1. Introduction

   The Message Tracking Models and Requirements document
   [RFC-MTRK-MODEL] discusses the models that message tracking solutions
   could follow, along with requirements for a message tracking solution
   that can be used with the Internet-wide message infrastructure.  This
   memo and its companions, [RFC-MTRK-ESMTP] and [RFC-MTRK-TSN],
   describe a complete message tracking solution that satisfies those
   requirements.  The memo [RFC-MTRK-ESMTP] defines an extension to the
   SMTP service that provides the information necessary to track
   messages.  This memo defines a protocol that can be used to query the
   status of messages that have been transmitted on the Internet via
   SMTP.  The memo [RFC-MTRK-TSN] describes the message/tracking-status
   [RFC-MIME] media type that is used to report tracking status
   information.  Using the model document's terminology, this solution
   uses active enabling and active requests with both request and
   chaining referrals.

1.1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC-KEYWORDS].

   All syntax descriptions use the ABNF specified by [RFC-ABNF].
   Terminal nodes not defined elsewhere in this document are defined in
   [RFC-ABNF], [RFC-URI], [RFC-MTRK-ESMTP], [RFC-SMTP], or
   [RFC-SMTPEXT].