rfc2822 April 2001

3.1. Introduction

   The syntax as given in this section defines the legal syntax of
   Internet messages.  Messages that are conformant to this standard
   MUST conform to the syntax in this section.  If there are options in
   this section where one option SHOULD be generated, that is indicated
   either in the prose or in a comment next to the syntax.

   For the defined expressions, a short description of the syntax and
   use is given, followed by the syntax in ABNF, followed by a semantic
   analysis.  Primitive tokens that are used but otherwise unspecified
   come from [RFC2234].

   In some of the definitions, there will be nonterminals whose names
   start with "obs-".  These "obs-" elements refer to tokens defined in
   the obsolete syntax in section 4.  In all cases, these productions
   are to be ignored for the purposes of generating legal Internet
   messages and MUST NOT be used as part of such a message.  However,
   when interpreting messages, these tokens MUST be honored as part of
   the legal syntax.  In this sense, section 3 defines a grammar for
   generation of messages, with "obs-" elements that are to be ignored,
   while section 4 adds grammar for interpretation of messages.