EAI: Simplified POP/IMAP downgrading

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Document Type Active Internet-Draft (eai WG)
Author Arnt Gulbrandsen 
Last updated 2012-03-02
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Network Working Group                                   Arnt Gulbrandsen
Internet-Draft                                                March 2012
Intended Status: Proposed Standard

                  EAI: Simplified POP/IMAP downgrading

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Internet-draft                                                 June 2011


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

1. Overview

   It may happen that an EAI-ignorant IMAP or POP client opens a mailbox
   containing EAI messages, or even read EAI messages, for instance when
   a user has both EAI-capable and EAI-ignorant MUAs.

   While the server can hide the existence of such messages entirely,
   doing that can be both tricky to implement and not very friendly to
   the user.

   This document specifies a way to present such messages to the client.
   It values simplicity of implementation over fidelity of
   representation, on the theory that anyone who wants accuracy should
   use EAI, and implementers' time should be used for implementing EAI

   The server is assumed to be EAI-capable internally. When it needs to
   present an EAI message (the "real message") to a non-EAI client, it
   synthesizes a non-EAI message containing most of the information and
   presents that (the "synthetic message").

2. Information preserved and lost

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

   The synthetic message is not intended to convey any EAI information
   to the MUA. Nothing parsable is added.

2.1 Email addresses

   Each regular EAI-specific email address in the 14 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.

   Given an EAI address "Fred <fred@EXAMPLE.com>", the rendering might

Gulbrandsen                Expires August 2012                  [Page 2]
Internet-draft                                                 June 2011

   be "fred@EXAMPLE.com <invalid@eai.invalid>" or "Fred

   Irregular email addresses (anything to which one cannot send mail,
   such as "unknown:;") are silently excised.

   The affected header fields are Bcc, Cc, From, ReplyTo, ResentBcc,
   ResentCc, ResentFrom, ResentSender, ResentTo, ReturnPath, Sender and
   To.  Any addresses present in other header fields are not regarded as
   addresses by this specification.

2.2 Mime values and comments

   Any mime field (whether in the message header or a bodypart header)
   which cannot be presented as-is to the client is silently excised
   along with its name.

   Given a field such as "Content-Disposition: attachment; filename=foo;
   signed-off-by=fred@EXAMPLE.com", the field is presented as "Content-
   Disposition: attachment; filename=foo".

2.3 Remaining header fields

   Any header field which cannot be presented to the client even after
   the modifications in sections 3.1 and 3.2 is silently excised.

3. IMAP-specific details

   IMAP offers a way to retrieve the message size without downloading
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