Internet Engineering Task Force (IETF) A. Gulbrandsen
Request for Comments: 6858 March 2013
Category: Standards Track
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
Status of This Memo
This is an Internet Standards Track document.
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
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Gulbrandsen Standards Track [Page 1]RFC 6858 POP and IMAP for Internationalized Email March 20131. 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
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.
example.com is a plain domain; EXAMPLE.com 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