An updated internet-draft is available from the IETF.
Abstract: «Over its thirty-four year history, Internet mail has undergone significant changes in scale and complexity. The first standardized architecture for email specified a simple split between the user world, in the form of Mail User Agents (MUA), and the transmission world, in the form of the Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA). Core aspects of the service, such as address and message style, have remained remarkably constant. However public discussion of the architecture has not kept pace with the real-world refinements. This document offers an enhanced Internet Mail architecture to reflect the current service.»