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SMTP                                                          D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Expires: September 29, 2005                               March 28, 2005


                       Internet Mail Architecture
                      draft-crocker-email-arch-04

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on September 29, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Over its thirty-four year history, Internet Mail has undergone
   significant changes in scale and complexity, as it has become a
   global infrastructure service.  The first standardized architecture
   for email specified a simple split between the user world, in the
   form of Mail User Agents (MUA), and the transmission world, in the
   form of the Mail Handling Service (MHS) composed of Mail Transfer
   Agents (MTA).  Core aspects of the service, such as address and



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   message style, have remained remarkably constant.  Today, Internet
   Mail is marked by many independent operators, many different
   components for providing users service and many others for performing
   message transfer.  Public discussion of the architecture has not kept
   pace with the real-world technical and operational refinements.  This
   document offers an enhanced Internet Mail architecture to reflect the
   current service.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Service Overview . . . . . . . . . . . . . . . . . . . . .  4
     1.2   Discussion venue . . . . . . . . . . . . . . . . . . . . .  5
     1.3   Changes  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Email Actor Roles  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1   User Actors  . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2   MHS Actors . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.3   Administrative Actors  . . . . . . . . . . . . . . . . . . 11
   3.  Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     3.1   Mailbox Addresses  . . . . . . . . . . . . . . . . . . . . 13
     3.2   Domain Names . . . . . . . . . . . . . . . . . . . . . . . 14
     3.3   Message Identifiers  . . . . . . . . . . . . . . . . . . . 15
     3.4   Identity Referencing Convention  . . . . . . . . . . . . . 15
   4.  Services . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.1   Message  . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.2   Mail User Agent (MUA)  . . . . . . . . . . . . . . . . . . 20
     4.3   Mail Submission Agent (MSA)  . . . . . . . . . . . . . . . 22
     4.4   Mail Transfer Agent (MTA)  . . . . . . . . . . . . . . . . 23
     4.5   Mail Delivery Agent (MDA)  . . . . . . . . . . . . . . . . 25
     4.6   Message Store (MS) . . . . . . . . . . . . . . . . . . . . 26
   5.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     5.1   Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 28
     5.2   ReSending  . . . . . . . . . . . . . . . . . . . . . . . . 30
     5.3   Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 31
     5.4   Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 34
     5.5   Boundary Filter  . . . . . . . . . . . . . . . . . . . . . 35
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     7.1   References - Normative . . . . . . . . . . . . . . . . . . 35
     7.2   Reference - Descriptive  . . . . . . . . . . . . . . . . . 38
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38
       Intellectual Property and Copyright Statements . . . . . . . . 39








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1.  Introduction

   Over its thirty-four year history, Internet Mail has undergone
   significant changes in scale and complexity, as it has become a
   global infrastructure service.

   The first standardized architecture for email specified a simple
   split between the user world, in the form of Mail User Agents (MUA),
   and the transmission world, in the form of the Mail Handling Service
   (MHS) composed of Mail Transfer Agents (MTA).  The MHS is responsible
   for accepting a message from one User and delivering it to one or
   more others.

                                 +--------+
               +---------------->|  User  |
               |                 +--------+
               |                      .
   +--------+  |          +--------+  .
   |  User  +--+--------->|  User  |  .
   +--------+  |          +--------+  .
       .       |               .      .
       .       |   +--------+  .      .
       .       +-->|  User  |  .      .
       .           +--------+  .      .
       .                .      .      .
       .                .      .      .
       .                .      .      .
   +--------------------------------------+
   |     Mail Handling Service (MHS)      |
   +--------------------------------------+

                Figure 1: Basic Internet Mail Service Model

   Today, Internet Mail is marked by many independent operators, many
   different components for providing users service and many other
   components for performing message transfer.  So it is not surprising
   that the operational service has sub-divided each of these "layers"
   into more specialized modules.  Core aspects of the service, such as
   address and message style, have remained remarkably constant.
   However public discussion of the architecture has not kept pace with
   the real-world refinements.  This document offers an enhanced
   Internet Mail architecture to reflect the current service.  The
   original distinction between user-level concerns and transfer-level
   concerns is retained, and the elaboration to each "level" of the
   architecture is discussed separately.  The term "Internet Mail" is
   used to refer to the entire collection of user and transfer
   components.




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   For Internet Mail, the term "end-to-end" usually refers to a single
   posting and the set of deliveries directly resulting from its single
   transiting of the MHS.  However, note that some uses of email
   consider the entire email service -- including Originator and
   Recipient -- as a subordinate component.  For these services, "end-
   to-end" refers to points outside of the email service.  Examples are
   voicemail over email [RFC2423], EDI over email [RFC1767], and
   facsimile over email.[ID-ffpim]

   The current draft seeks to:

   o  Document refinements to the email model

   o  Clarify functional roles for the architectural components

   o  Clarify identity-related issues, across the email service

   o  Provide a document that serves as a common venue for further
      defining and citing modern Internet Mail architecture

   NOTE:

      Any attempt to provide a retroactive description, for a service
      that evolved so extensively, is certain to claim definitions and
      relationships that do not match the equally reasonable views of
      some portion of the technical community.  Ultimately, the
      "correct" choices are determined solely by the willingness of that
      community to use the descriptions.


