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Versions: 00 01 02 03 04 05 06 07 rfc5983                               
Intended Status: Experimental.
Internet Draft: Mailing Lists and Internationalized          R. Gellens
                Email Addresses                                Qualcomm
Document: draft-ietf-eai-mailinglist-04.txt
Expires: May 20, 2009                                 November 20, 2008

          Mailing Lists and Internationalized Email Addresses

Status of this Memo

    By submitting this Internet-Draft, each author represents that any
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Copyright Notice

    Copyright (C) The IETF Trust (2008).  All Rights Reserved.


    This document describes considerations for mailing lists with the
    introduction of internationalized email addresses.

    This document makes some specific recommendations on how mailing
    lists should act in various situations.

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Table of Contents

     1.  Conventions Used in this Document  . . . . . . . . . . . . .  2
     2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . .   2
     3.  Scenarios Involving Mailing Lists  . . . . . . . . . . . . .  4
     4.  Capabilities and Requirements . . . . . . . . . . . . . . .   5
     5.  List Header Fields   . . . . . . . . . . . . . . . . . . . .  6
     6.  Further Discussion  . . . . . . . . . . . . . . . . . . . .   7
     7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  8
     8.  Security Considerations   . . . . . . . . . . . . . . . . .   8
     9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  8
    10.  Normative References  . . . . . . . . . . . . . . . . . . .   8
    11.  Informative References . . . . . . . . . . . . . . . . . . .  9
    12.  Authors' Addresses  . . . . . . . . . . . . . . . . . . . .   9
       Appendix A: Changes from Previous Version  . . . . . . . . . .  9
       Intellectual Property Statement . . . . . . . . . . . . . . .  10
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11

1.  Conventions Used in this Document

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    document are to be interpreted as described in [KEYWORDS].

2.  Introduction

    Mailing lists are an important part of email usage and collaborative
    communications.  The introduction of internationalized email
    addresses affects mailing lists in three main areas: (1) transport
    (receiving and sending messages); (2) message headers of received
    and retransmitted messages; and (3) mailing list operational

    A mailing list is a mechanism whereby a message may be distributed
    to multiple recipients by sending to one address.  An agent
    (typically not a human being) at that single address receives the
    message and then causes the message to be redistributed to a list of
    recipients.  This agent sets the envelope return address of the
    redistributed message to a different address from that of the
    original message.  Using a different envelope return address
    (reverse-path) directs error (and other automatically generated)
    messages to an error handling address associated with the mailing
    list. (This avoids having error and other automatic messages go to
    the original sender, who typically doesn't control the list and
    hence can't do anything about them.)

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    In most cases, the mailing list agent redistributes a received
    message to its subscribers as a new message, that is, conceptually
    it uses message submission [submit] (as did the sender of the
    original message).  The exception, where the mailing list is not a
    separate agent that receives and redistributes messages in separate
    transactions, but is instead an expansion step within an SMTP
    transaction where one local address expands to multiple local or
    non-local addresses, is out of scope for this document.

    Some mailing lists alter the message header, while others do not.  A
    number of standardized list-related header fields have been defined,
    and many lists add one or more of these headers.  Separate from
    these standardized list-specific header fields, and despite a
    history of interoperability problems from doing so, some lists alter
    or add header fields in an attempt to control where replies are
    sent.  Such lists typically add or replace the "Reply-To" field and
    some add or replace the "Sender" field.  Poorly-behaved lists may
    alter or replace other fields, including "From".

    Among these list-specific header fields are those specified in
    RFC2369 -- The Use of URLs as Meta-Syntax for Core Mail List
    Commands and their Transport through Message Header Fields [List-*]
    and RFC2919 -- List-Id:  A Structured Field and Namespace for the
    Identification of Mailing Lists [List-ID].  For more information,
    see Section 5.

    While the mail transport protocol does not differ between regular
    email recipients and mailing list recipients, lists have special
    considerations with internationalized email addresses because they
    retransmit messages composed by other agents to potentially many

    There are considerations for internationalized email addresses in
    the envelope as well as header fields of redistributed messages.  In
    particular, an internationalized message cannot be downgraded unless
    envelope addresses are in ASCII (which includes use of ALT-ADDRESS).

