rfc2822 April 2001

3.2.2. Quoted characters

   Some characters are reserved for special interpretation, such as
   delimiting lexical tokens.  To permit use of these characters as
   uninterpreted data, a quoting mechanism is provided.

quoted-pair     =       ("\" text) / obs-qp

   Where any quoted-pair appears, it is to be interpreted as the text
   character alone.  That is to say, the "\" character that appears as
   part of a quoted-pair is semantically "invisible".

   Note: The "\" character may appear in a message where it is not part
   of a quoted-pair.  A "\" character that does not appear in a
   quoted-pair is not semantically invisible.  The only places in this
   standard where quoted-pair currently appears are ccontent, qcontent,
   dcontent, no-fold-quote, and no-fold-literal.