1.1  Service Overview

   End-to-end Internet Mail exchange is accomplished by using a
   standardized infrastructure comprising:

   o  An email object

   o  Global addressing

   o  An asynchronous sequence of point-to-point transfer mechanisms

   o  No prior arrangement between Originator and Recipient

   o  No prior arrangement between point-to-point transfer services,
      over the open Internet

   o  No requirement for Originator and Recipient to be online at the
      same time.



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   The end-to-end portion of the service is the email object, called a
   message.  Broadly the message, itself, is divided between handling
   control information and user message content.

   A precept to the design of Internet Mail is permitting user-to-user
   and MTA-to-MTA interoperability with no prior, direct administrative
   arrangement.  That is, all participants rely on having the core
   services be universally supported, either directly or through
   Gateways that translate between Internet Mail standards and other
   email conventions.

   For localized environments (Edge networks) prior, administrative
   arrangement can include access control, routing constraints and
   lookup service configuration.  In recent years one change to local
   environments is an increased requirement for authentication or, at
   least, accountability.  In these cases, the server performs explicit
   validation of the client's identity.

1.2  Discussion venue

   Discussion about this document should be directed to the IETF-SMTP
   mailing list <http://www.imc.org/ietf-smtp>.  It is the most active,
   long-standing venue for discussing email architecture.  Although it
   is primarily for discussing only the SMTP protocol, it is recommended
   that discussion of this draft take place on that mailing list because
   it attends to end-to-end infrastructure and architecture issues more
   than other email-related mailing lists.

1.3  Changes

   This is intended to be the last major revision, prior to seeking
   publication.

   Significant changes to this version:

   Administrative Unit:   Changed from Administrative Domain to
      Administrative Unit, to remove possible confusion with "domain
      name".  Added Tussle reference

   Sieve:   Noted ability to have other places to run sieve
      instructions.

   Word Smithing:   Assorted small tweaks to definitions, diagrams and
      comments.







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   Notices, Bounces and Disp:   Added Bounce module to Services diagram,
      to make clear that MHS return messages can go to an independent
      address.  Dotted link to MSA shows responsibility for setting
      Notices address.  Changed "Notification" to "Bounce", to use more
      popular term and to avoid confusion with MDN notices.  Added Disp
      module to Services, to distinguish DSN traffic from MDN.


2.  Email Actor Roles

   Internet Mail is a highly distributed service, with a variety of
   actors serving different roles.  These divide into:

   o  User

   o  Mail Handling Service (MHS)

   o  Administrative Unit

   Although related to a technical architecture, the focus on Actors
   concerns participant responsibilities, rather than on functional
   modules.  Hence the labels used are different than for classic email
   architecture diagrams.  Actors often will be associated with
   different organizations.  This operational independence provides the
   motivation for distinguishing Administrative Units.

2.1  User Actors

   Users are the sources and sinks of messages.  They may be humans or
   processes.  They may have an exchange that iterates and they may
   expand or contract the set of Users participating in a set of
   exchanges.  In Internet Mail there are three types of user-level
   Actors:

   o  Originators

   o  Recipients

   o  Mediators

   From the User-level perspective all mail transfer activities are
   performed by a monolithic Mail Handling Service (MHS), even though
   the actual service may be provided by many independent organizations.
   Users are customers of this service.







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   The following depicts the flow of messages among Actors.

   +------------+
   | Originator |<--------------+
   +-+---+----+-+               |
     |   |    |                 |
     |   |    V                 |
     |   |  +-----------+       |
     |   |  | Recipient |       |
     |   |  +-----------+       |
     |   |                      |
     |   |       +----------+   |
     |   |       |          |   |
     |   V       V          |   |
     | +-----------+    +---+---+---+
     | | Mediator  +--->| Recipient |
     | +-----------+    +-----------+
     |
     V
   +-----------+    +-----------+    +-----------+
   | Mediator  +--->| Mediator  +--->| Recipient |
   +-----------+    +-----------+    +-----------+

                 Figure 2: Relationships Among User Actors


2.1.1  Originator

   Also called "Author", this is the user-level participant responsible
   for creating original content and requesting its transmission.  The
   MHS operates to send and deliver mail among Originators and
   Recipients.  As described below, the MHS has a "Source" role, that
   correlates with the Author role.

2.1.2  Recipient

   The Recipient is a consumer of delivered content.  As described
   below, the MHS has a "Dest" role, that correlates with the Recipient
   role.

   A Recipient may close the user-level communication loop by creating
   and submitting a new message that replies to an Originator.  An
   example of an automated form of reply is the Message Disposition
   Notification, which informs the Originator about the Recipient's
   disposition of the message.  See Section 4.1.






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2.1.3  Mediator

   A Mediator receives, aggregates, reformulates and redistributes
   messages as part of a potentially-protracted, higher-level exchange
   among Users.  Example uses of Mediators include group dialogue and
   organizational message flow, as occurs with a purchase approval
   process.  Note that it is easy to confuse this user-level activity
   with the underlying MHS transfer exchanges.  However they serve very
   different purposes and operate is very different ways.  Mediators are
   considered extensively in Section 5.

