Internet Engineering Task Force (IETF)                        R. Gellens
Request for Comments: 5983                                      Qualcomm
Category: Experimental                                      October 2010
ISSN: 2070-1721


          Mailing Lists and Internationalized Email Addresses

Abstract

   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.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see 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
   http://www.rfc-editor.org/info/rfc5983.

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Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................4
   3. Scenarios Involving Mailing Lists ...............................4
   4. Capabilities and Requirements ...................................5
   5. List Header Fields ..............................................6
   6. Further Discussion ..............................................8
   7. Security Considerations .........................................8
   8. Acknowledgments .................................................9
   9. References ......................................................9
      9.1. Normative References .......................................9
      9.2. Informative References ....................................10

1.  Introduction

   This document describes considerations for mailing lists with the
   introduction of internationalized email addresses [RFC5335].

   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 policies.

   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 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 header fields.
   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
   recipients.

   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 cannot 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 a
   missed email in one-to-one communications might not be appropriate
   when dealing with mailing lists.  For example, if a user sends a
   message that 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 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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 it 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
   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 an 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
   addresses.

   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 this 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
   document.

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 Message Submission Agent (MSA)
      [submission] 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 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
      reports).

      There are also implications for the List-* header fields (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 Message Transfer Agents
      (MTAs) and Mail Delivery Agent (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 RFCs 2369 and 2919.  For example, these include:

   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-*].  For List-ID, RFC 2919
   specifies that, "The list identifier will, in most cases, appear like
   a host name in a domain of the list owner" [List-ID].

   Except for the List-ID header field, these mailing list header fields
   contain URLs [RFC3986].  The most common schemes are generally HTTP,
   HTTPS, mailto, and FTP.  Schemes that permit both URI and
   Internationalized Resource Identifier (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 field contains a UTF-8 mailto (even
   encoded in ASCII) followed by an ASCII mailto, it may be advisable
   not only to copy and preserve the original header field as usual
   (ENCAPSULATION method of [EAI-Downgrade]), but also to edit the
   header field 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 field, an
   ASCII form SHOULD also be used.  These header fields are vital to
   good operations and use of mailing lists; caution is called for when
   considering how to form and use these header fields in a non-ASCII
   environment.

   The most commonly used URI schemes in List-* header fields 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 provides an opaque value that uniquely
   identifies a list.  The intent is that the value of this header field
   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 that 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
   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 header fields 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, which 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-* header fields will be easier to downgrade
      if they contain an alt-address [UTF8SMTP].

7.  Security Considerations

   Because use of both a UTF-8 address and an alt-address for the same
   entity introduces a potential ambiguity regarding the identity of
   list subscribers and message senders, implementers are advised to

   carefully handle authorization decisions regarding subscriptions,
   sender filters, and other common list administration features.  For
   example, a binding between a UTF-8 address and an ASCII alt-address
   can be used by an attacker to deny another person admission to an
   Email Address Internationalization (EAI) mailing list.

   Other relevant security considerations are discussed in the Framework
   document [EAI-Framework].

8.  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.

9.  References

9.1.  Normative References

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

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

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC5335]  Abel, Y., Ed., "Internationalized Email Headers", RFC
              5335, September 2008.

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

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

9.2.  Informative References

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

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

   [IRI-bis]  Duerst, M., Suignard, M., and L. Masinter,
              "Internationalized Resource Identifiers (IRIs)", Work in
              Progress, July 2010.

   [mailto-bis]
              Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", Work in Progress, May 2010.

Author's Address

   Randall Gellens
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121
   rg+ietf@qualcomm.com