    With mailing lists, there are two different types of considerations:
    first, the purely technical ones involving message handling, error
    cases, downgrades, and the like, and second, those that arise from
    the fact that humans use mailing lists to communicate.  As an
    example of the first, mailing lists might choose to reject all
    messages from internationalized addresses that lack an alt-address,
    or even all internationalized messages that can not be downgraded.
    As an example of the second, a user who sends a message to a list
    often is unaware of the list membership.  In particular, the user
    often doesn't know if the members are i18mail users or not, and
    often neither the original sender nor the recipients personally know
    each other.  As a consequence of this, remedies that may be readily

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    available for one-to-one communication might not be appropriate when
    dealing with mailing lists.  For example, if a user sends a message
    which is undeliverable, normally the telephone, instant messaging,
    or other forms of communication are available to obtain a working
    address.  With mailing lists, the users may not have any recourse.
    Of course, with mailing lists, the original sender usually does not
    know if the message was successfully received by any list members,
    or if it was undeliverable to some.

    Conceptually, a mailing list's internationalization can be divided
    into three capabilities:  First, does it have a UTF8 submission or
    return-path address?  Second, does it accept subscriptions to UTF8
    addresses?  And third, does it accept UTF8SMTP messages?  This is
    explored in Section 4.

    A brief discussion on a few additional considerations for mailing
    list operation is in Section 6.

3.  Scenarios Involving Mailing Lists

    Generally (and exclusively within the scope of this document), an
    original message is sent to a mailing list as a completely separate
    and independent transaction from the mailing list agent sending the
    retransmitted message to one or more list recipients.  In both
    cases, the message might have only one recipient, or might have
    multiple recipients.  That is, the original message might be sent to
    additional recipients as well as the mailing list agent, and the
    mailing list might choose to send the retransmitted message to each
    list recipient in a separate message submission transaction, or
    might choose to include multiple recipients per transaction.
    (Often, mailing lists are constructed to work in cooperation with,
    rather than include the functionality of, a message submission
    server, and hence the list transmits to a single submission server
    one copy of the retransmitted message, with all list recipients
    specified in the SMTP envelope.  The submission server then decides
    which recipients to include in whi ch transaction.)

    The retransmitted message sent by the mailing list to its
    subscribers might need to be downgraded [EAI-Downgrade].  In order
    for a downgrade to be possible, the return path set by the mailing
    list agent must be an ASCII address or have ALT-ADDRESS specified.
    In addition, the recipient addresses need to have ASCII addresses
    available.  It may be advisable for mailing list operators to
    pre-obtain an alt-address for all its internationalized member

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    In the case where a member or non-member with an internationalized
    email address is sending to a mailing list, no alt-address is
    specified, and a downgrade is required, the message cannot be
    delivered.  To protect against this, a UTF8SMTP-aware mailing list
    might prefer to reject submissions from internationalized email
    addresses that lack an alt-address.

    (Note that the situation is not unique to mailing lists.  Mail
    relays that are UTF8SMTP- aware will potentially encounter the same
    situation.) Further discussions are included in section 6 of this

4.  Capabilities and Requirements

    There are three primary internationalization capabilities of mailing
    lists:  First, does it have a UTF8 submission or return-path
    address?  Second, does it allow subscriptions from UTF8 addresses?
    And third, does it accept UTF8SMTP messages?

    In theory, any list can support any combination of these.  In
    practice, only some offer any benefit.  For example, neither
    allowing UTF8 addresses to subscribe, nor accepting UTF8SMTP
    messages, makes much sense without the other (an all-ASCII address
    might or might not be capable of receiving UTF8SMTP messages, but a
    UTF8 address of necessity needs to accept UTF8SMTP messages).
    Likewise, there is no real benefit to a list in using a UTF8
    submission address unless it also accepts UTF8SMTP messages and
    permits UTF8 addresses to subscribe.

    However, requirements for lists can be discussed separately for each
    of the three capabilities.

    1.  If the list uses a UTF8 submission or return-path address, it
    SHOULD specify an alt-address for it.  Clearly, it needs to sit
    behind a UTF8SMTP-enabled final-delivery SMTP server and delivery
    agent.  Likewise, if a list uses a UTF8 return-path address, then
    its MSA needs to support UTF8SMTP.