   When mail is delivered to an envelope address, a Mediator is viewed
   by the Mail Handling Service as a Recipient.  When submitting
   messages, the Mediator is an Originator.  What is distinctive is that
   a Mediator preserves the Originator information of the message it
   reformulates, but may make meaningful changes to the content.  Hence
   the MHS sees a new message, but Users receive a message that is
   interpreted as primarily being from -- or, at least, initiated by --
   the author of the original message.  The role of a Mediator permits
   distinct, active creativity, rather than being limited to the more
   constrained job of merely connecting together other participants.
   Hence it is really the Mediator that is responsible for the new
   message.

   A Mediator's task may be complex and contingent, such as by modifying
   and adding content or regulating which users may participate and
   when.  The popular example of this role is a group mailing list.  A
   sequence of mediators may even perform a series of formal steps, such
   as reviewing, modifying and approving a purchase request.

   Because a Mediator originates messages, it might also receive
   replies.  So, a Mediator really is a full-fledged User.

   Gateway:   A Gateway is a particularly interesting form of Mediator.
      It is a hybrid of User and Relay that interconnects heterogeneous
      mail services.  Its goal is to emulate a Relay, so Gateway is
      described in more detail, in the next section.


2.2  MHS Actors

   The Mail Handling Service (MHS) has the task of performing a single,
   email-level end-to-end transfer, on behalf of the Originator and
   reaching the Recipient address(es) specified in the envelope.
   Mediated or protracted, iterative exchanges, such as those used for
   collaboration over time, are part of the User-level service, and are
   not part of this Transfer-level Handling Service.




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   The following depicts the relationships among transfer participants
   in Internet Mail.  It shows the Source as distinct from the
   Originator, and Destination as distinct from Recipient, although it
   is common for each pair to be the same actor.  The figure also shows
   multiple Relays in the sequence.  It is legal to have no separate
   Relay, where the Source and Dest interact directly.  For intra-
   organization mail services, it is common to have only one Relay.

   +------------+                         +-----------+
   | Originator |                         | Recipient |
   +-----+------+                         +-----------+
         |                                      ^
         |         Mail Handling Service        |
   /+=================================================+\
   ||    |                                      |     ||
   ||    |                                      |     ||
         V                                      |
     +---------+    +--------+             +----+----+
     |         |    |        |<------------+         |
     | Source  +...>| Bounce |             |  Dest   |
     |         |    |        |<---+        |         |
     +----+----+    +--------+    |        +---------+
          |                       |             ^
          V                       |             |
     +---------+             +----+----+   +----+----+
     |  Relay  +-->.......-->|  Relay  +-->|  Relay  |
     +---------+             +----+----+   +---------+
                                  |
                                  V
                             +---------+
                             | Gateway +-->...
                             +---------+

                 Figure 3: Relationships Among MHS Actors


2.2.1  Source

   The Source role is responsible for ensuring that a message is valid
   for posting and then submitting it to a Relay.  Validity includes
   conformance with Internet Mail standards, as well as with local
   operational policies.  The source may simply review the message for
   conformance, and reject it if there are errors, or it may create some
   or all of the necessary information.

   The Source operates with dual "allegiance".  It serves the Originator
   and often it is the same entity.  However its role in assuring
   validity means that it must also represent the local operator of the



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   MHS, that is, the local Administrative Domain.

   The Source also has the responsibility for any post-submission,
   Originator-related administrative tasks associated with message
   transmission and delivery.  Notably this pertains to error and
   delivery notices.  Hence, Source is best held accountable for the
   message content, even when they did not create any or most of it.

2.2.2  Bounce Handler

   The Bounce Handler processes service notifications that are generated
   by the MHS, as a result of its efforts to transfer or deliver the
   message.  Notices may be about failures or completions and are sent
   to an address that is specified by the Source.  This Bounce handling
   address (also known as a Return address) might have no visible
   characteristics in common with the address of the Originator or
   Source.

2.2.3  Relay

   A mail Relay performs email transfer-service routing and store-and-
   forward.  It adds envelope-level handling information and then
   (re-)transmits the message on towards its Recipient(s).  A Relay may
   add information to the envelope, such as with trace information.
   However it does not modify existing envelope information or the
   message content semantics.  It may modify message content syntax,
   such as a change from text to binary transfer-encoding form, only as
   required to meet the capabilities of the next hop in the MHS.

   A set of Relays composes a Mail Handling Service network.  This is
   above any underlying packet-switching network that they might be
   using and below any gateways or other user-level Mediators.

   In other words, interesting email scenarios can involve three,
   distinct architectural layers of store-and-forward service:

   o  User Mediators

   o  MHS Relays

   o  Packet Switches

   with the bottom-most usually being the Internet's IP service.  The
   most basic email scenarios involve Relays and Switches.

   Aborting a message transfer results in having the Relay become an
   Originator and send an error message to the Bounce (Bounce) address.
   (The potential for looping is avoided by having this message, itself,



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   contain no Bounce address.)

2.2.4  Gateway

   A Gateway is a hybrid form of User and Relay that interconnects
   heterogeneous mail services.  Its purpose is simply to emulate a
   Relay and the closer it comes to this, the better.  However it
   operates at the User level, because it must be able to modify message
   content.

