rfc2822 April 2001

2.2.2. Structured Header Field Bodies

   Some field bodies in this standard have specific syntactical
   structure more restrictive than the unstructured field bodies
   described above. These are referred to as "structured" field bodies.
   Structured field bodies are sequences of specific lexical tokens as
   described in sections 3 and 4 of this standard.  Many of these tokens
   are allowed (according to their syntax) to be introduced or end with
   comments (as described in section 3.2.3) as well as the space (SP,
   ASCII value 32) and horizontal tab (HTAB, ASCII value 9) characters
   (together known as the white space characters, WSP), and those WSP
   characters are subject to header "folding" and "unfolding" as
   described in section 2.2.3.  Semantic analysis of structured field
   bodies is given along with their syntax.