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Versions: 00 01 02 03 04 05 06 07 rfc5983                   Experimental
Intended Status: Experimental
Internet Draft: Mailing Lists and Internationalized          R. Gellens
                Email Addresses                                Qualcomm
Document: draft-ietf-eai-mailinglist-07.txt
Expires: December 28, 2010                                June 28, 2010

          Mailing Lists and Internationalized Email Addresses

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    This document may contain material from IETF Documents or IETF
    Contributions published or made publicly available before November
    10, 2008.  The person(s) controlling the copyright in some of this
    material may not have granted the IETF Trust the right to allow
    modifications of such material outside the IETF Standards Process.
    Without obtaining an adequate license from the person(s) controlling
    the copyright in such materials, this document may not be modified
    outside the IETF Standards Process, and derivative works of it may
    not be created outside the IETF Standards Process, except to format
    it for publication as an RFC or to translate it into languages other
    than English.


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

1.  Introduction

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

    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.)

    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 [submission] (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

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    non-local addresses, is out of scope for this document.

    Some mailing lists alter message header fields, 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 RFC
    2369 ("The Use of URLs as Meta-Syntax for Core Mail List Commands
    and their Transport through Message Header Fields") [List-*] and RFC
    2919 ("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 in header fields of redistributed messages.
    In particular, an internationalized message cannot be downgraded
    unless all envelope addresses are available in ASCII (that is, each
    address either is ASCII or has an alt-address [UTF8SMTP]).

    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 UTF-8 mail 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
    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

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    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 UTF-8 submission or
    return-path address?  Second, does it accept subscriptions to UTF-8
    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.

2.  Conventions Used in this Document

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

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 [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 [submission], 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 which

    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 [UTF8SMTP]
    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
    [UTF8SMTP] is specified, and a downgrade is required, the message
    cannot be delivered.  To protect against this, a UTF8SMTP-aware
    [UTF8SMTP] 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 UTF-8 submission or return-path
    address?  Second, does it allow subscriptions from UTF-8 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 UTF-8 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
    UTF-8 address of necessity needs to accept UTF8SMTP messages).
    Likewise, there is no real benefit to a list in using a UTF-8
    submission address unless it also accepts UTF8SMTP messages and
    permits UTF-8 addresses to subscribe.

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

    1.  If the list uses a UTF-8 submission or return-path address, it
    SHOULD specify an alt-address [UTF8SMTP] for it.  Clearly, it needs
    to sit behind a UTF8SMTP-enabled final-delivery SMTP server
    [UTF8SMTP] and delivery agent.  Likewise, if a list uses a UTF-8
    return-path address, then its MSA [submission] needs to support

    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 UTF-8 submission address.  If the list does use a
    UTF-8 return path, it MUST specify an alt-address [UTF8SMTP] (or
    else there is a high risk of being unable to receive non-delivery

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

    2.  If it allows UTF-8 addresses to subscribe, it MAY require an
    alt-address [UTF8SMTP] to be specified for each UTF-8 subscriber.

    Naturally, if it permits UTF-8 addresses to subscribe, it needs a
    mechanism to accept subscription requests from such addresses
    (preferably specified in the form <utf8@utf8 <ascii@ascii>>).  In
    order to send email to its subscribers who have UTF-8 addresses, its
    MSA needs to support [UTF8SMTP].

    3.  If it accepts UTF8SMTP messages, the MTAs and MDA in its inbound
    path need to support UTF8SMTP.

5.  List Header Fields

    A number of header fields specifically for mailing lists have been
    introduced in RFC 2369 and RFC 2919.  These include, for example:

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

    As described in RFC 2369, "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.  Schemes which
    permit both URI and IRI [IRI] forms should use the URI-encoded form
    described in [IRI].  Future work may extend these header fields or
    define replacements to directly support non-encoded UTF-8 in IRIs
    (for example, [mailto-bis]), but in the absence of such extension or
    replacement, non-ASCII characters can only appear within when
    encoded as ASCII.  Note that discussion on whether internationalized
    domain names should be percent-encoded or puny-coded, is ongoing;
    see [IRI-bis]

    Even without these header fields being extended to support UTF-8,
    some special provisions may be helpful when downgrading.  In
    particular, if a List-* header contains a UTF-8 mailto (even encoded
    in ASCII) followed by an ASCII mailto, it may be advisable to not

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    only copy and preserve the original header as usual (ENCAPSULATION
    method of [EAI-Downgrade]), but also to edit the header to remove
    the UTF-8 address.  Otherwise, a client might run into trouble if
    the decoded mailto results in a non-ASCII address.