   Differences between mail services can be as small as minor syntax
   variations, but usually encompass significant, semantic distinctions.
   For example, the concept of an email address might be as different as
   a hierarchical, machine-specific address versus a flat, global name
   space.  Or between text-only content and multi-media.  Hence the
   Relay function in a Gateway offers the minor challenge in design.
   The more significant challenge is in ensuring the user-to-user
   functionality that matches syntax and semantics of independent email
   standards suites.

   The basic test of a Gateway's adequacy is, of course, whether an
   Originator on one side of a Gateway can send a message to a Recipient
   on the other side, without requiring changes to any of the components
   in the Originator's or Recipient's mail services, other than adding
   the Gateway.  To each of these otherwise independent services, the
   Gateway will appear to be a "native" participant.  However the
   ultimate test of a Gateway's adequacy is whether the Originator and
   Recipient can sustain a dialogue.  In particular, can a Recipient's
   MUA automatically formulate a valid Reply?

2.3  Administrative Actors

   Operation of Internet Mail services is apportioned to different
   providers (or operators).  Each can be composed of an independent
   Administrative Unit (AU).  Examples include an end-user operating
   their desktop client, a department operating a local Relay, an IT
   department operating an enterprise Relay, and an ISP operating a
   public, shared email service.  These can be configured into many
   combinations of administrative and operational relationships, with
   each Administrative Unit potentially having a complex arrangement of
   functional components.  Figure 4 depicts the relationships among AUs.
   Perhaps the most salient aspect of an AU is the differential trust
   that determines its policies for activities within the AU, versus
   those involving interactions with other AUs.  The architectural
   impact of needing to have boundaries between AU's is discussed in
   [Tussle]

   Basic components of AU distinction include:



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   Edge:   Independent transfer services, in networks at the edge of the
      Internet Mail service.

   User:   End-user services.  This might be subsumed under the Edge
      service, such as is common for web-based email access.

   Transit:   These are Mail Service Providers (MSP) offering value-
      added capabilities for Edge AUs, such as aggregation and
      filtering.

   Note that Transit services are quite different from packet-level
   transit operation.  Whereas end-to-end packet transfers usually go
   through intermediate routers, email exchange across the open Internet
   is often directly between the Edge AUs, at the email level.

   +------                             +------+     +------+
   | AU-1 |                            | AU-3 |     | AU-4 |
   | ---- |                            | ---- |     | ---- |
   |      |    +---------------------->|      |     |      |
   | User |    |                       |-Edge-+---->|-User |
   |  |   |    |                  +--->|      |     |      |
   |  V   |    |                  |    +------+     +------+
   | Edge-+----+                  |
   |      |    |    +---------+   |
   +------+    |    | AU-2    |   |
               |    | ------- |   |
               |    |         |   |
               +--->|-Transit-+---+
                    |         |
                    +---------+

                Figure 4: Administrative Units (AU) Example

   Edge networks may use proprietary email standards internally.
   However the distinction between Transit network and Edge network
   transfer services is primarily significant because it highlights the
   need for concern over interaction and protection between independent
   administrations.  In particular, this distinctions calls for
   additional care in assessing transitions of responsibility, as well
   as the accountability and authorization relationships among
   participants in email transfer.

   The interactions between functional components within an
   Administrative Unit are subject to the policies of that domain.
   Policies can cover such things as reliability, access control,
   accountability and even content evaluation and modification.  They
   may be implemented in different functional components, according to
   the needs of the Administrative Unit.  For example, see [ID-spamops].



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   User, Edge and Transit services can be offered by providers that
   operate component services or sets of services.  Further, it is
   possible for one AU to host services for other AUs.  Common AU
   examples are:

   Enterprise Service Providers:

      Operating an organization's internal data and/or mail services.

   Internet Service Providers:

      Operating underlying data communication services that, in turn,
      are used by one or more Relays and Users.  It is not their job to
      perform email functions, but to provide an environment in which
      those functions can be performed.

   Mail Service Providers:

      Operating email services, such as for end-users, or mailing lists.

   Operational pragmatics often dictate that providers be involved in
   detailed administration and enforcement issues, to help ensure the
   health of the overall Internet Mail Service.  This can include
   operators of lower-level packet services.

3.  Identities

   Internet Mail uses three forms of identity.  The most common is the
   mailbox address <addr-spec> [RFC2822] Also see <address> and
   <mailbox> in [RFC2821].  The other two are the domain name <domain>
   Section 3.2 and message identifier <msg-id> [RFC2822].

3.1  Mailbox Addresses

      "A mailbox sends and receives mail.  It is a conceptual entity
      which does not necessarily pertain to file storage."  [RFC2822]

   A mailbox is specified as an Internet Mail address <addr-spec>.  It
   has two distinct parts, divided by an at-sign ("@").  The right-hand
   side contains a globally interpreted name for an Administrative Unit.
   Domain Names are discussed in Section 3.2.

   The portion to the left of the at-sign contains a string that is
   globally opaque and is called the <local-part>.  It is to be
   interpreted only by the entity specified in the address's right-hand
   side.  All other entities must treat the local-part as a
   uninterpreted, literal string and must preserve all of its original
   details.  As such, its public distribution is equivalent to sending a



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   "cookie" that is only interpreted upon being returned to its
   originator.

