There are many more pieces of software than Usefor gateways which get
stuck because they do not know the correct way to parse some field. The
design flaw was in RFC 2047 which allowed that situation to be gotten into
in the first place.
rfc 2047 doesn't affect how fields are parsed. it only affects how
fields are displayed. if a viewer doesn't realize that a field
is structured and/or doesn't know how it should be parsed, the worst
that should happen is that encoded-words are not decoded before display.
even the, encoded-words were specifically chosen to be unlikely to appear
in ordinary input. so simply matching =\?[^?]+\?[^?]+\?[^?]+\?=
(or something like that), while it does have a potential for error,
has a low failure rate in practice.
most of the problems result from using rfc 2047 for purposes other
than display.
Keith