A must leave the Newsgroup line unchanged. If it were to remove "btl.general", the edited header could eventually reenter the "btl.all" class, resulting in an article that is not shown to users subscribing to "btl.general". Also, followups from outside "btl.all" would not be shown to such users.