3.1.1  Global Standards for Local-Part

   It is common for sites to have local structuring conventions for the
   left-hand side (local-part) of an addr-spec.  This permits sub-
   addressing, such as for distinguishing different discussion groups by
   the same participant.  However it must be stressed that these
   conventions are strictly private to the user's organization and must
   not be interpreted by any domain except the one listed in the right-
   hand side of the addr-spec.

   A small class of addresses has an elaboration on basic email
   addressing, with a standardized, global schema for the local-part.
   These are conventions between originating end-systems and Recipient
   Gateways, and they are invisible to the public email transfer
   infrastructure.  When an Originator is explicitly sending via a
   Gateway out of the Internet, there are coding conventions for the
   local-part, so that the Originator can formulate instructions for the
   Gateway.  Standardized examples of this are the telephone numbering
   formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com",
   and iFax [RFC2304], such as
   "FAX=+12027653000/T33S=1387@ifax.example.com".

3.1.2  Scope of Email Address Use

   Email addresses are being used far beyond their original email
   transfer and delivery role.  In practical terms, email strings have
   become a common form of user identity on the Internet.  What is
   essential, then, is to be clear about the nature and role of an
   identity string in a particular context and to be clear about the
   entity responsible for setting that string.

3.2  Domain Names

   A domain name is a global reference to an Internet resource, such as
   a host, a service or a network.  A name usually maps to one or more
   IP Addresses.  Conceptually, the name might encompass an entire
   organization, or a collection of machines integrated into a
   homogeneous service, or only a single machine.  A domain name can be
   administered to refer to individual users, but this is not common
   practice.  The name is structured as a hierarchical sequence of sub-
   names, separated by dots (".").  Domain names are defined and
   operated through the Domain Name Service (DNS) [RFC1034], [RFC1035],
   [RFC2181].

   When not part of a mailbox address, a domain name is used in Internet



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   Mail to refer to a node that took action upon the message, such as
   providing the administrative scope for a message identifier, or
   performing transfer processing.

3.3  Message Identifiers

   Like mailbox addresses, message identifiers have two distinct parts,
   divided by an at-sign ("@").  The right-hand side is globally
   interpreted and specifies the Administrative Unit assigning the
   identifier.  The left-hand side of the at-sign contains a string that
   is globally opaque and serves to uniquely identify the message within
   the domain referenced on the right-hand side.  The duration of
   uniqueness for the message identifier is undefined.

   The identifier may be assigned by the user or by any component of the
   system along the path, within the AU responsible for the indicated
   domain.  Although Internet Mail standards provide for a single
   identifier, more than one is sometimes assigned.

3.4  Identity Referencing Convention

   In this document, fields references to identities are labeled in a
   two-part, dotted notation.  The first part cites the document
   defining the identity and the second defines the name of the
   identity.  Hence, <RFC2822.From> is the From field in an email
   content header, and <RFC2821.MailFrom> is the address in the SMTP
   "Mail From" command.

4.  Services

   The Internet's MHS architecture distinguishes six types of functional
   components, arranged to support a store-and-forward service
   architecture:

   o  Message

   o  Mail User Agent (MUA)

   o  Message Submission Agent (MSA)

   o  Message Transfer Agent (MTA)

   o  Message Delivery Agent (MDA)

   o  Message Store (MS)

   This section describes the specific functional components for
   Internet Mail, and the standard protocols associated with performing



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   them.

   Software implementations of these architectural components often
   compress them, such as having the same software do MSA, MTA and MDA
   functions.  However the requirements for each of these components of
   the service are becoming more extensive.  So, their separation is
   increasingly common.

   NOTE:

      A discussion about any interesting system architecture is often
      complicated by confusion between architecture versus
      implementation.  An architecture defines the conceptual functions
      of a service, divided into discrete conceptual modules.  An
      implementation of that architecture may combine or separate
      architectural components, as needed for a particular operational
      environment.  It is important not to confuse the engineering
      decisions that are made to implement a product, with the
      architectural abstractions used to define conceptual functions.

   This figure shows function modules and the protocols used between
   them.  Additional protocols and configurations are possible.





























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                        +------+   +---------+
         ...............+ oMUA |...|  Disp   |<----------------+
         .              +--+---+   +---------+                 |
         .                 |    {smtp,                         |
         .                 V    {submission                    |
         .              +------+   +---------+                 |
         .              | MSA  |...| Bounces |< -----+         |
         .              +--+---+   +---------+       |         |
         .                 |                         |         |
         .                 V    {smtp                |         |
         .              +------+                /+===+===+\    |
         .              | MTA  |                ||  dsn  ||    |
   /+==========+\       +--+---+                \+=======+/    |
   || MESSAGE  ||          .                       ^   ^       |
   ||----------||          .    {smtp              |   |       |
   || Envelope ||          .                       |   |       |
   ||  SMTP    ||          V                       |   |       |
   ||  RFC2822 ||       +------+                   |   |   /+==+==+\
   || Content  ||       | MTA  +-------------------+   |   || mdn ||
   ||  RFC2822 ||       +--+---+                       |   \+=====+/
   ||  MIME    ||          |    {local, smtp,          |       |
   \+==========+/          V    {lmtp                  |       |
         .              +------+                       |       |
         .              |      +-----------------------+       |
         .              | MDA  |                               |
         .              |      |<--------------------+         |
         .              +-+--+-+                     |         |
         .        local}  |  |                       |         |
         .                V  |                       |         |
         .          +------+ |                  /+===+===+\    |
         .          | sMS  | |                  || sieve ||    |
         .          +-+--+-+ |                  \+=======+/    |
         .            |  |   |  {pop, imap           ^         |
         .            |  V   V                       |         |
         .            | +------+                     |         |
         .            | | uMS  |                     |         |
         .            | +--+---+                     |         |
         .            |    |    {pop, imap,          |         |
         .            V    V    {local               |         |
         .           +------+                        |         |
         .           |      +---- -------------------+         |
         ...........>| rMUA |                                  |
                     |      +----------------------------------+
                     +------+