    The list's return-path address is usually separate from its
    submission address (so that delivery reports and other
    automatically-generated messages are not sent to the submission
    address).  For reliability in receiving delivery status
    notifications, a list MAY choose to use an all-ASCII return-path
    even if it uses a UTF8 submission address.  If the list does use a
    UTF8 return path, it MUST specify an alt-address (or else there is a
    high risk of being unable to receive non-delivery reports).

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    There are also implications for the List-* headers (see below).

    2.  If it allows UTF8 addresses to subscribe, it MAY require an
    alt-address to be specified for each UTF8 subscriber.

    Naturally, if it permits UTF8 addresses to subscribe, it needs a
    mechanism to accept subscription requests from such addresses
    (preferably specified in the form <utf8@utf8 <ascii@ascii>>).
    Likewise, its MSA needs to support UTF8SMTP.

    3.  If it accepts UTF8SMTP messages, its MSA needs to support

5.  List Header Fields

    A number of header fields specifically for mailing lists have been
    introduced in RFC2369 and RFC2919.  These include, for example:

    List-Id: List Header Mailing List <list-header.nisto.com>
    List-Help: <mailto:list@host.com?subject=help> (List Instructions)
    List-Unsubscribe: <mailto:list@host.com?subject=unsubscribe>
    List-Subscribe: <mailto:list@host.com?subject=subscribe>
    List-Post: <mailto:list@host.com>
    List-Owner: <mailto:listmom@host.com> (Contact Person for Help)
    List-Archive: <mailto:archive@host.com?subject=index%20list>

    As described in RFC2369, "The contents of the list header fields
    mostly consist of angle-bracket ('<', '>') enclosed URLs, with
    internal whitespace being ignored." [List-*] Whereas RFC2919
    specifies that, "The list identifier will, in most cases, appear
    like a host name in a domain of the list owner." [List-ID]

    These mailing list header fields contain URLs.  The most common
    schemes are generally HTTP, HTTPS, mailto, and FTP.  The URLs in
    these fields can use RFC3987 "Internationalized Resource Identifier
    (IRI)" [IRI] encoded as URLs.  Future work may extend these header
    fields or define replacements to directly support non-encoded UTF8
    in IRIs (for example, [mailto-bis]), but in the absence of such
    extension or replacement, non-ASCII characters can only appear
    within IRIs when properly encoded.  Note that internationalized
    domain names could potentially be either percent-encoded or
    puny-coded, but punycode is likely to have better results.

    Even without these header fields being extended to support UTF8,
    some special provisions may be helpful when downgrading.  In
    particular, when a List-* header contains a UTF8 mailto (even
    encoded in ASCII) followed by an ASCII mailto, it may be advisable
    to not only copy and preserve the original header as usual, but also

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    to edit the header to remove the UTF8 address.  Otherwise, a
    non-UTF8-aware client might run into trouble if the decoded mailto
    results in a non-ASCII address. [[[EDITOR'S NOTE:  This needs to be
    vetted by the eai list.]]]

    When mailing lists use a UTF8 form of a List-* header, an ASCII form
    SHOULD also be used.  These headers are vital to good operations and
    use of mailing lists; caution is called for when considering how to
    form and use these headers in a non-ASCII environment.

    The most commonly-used URI schemes in List-* headers tend to be HTTP
    and mailto.  The current specification for mailto does not permit
    unencoded UTF8 characters, although work has been proposed to extend
    or more likely replace mailto in order to permit this.  For mailto
    URIs, a separate consideration is how to include an alternate ASCII
    address (alt-address) for a UTF8 address.  Note that the existing
    ability to specify multiple URLs within each List-* header field
    provides one solution.

    [List-*] says:
       A list of multiple, alternate, URLs MAY be specified by a comma-
       separated list of angle-bracket enclosed URLs.  The URLs have
       order of preference from left to right.  The client application
       should use the left most protocol that it supports, or knows how
       to access by a separate application.

    When a UTF8 mailto is used in a List-* header field, an alt-address,
    if available, SHOULD immediately follow it.

    The List-ID header filed uniquely identifies a list.  The intent is
    that the value of this header remain constant, even if the machine
    or system used to operate and host the list changes.  This header
    field is often used in various filters and tests, such as
    client-side filters, Sieve filters, and so forth.  Because of this,
    great care should be taken, as a non-ASCII value might not match
    when encoded into ASCII.  It is generally desirable that this header
    field contain something meaningful that users can type in.  However,
    non-ASCII characters encoded into ASCII are unlikely to be
    meaningful to users or easy for them to accurately type.

