message-required-news-headers

message-required-news-headers is a variable defined in `message.el'.
Its value is

(From Newsgroups Subject Date Message-ID
      (optional . Organization)
      (optional . User-Agent))

Documentation:
*Headers to be generated or prompted for when posting an article.
RFC977 and RFC1036 require From, Date, Newsgroups, Subject,
Message-ID. Organization, Lines, In-Reply-To, Expires, and
User-Agent are optional. If you don't want message to insert some
header, remove it from this list.

You can customize this variable.