                     Figure 5: Protocols and Services





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4.1  Message

   The purpose of the Mail Handling Service is to exchange a message
   object among participants.  Hence, all of the underlying mechanisms
   are merely in the service of getting that message from its Originator
   to its Recipients.  A message may be explicitly labeled as to its
   nature.  [RFC3458]

   A message comprises a transit handling envelope and the end-user
   message content.  The envelope contains handling information used by
   the Message Handling Service, or generated by it.  The content is
   divided into a structured header and the body.  The body may be
   unstructured, simple text, or it may be a tree of multi-media
   subordinate objects, called body-parts.

   Internet Mail has distinguished some special versions of messages,
   for exchanging control information:

   Delivery Status Notification (DSN):

      A Delivery Status Notification (DSN) may be generated by the Mail
      Handling Service (MSA, MTA or MDA) and sent to the
      RFC2821.MailFrom address.  It provides information about message
      transit, such as transmission errors or successful delivery.
      [RFC3461]

   Message Disposition Notification (MDN):

      A Message Disposition Notification (MDN) may be generated by an
      rMUA and is sent to the Disposition-Notification-To address(es).
      It provides information about user-level, Recipient-side message
      processing, such as indicating that the message has been read
      [RFC2298] or the form of content that can be supported.  [RFC3297]

   Message Filtering (SIEVE):

      SIEVE provides a means of specifying conditions for differential
      handling of mail, at the time of delivery [RFC3028].  Figure 5
      shows a Sieve specification going from the rMUA to the MDA.
      However filtering can be done at many different points along the
      transit path and any one or more of them might be subject to Sieve
      directives, especially within a single AU.  Hence, the Figure
      shows only one relationship, for simplicity.


4.1.1  Envelope

   Information that is directly used by, or produced by, the MHS is



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   called the "envelope".  It controls and records handling activities
   by the transfer service.  Internet Mail has a fragmented framework
   for handling this "handling" information.  The envelope exists partly
   in the transfer protocol SMTP [RFC2821] and partly in the message
   object [RFC2822].  The SMTP specification uses the term to refer only
   to the transfer-protocol information.

   NOTE:

      Due to the frequent use of the term "envelope" to refer only to
      SMTP constructs, there has been some call for using a different
      term, to label the larger set of information defined here.  So
      far, no alternative term has developed any community support.

   Direct envelope addressing information, as well as optional transfer
   directives, are carried within the SMTP control channel.  Other
   envelope information, such as trace records, is carried within the
   content header fields.  Upon delivery, some SMTP-level envelope
   information is typically encoded within additional content header
   fields, such as Return-Path.

4.1.2  Message Header Fields

   Header fields are attribute/value pairs covering an extensible range
   of email service, user content and user transaction meta-information.
   The core set of header fields is defined in [RFC2822], [RFC0822].  It
   is common to extend this set, for different applications.  Procedures
   for registering headers are provided in [RFC4021].  A complete set of
   registered header fields is being developed through [ID-hdr-reg].

   One danger with placing additional information in header fields is
   that Gateways often alter or delete them.

4.1.3  Body

   The body of a message might simply be lines of ASCII text or it might
   be structured into a composition of multi-media, body-part
   attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],
   and [RFC2049].  MIME structures each body-part into a recursive set
   of MIME header field meta-data and MIME Content sections.

4.1.4  Identity References in a Message

   For a message in transit, the core uses of identity references
   combine into:






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     +-----------------------+-------------+---------------------+
     | Layer                 | Field       | Set By              |
     +-----------------------+-------------+---------------------+
     | Message Body          | MIME Header | Originator          |
     | Message header fields | From        | Originator          |
     |                       | Sender      | Source              |
     |                       | Reply-To    | Originator          |
     |                       | To, CC, BCC | Originator          |
     |                       | Message-ID  | Source              |
     |                       | Received    | Source, Relay, Dest |
     |                       | Return-Path | MDA, from MailFrom  |
     |                       | Resent-*    | Mediator            |
     | SMTP                  | HELO        | Latest Relay Client |
     |                       | MailFrom    | Source              |
     |                       | RcptTo      | Originator          |
     | IP                    | IP Address  | Latest Relay Client |
     +-----------------------+-------------+---------------------+


4.2  Mail User Agent (MUA)

   A Mail User Agent (MUA) works on behalf of end-users and end-user
   applications.  It is their "representative" within the email service.

