rfc2822 April 2001

2.2. Header Fields

   Header fields are lines composed of a field name, followed by a colon
   (":"), followed by a field body, and terminated by CRLF.  A field
   name MUST be composed of printable US-ASCII characters (i.e.,
   characters that have values between 33 and 126, inclusive), except
   colon.  A field body may be composed of any US-ASCII characters,
   except for CR and LF.  However, a field body may contain CRLF when
   used in header "folding" and  "unfolding" as described in section
   2.2.3.  All field bodies MUST conform to the syntax described in
   sections 3 and 4 of this standard.