    When mailing lists use a UTF-8 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

    The most commonly-used URI schemes in List-* headers tend to be HTTP
    and mailto.  The current specification for mailto does not permit
    unencoded UTF-8 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) [UTF8SMTP] for a UTF-8 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 UTF-8 mailto is used in a List-* header field, an alt-address
    [UTF8SMTP], if available, SHOULD be supplied.

    The List-ID header field uniquely identifies a list.  The intent is
    that the value of this header remain constant, even if the machine
    or system used to operate or 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.  Such filters and
    tests may not properly compare a non-ASCII value which has been
    encoded into ASCII.  In addition to these comparison considerations,
    it is generally desirable that this header field contain something
    meaningful that users can type in.  However, ASCII encodings of
    non-ASCII characters 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

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    mailing list upgrades to internationalized mail.

    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 [UTF8SMTP], and redistribute it to UTF-8 mail and
    all-ASCII mail 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 [UTF8SMTP]
    is not required from the sending MTA for the transport to be
    complete.  When the mailing list then retransmits the message to its
    subscribers, it may encounter paths where a downgrade is needed (if
    a relay or final MSA does not supports UTF8SMTP).  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 -- UTF-8 addresses in
    mailto links in List-* headers will be easier to downgrade if they
    contain an alt-address [UTF8SMTP].

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 extensive comments.  Ted Hardie
    contributed helpful text on IRIs.  Last-call comments from S.
    Moonesamy and Amanda Baber, plus shepherd review by Pete Resnick,
    improved the document.

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10.  Normative References

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

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

    [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", RFC 2369, July 1998

    [List-ID] R. Chandhok and G. Wenger, "List-Id:  A Structured Field
    and Namespace for the Identification of Mailing Lists", RFC 2919,
    March 2001

    [submission] R. Gellens and J. Klensin, "Message Submission for
    Mail", RFC 4409, April 2006.

    [UTF8SMTP] J. Yao and W. Mao, "SMTP Extension for Internationalized
    Email Addresses", RFC 5336, September 2008.

11.  Informative References

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

    [IRI] M. Duerst and M. Suignard,"Internationalized Resource
    Identifiers (IRIs)", RFC 3987, January 2005

    [IRI-bis], Internationalized Resource Identifiers IETF Working
    Group, http://datatracker.ietf.org/wg/iri/

    [EAI-Downgrade] K. Fujiwara and Y. Yoneya, "Downgrading Mechanism
    for Email Address Internationalization", RFC 5504, March 2009.

12.  Authors' Addresses

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

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Appendix A:  Changes from Previous Version


    Changes made from version -06 to -07:
    o   Added explanatory text in section 2 about why a list server's
        MSA needs to support UTF8SMTP.
    o   Changed section 5 to say that an alt-address should "be
        supplied" instead of "immediately follow" the UTF-8 address.
    o   Moved [List-*], [List-ID], [submission], and [UTF8SMTP]
        references from Informative to Normative.
    o   Switched former sections 1 ("Conventions used in this document")
        and 2 ("Introduction").
    o   Added new sentence to start of Introduction.
    o   Changed "the message header" to "message header fields" in
    o   Added reference to RFC 5336 after some uses of "alt-address".
    o   Made case consistent in "alt-address".
    o   Used sanctioned instead of ad-hoc example domains.
    o   Replaced "i18mail" with "UTF-8 mail".
    o   Added additional people to Acknowledgments.

    Changes made from version -05 to -06:
    o   Reworded IRI text per Ted Hardie's suggestion.
    o   Added reference to IRI WG.
    o   Deleted remaining fEditor's Note.

    Changes made from version -04 to -05:
    o   Added [submission] as informative reference.
    o   Normalized "UTF-8" for descriptive terms except as part of
        hyphenated compounds.
    o   Reworded Introduction text on envelope addresses and ASCII.
    o   Added informative reference to RFC 5336.
    o   Reworded discussion of List-ID in "List Header Fields" section
        to more clearly explain the risks of using ASCII encodings of
        non-ASCII characters.
    o   Added text to "Further Discussion" clarifying that downgrade
        might be needed if an MAA between the list and a subscriber does
        not support UTF8SMTP.
    o   Corrected some references.

    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.

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    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.

    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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