   At the origination side of the service, the oMUA is used to create a
   message and perform initial "submission" into the transfer
   infrastructure, via a Mail Submission Agent (MSA).  It may also
   perform any creation- and posting-time archival.  An MUA outbox is
   part of the origination-side MUA.

   The Recipient-side rMUA works on behalf of the end-user Recipient to
   process received mail.  This includes generating user-level return
   control messages, display and disposition of the received message,
   and closing or expanding the user communication loop, by initiating
   replies and forwarding new messages.

   An MUA may, itself, have a distributed implementation, such as a
   "thin" user interface module on a limited end-user device, with the
   bulk of the MUA functionality operated remotely on a more capable
   server.  An example of such an architecture might use IMAP [RFC3501]
   for most of the interactions between an MUA client and an MUA server.

   A Mediator is special class of MUA.  It performs message re-posting,
   as discussed in Section 2.1.

   Identity fields relevant to the MUA include:





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   RFC2822.From

      Set by: Originator

      Names and addresses for author(s) of the message content are
      listed in the From field

   RFC2822.Reply-To

      Set by: Originator

      If a message Recipient sends a reply message that would otherwise
      use the RFC2822.From field address(es) contained in the original
      message, then they are instead to use the address(es) in the
      RFC2822.Reply-To field.  In other words, this field is a direct
      override of the From field, for responses from Recipients.

   RFC2822.Sender

      Set by: Source

      This specifies the address responsible for submitting the message
      into the transfer service.  For efficiency, this field should be
      omitted if it contains the same address as RFC2822.From.  However
      this does not mean there is no Sender specified.  Rather, it means
      that that header field is virtual and that the address in the From
      field must be used.  Specification of the error return addresses
      -- the "Bounce" address, contained in RFC2821.MailFrom -- is made
      by the RFC2822.Sender.  Typically the Bounce address is the same
      as the Sender address.  However some usage scenarios require it to
      be different.

   RFC2822.To, RFC2822.CC

      Set by: Originator

      These specify MUA Recipient addresses.  The addresses in the
      fields might not be present in the RFC2821.RcptTo command.  The
      distinction between To and CC is subjective.  Generally, a To
      addressee is considered primary and is expected to take action on
      the message.  A CC addressee typically receives a copy only for
      their information.

   RFC2822.BCC







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      Set by: Originator

      A message might be copied to an addressee whose participation is
      not to be disclosed to the RFC2822.To or RFC2822.CC Recipients
      and, usually, not to the other BCC Recipients.  The BCC header
      field indicates a message copy to such a Recipient.  Typically,
      the field lists no addresses or only lists the address of the
      Recipient receiving this copy.  An MUA will typically make
      separate postings for TO and CC Recipients, versus BCC Recipients.
      The former will see no indication that any BCCs were sent, whereas
      the latter have a BCC field present.  It might be empty, contain a
      comment, or contain one or more BCC addresses, depending upon the
      preferences or the Originator.


4.3  Mail Submission Agent (MSA)

   A Mail Submission Agent (MSA) accepts the message submission from the
   oMUA and enforces the policies of the hosting AU and the requirements
   of Internet standards.  Enforcement might be passive, involving
   review and approval or rejection, or it might be active, involving
   direct modification of the message.  An MSA implements a server
   function to MUAs and a client function to MTAs (or MDAs).

   Examples of MSA-styled functions, in the world of paper mail, might
   range across the very different capabilities of administrative
   assistants, postal drop boxes, and post office front-counter
   employees.

   The MUA/MSA interface can be implemented within a single host and use
   private conventions for its interactions.  Historically, standards-
   based MUA/MSA interactions have used SMTP [RFC2821].  However a
   recent alternative is SUBMISSION [RFC2476].  Although SUBMISSION
   derives from SMTP, it operates on a separate TCP port, and will
   typically impose distinct requirements, such as access authorization.

   Identities relevant to the MSA include:

   RFC2821.HELO or RFC2821.EHLO

      Set by: Source

      The MSA may specify its hosting domain identity for the SMTP HELO
      or EHLO command operation.







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   RFC2821.MailFrom

      Set by: Source

      This is an end-to-end string that specifies an email address for
      receiving return control information, such as "bounces".  The name
      of this field is misleading, because it is not required to specify
      either the author or the agent responsible for submitting the
      message.  Rather, the agent responsible for submission specifies
      the RFC2821.MailFrom address.  Ultimately the simple basis for
      deciding what address needs to be in the RFC2821.MailFrom is to
      determine what address needs to be informed about transmission-
      level problems (and, possibly, successes.)

   RFC2821.RcptTo

      Set by: Originator

      This specifies the MUA mailbox address of a recipient.  The string
      might not be visible in the message content header.  For example,
      the message destination address header fields, such as RFC2822.To,
      might specify a mailing list address, while the RFC2821.RcptTo
      address specifies a member of that list.

   RFC2821.Received

      Set by: Source

      An MSA may record a Received header field, to indicate initial
      submission trace information, including originating host and MSA
      host domain names and/or IP Addresses.


