Simplified POP and IMAP Downgrading for Internationalized Email
RFC 6858

Document Type RFC - Proposed Standard (March 2013; No errata)
Updates RFC 3501
Author Arnt Gulbrandsen 
Last updated 2015-10-14
Stream IETF
Formats plain text html pdf htmlized bibtex
Stream WG state WG Document
Document shepherd John Klensin
Shepherd write-up Show (last changed 2012-08-24)
IESG IESG state RFC 6858 (Proposed Standard)
Consensus Boilerplate Unknown
Telechat date
Responsible AD Pete Resnick
Send notices to (None)
Internet Engineering Task Force (IETF)                    A. Gulbrandsen
Request for Comments: 6858                                    March 2013
Updates: 3501
Category: Standards Track
ISSN: 2070-1721

    Simplified POP and IMAP Downgrading for Internationalized Email


   This document specifies a method for IMAP and POP servers to serve
   internationalized messages to conventional clients.  The
   specification is simple, easy to implement, and provides only
   rudimentary results.

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

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
   ( 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.

Gulbrandsen                  Standards Track                    [Page 1]
RFC 6858        POP and IMAP for Internationalized Email      March 2013

1.  Overview

   A conventional IMAP or POP client may open a mailbox containing
   internationalized messages or may even attempt to read
   internationalized messages, for instance, when a user has both
   internationalized and conventional Mail User Agents (MUAs).

   Some operations cannot be performed by conventional clients.  Most
   importantly, an internationalized message usually contains at least
   one internationalized address, so address-based operations are rarely
   possible.  This includes displaying the addresses, replying to
   messages, and the processing of most types of address-based signature
   or security.

   However, the sender's name, message subject, body of text, and
   attachments can easily be displayed, so a helpful IMAP or POP server
   may prefer to display as much of the message as possible, rather than
   hide the message entirely.

   This document specifies a way to present such messages to the client.
   It values simplicity of implementation over fidelity of
   representation, since implementing a high-fidelity downgrade
   algorithm such as the one specified in a companion document [RFC6857]
   is likely more work than implementing proper UTF-8 support for POP
   [RFC6856] and/or IMAP [RFC6855].

   The server is assumed to be internationalized internally and to store
   messages that are internationalized messages natively.  When it needs
   to present an internationalized message to a conventional client, the
   server synthesizes a conventional message containing most of the
   information and presents the "surrogate message".

   This specification modifies the base IMAP specification [RFC3501] by
   relaxing a requirement that sizes be exact and adding a reporting
   requirement as discussed in Section 3 below.

2.  Information Preserved and Lost

   The surrogate message is intended to convey the most important
   information to the user.  Where information is lost, the user should
   consider the message incomplete rather than modified.

   The surrogate message is not intended to convey any information to
   the client software that would require or enable it to apply special
   handling to the message.  Client authors who wish to handle
   internationalized messages are encouraged to implement POP [RFC6856]
   and/or IMAP [RFC6855] support for UTF-8.

Gulbrandsen                  Standards Track                    [Page 2]
RFC 6858        POP and IMAP for Internationalized Email      March 2013

   Uppercase letters in examples represent non-ASCII characters. is a plain domain; represents a non-ASCII
   domain in the .com top-level domain.

2.1.  Email Addresses

   Each internationalized email address in the header fields listed
   below is replaced with an invalid email address whose display-name
   tells the user what happened.

   The format of the display-name is explicitly unspecified.  Anything
   that tells the user what happened is good.  Anything that produces an
   email address that might belong to someone else is bad.
Show full document text