rfc2822 April 2001

4. Obsolete Syntax

   Earlier versions of this standard allowed for different (usually more
   liberal) syntax than is allowed in this version.  Also, there have
   been syntactic elements used in messages on the Internet whose
   interpretation have never been documented.  Though some of these
   syntactic forms MUST NOT be generated according to the grammar in
   section 3, they MUST be accepted and parsed by a conformant receiver.
   This section documents many of these syntactic elements.  Taking the
   grammar in section 3 and adding the definitions presented in this
   section will result in the grammar to use for interpretation of
   messages.

   Note: This section identifies syntactic forms that any implementation
   MUST reasonably interpret.  However, there are certainly Internet
   messages which do not conform to even the additional syntax given in
   this section.  The fact that a particular form does not appear in any
   section of this document is not justification for computer programs
   to crash or for malformed data to be irretrievably lost by any
   implementation.  To repeat an example, though this document requires
   lines in messages to be no longer than 998 characters, silently
   discarding the 999th and subsequent characters in a line without
   warning would still be bad behavior for an implementation.  It is up
   to the implementation to deal with messages robustly.

   One important difference between the obsolete (interpreting) and the
   current (generating) syntax is that in structured header field bodies
   (i.e., between the colon and the CRLF of any structured header
   field), white space characters, including folding white space, and
   comments can be freely inserted between any syntactic tokens.  This
   allows many complex forms that have proven difficult for some
   implementations to parse.

   Another key difference between the obsolete and the current syntax is
   that the rule in section 3.2.3 regarding lines composed entirely of
   white space in comments and folding white space does not apply.  See
   the discussion of folding white space in section 4.2 below.

   Finally, certain characters that were formerly allowed in messages
   appear in this section.  The NUL character (ASCII value 0) was once
   allowed, but is no longer for compatibility reasons.  CR and LF were
   allowed to appear in messages other than as CRLF; this use is also
   shown here.

   Other differences in syntax and semantics are noted in the following
   sections.