1. Introduction

   Email messages can be used to convey many different forms of
   messages, and the user will interact with different types in
   different ways.  As explained in RFC 3458prop [1], the "message context"
   of the message conveys information about the way the user expects to
   interact with the message, such as which icon to display.  RFC 3458prop
   then registers the message contexts for a "voice-message", "fax-
   message", "pager-message", "multimedia-message", "text-message", and
   "none".