Advanced Usenet Newsgroups Use

Since the ARPANET format is extensible, extensions to meet the additional needs of USENET are easily made within the ARPANET standard. Therefore, the rule is adopted that all USENET news articles must be formatted as valid ARPANET mail messages, according to the ARPANET standard RFC 822.

- Mark Horton; Standard for Interchange of USENET Messages; RFC 850; June 1983.