6.  Further Discussion

    While mailing lists do not create a significant additional burden to
    the deployment of internationalized email address functionalities,
    there are some specific areas that need to be considered when the
    operator of a mailing list or of a final delivery MTA that serves a
    mailing list upgrades to internationalized mail.

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    Mailing lists face additional complexity since they redistribute
    messages composed by other agents.  Hence, they may be asked to
    accept a message with non-ASCII headers composed by a UTF8SMTP-aware
    user agent, and redistribute it to i18mail and non-i18mail users via
    systems that are not UTF8SMTP-aware.

    1.  Obtaining Downgrade Information -- for a mailing list, or mail
    relay server for that matter, that is UTF8SMTP-aware, receiving mail
    from an internationalized email address, the alt-address is not
    required from the sending MTA for the transport to be complete.
    Thereupon when the mailing list retransmits the message to its
    subscribers, it may encounter paths where a downgrade is called for.
    In order to mitigate this situation, the mailing list might perhaps
    decide to reject all incoming mail from an internationalized email
    address that lacks an alt-address.  However, note that in general,
    downgrades are not expected to be the normal case.

    2.  Downgrading Considerations for mailto URLs -- UTF8 addresses in
    mailto links in List-* headers will be easier to downgrade if they
    contain an alt-address.

7.  IANA Considerations


8.  Security Considerations

    Security considerations are discussed in the Framework document
    [EAI-Framework].  No further security considerations are raised by
    this document.

9.  Acknowledgments

    Edmon Chung of Afilias wrote the original version of this document.
    Thanks to Harald Alvestrand for his comments.

10.  Normative References

    [EAI-Framework] J. Klensin and Y. Ko, "Overview and Framework for
    Internationalized Email", RFC 4952, July 2007.

    [EAI-Downgrade] Y. YONEYA and K. Fujiwara, "Downgrading mechanism
    for Internationalized eMail Address (IMA)",
    draft-ietf-eai-downgrade-09.txt (work in progress).

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    [KEYWORDS] "Key words for use in RFCs to Indicate Requirement
    Levels", S. Bradner, RFC 2119, BCP 14, March 1997.

11.  Informative References

    [mailto-bis] M. Duerst and L. Masinter, "The mailto URI scheme",
    draft-duerst-mailto-bis-xx (work in progress).

    [List-*] G. Neufeld and J. Baer, "The Use of URLs as Meta-Syntax for
    Core Mail List Commands and their Transport through Message Header
    Fields", July 1998

    [List-ID] R. Chandhok and G. Wenger, "List-Id:  A Structured Field
    and Namespace for the Identification of Mailing Lists", March 2001

    [IRI] M. Duerst and M. Suignard,"Internationalized Resource
    Identifiers (IRIs)", January 2005

12.  Authors' Addresses

    Randall Gellens
    QUALCOMM Incorporated
    5775 Morehouse Drive
    San Diego, CA  92121

Appendix A:  Changes from Previous Version


    Changes made from version -03 to -04:
    o   Rewrote text in Section 5 on List-* headers.  Added new text
        specifically on List-ID.  Noted that currently, IRIs in List-*
        headers must be encoded as ASCII.

    Changes made from version -02 to -03:
    o   Deleted Section 6.
    o   Restored missing text in third paragraph from the end of Section
    o   Deleted broken suggestion in Section 5.
    o   Additional text fixes.
    o   Reworked text on List-* header fields.
    o   Removed most Editor's Notes, including deletion of all text that
        had been followed by an Editor's Note asking if it was useful.
    o   Modified Abstract.
    o   Edited Sections 3, 4, and 5.

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    Changes made from version -01 to -02:
    o   Significant changes throughout the document.  Sorry.

    Changes made from version -00 to -01:
    o   Fixed SMTP envelope versus message header confusion.
    o   Fixed erroneous mailing list operation text.
    o   Removed references to ATOMIC.
    o   Removed unneeded scenarios.
    o   Added discussion of human considerations which arise with lists.
    o   Fixed some typos.

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Full Copyright Statement

    Copyright (C) The IETF Trust (2008).

    This document is subject to the rights, licenses and restrictions
    contained in BCP 78, and except as set forth therein, the authors
    retain all their rights.

    This document and the information contained herein are provided on

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