Post-Delivery Message Downgrading for Internationalized Email Messages
RFC 6857

Internet Engineering Task Force (IETF)                       K. Fujiwara
Request for Comments: 6857                                          JPRS
Category: Standards Track                                     March 2013
ISSN: 2070-1721

 Post-Delivery Message Downgrading for Internationalized Email Messages

Abstract

   The Email Address Internationalization (SMTPUTF8) extension to SMTP
   allows Unicode characters encoded in UTF-8 and outside the ASCII
   repertoire in mail header fields.  Upgraded POP and IMAP servers
   support internationalized messages.  If a POP or IMAP client does not
   support Email Address Internationalization, a POP or IMAP server
   cannot deliver internationalized messages to the client and cannot
   remove the message.  To avoid that situation, this document describes
   a mechanism for converting internationalized messages into the
   traditional message format.  As part of the conversion process,
   message elements that require internationalized treatment are recoded
   or removed, and receivers are able to recognize that they received
   messages containing such elements, even if they cannot process the
   internationalized elements.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6857.

Fujiwara                     Standards Track                    [Page 1]
RFC 6857                  POP or IMAP Downgrade               March 2013

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Fujiwara                     Standards Track                    [Page 2]
RFC 6857                  POP or IMAP Downgrade               March 2013

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Problem Statement  . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Possible Solutions . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Approach Taken in This Specification . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Email Message Header Field Downgrading . . . . . . . . . . . .  7
     3.1.  Downgrading Method for Each ABNF Element . . . . . . . . .  7
       3.1.1.  Unstructured Downgrading . . . . . . . . . . . . . . .  7
       3.1.2.  Word Downgrading . . . . . . . . . . . . . . . . . . .  7
       3.1.3.  Comment Downgrading  . . . . . . . . . . . . . . . . .  7
       3.1.4.  MIME-Value Downgrading . . . . . . . . . . . . . . . .  7
       3.1.5.  Display-Name Downgrading . . . . . . . . . . . . . . .  7
       3.1.6.  Domain Downgrading . . . . . . . . . . . . . . . . . .  8
       3.1.7.  Group Downgrading  . . . . . . . . . . . . . . . . . .  8
       3.1.8.  Mailbox Downgrading  . . . . . . . . . . . . . . . . .  8
       3.1.9.  Type-Addr Downgrading  . . . . . . . . . . . . . . . .  9
       3.1.10. Encapsulation: A Last Resort . . . . . . . . . . . . .  9
     3.2.  Downgrading Method for Each Header Field . . . . . . . . . 10
       3.2.1.  Address Header Fields That Contain <address>
               Elements . . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.2.  Non-ASCII Strings in <comment> Elements  . . . . . . . 11
       3.2.3.  Message-ID Header Fields . . . . . . . . . . . . . . . 11
       3.2.4.  Received Header Field  . . . . . . . . . . . . . . . . 11
       3.2.5.  MIME Content Header Fields . . . . . . . . . . . . . . 12
       3.2.6.  Non-ASCII Characters in <unstructured> Elements  . . . 12
       3.2.7.  Non-ASCII Characters in <phrase> Elements  . . . . . . 12
       3.2.8.  Other Header Fields  . . . . . . . . . . . . . . . . . 12
   4.  MIME Body Parts and Delivery Status Notifications  . . . . . . 12