4.4  Mail Transfer Agent (MTA)

   A Mail Transfer Agent (MTA) relays mail for one, application-level
   "hop".  It is like a packet-switch or IP router in that its job is to
   make routing assessments and to move the message closer to the
   Recipient(s).  Relaying is performed by a sequence of MTAs, until the
   message reaches its destination MDA(s).  Hence an MTA implements both
   client and server MTA functionality.  It does not make changes to
   addresses in the envelope or reformulate the content, except as
   transfer-encoding requirements dictate.  Also it may add trace
   information.  Of course email objects are typically much larger than
   the payload of a packet or datagram, and the end-to-end latencies are
   typically much higher.

   Internet Mail primarily uses SMTP [RFC2821], [RFC0821] to effect



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   point-to-point transfers between peer MTAs.  Other transfer
   mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645].  As with
   most network layer mechanisms Internet Mail's SMTP supports a basic
   level of reliability, by virtue of providing for retransmission after
   a temporary transfer failure.  Contrary to typical packet switches
   (and Instant Messaging services) Internet Mail MTAs typically store
   messages in a manner that allows recovery across service
   interruptions, such as host system shutdown.  However the degree of
   such robustness and persistence by an MTA can be highly variable.

   The primary "routing" mechanism for Internet Mail is the DNS MX
   record [RFC1035], which specifies a host, through which the queried
   domain can be reached.  This presumes a public -- or at least a
   common -- backbone that permits any attached host to connect to any
   other.

   An important characteristic of MTA-MTA communications, over the open
   Internet, is that they do not require prior arrangement between the
   independent administrations operating the different MTAs.  Given the
   importance of spontaneity and serendipity in the world of human
   communications, this lack of prearrangement, between the
   participants, is a core benefit of Internet Mail and remains a core
   requirement for it.

   Identities relevant to the MTA include:

   RFC2821.HELO

      Set by: Relay

      The MTA may specify its hosting domain identity for the SMTP HELO
      or EHLO command.  This is the only standardized way of identifying
      the agent responsible for operation of the Relay, during the
      transfer operation.

   RFC2821.MailFrom

      Set by: Source

      This is an MHS end-to-end string that specifies an email address
      for receiving return control Bounce, such as delivery
      confirmations and error notices.  The protocol name of this field
      is misleading, because it is not required to specify either the
      author or the agent responsible for submitting the message.
      Rather, the agent responsible for submission specifies the
      MailFrom address.  Ultimately the simple basis for deciding what
      address needs to be in the RFC2821.MailFrom is to determine what
      address needs to be informed about transmission-level problems



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      (and, possibly, successes.)

   RFC2821.RcptTo

      Set by: Originator

      This specifies the MUA mailbox address of a Recipient.  The string
      might not be visible in the message content header.  For example,
      the message destination address header fields, such as RFC2822.To,
      might specify a mailing list address, while the RFC2821.RcptTo
      address specifies a member of that list.

   RFC2822.Received

      Set by: Relay

      An MTA must record a Received header field, to indicate trace
      information, including source host and receiving host domain names
      and/or IP Addresses.


4.5  Mail Delivery Agent (MDA)

   A Mail Delivery Agent (MDA) delivers email to the Recipient's
   mailbox.  It can provide distinctive, address-based functionality,
   made possible by its detailed knowledge of the properties of the
   destination address.  This knowledge might also be present elsewhere
   in the Recipient's Administrative Unit, such as at an organizational
   border Relay.  However it is required for the MDA, if only because
   the MDA must know where to deliver the message.

   Using Internet protocols, delivery can be effected by a variety of
   standard protocols.  When coupled with an internal, local mechanism,
   SMTP [RFC2821] and LMTP [RFC2033] permit "push" delivery to the
   Recipient system, at the initiative of the upstream email service.
   POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the
   initiative of the Recipient system.  POP and IMAP can also be used
   for repeated access to messages on a remote MS.

   Identities relevant to the MDA include:

   RFC2821.Return-Path

      Set by: Source







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      The MDA records the RFC2821.MailFrom address into the
      RFC2822.Return-Path field.

   RFC2822.Received

      Set by: Destination

      An MDA must record a Received header field, to indicate trace
      information, including source host and receiving host domain names
      and/or IP Addresses.


4.6  Message Store (MS)

   An MUA can use a long-term Message Store (MS).  A rich set of choices
   for the use of that store derives from permitting more than one to be
   associated with a single user, demonstrated as a server-based MS
   (sMS) and user-based MS (uMS) in Figure 5. sMS is shown as being
   remote from the MUA and uMS as being local.  Further the relationship
   between two message store may vary.  Between the MDA and the MUA,
   these choices are supported by a wide variety of protocol options.

   The operational relationship among two MSs can be:

   Online:

      Only a remote MS is used, with messages being accessible only when
      the MUA is attached to the MS, and the MUA repeatedly fetches all
      or part of a message, from one session to the next.

   Offline:

      The MS is local to the user, and messages are moved from any
      remote store, rather than (also) being retained there.

   Disconnected:

      A remote MS and a local MS synchronize all or parts of their
      contents, while connected.  The user may make changes while
      disconnected, and the two stores are re-synchronized upon
      reconnection.


5.  Mediators

   Basic email transfer is accomplished with an asynchronous store-and-
   forward communication infrastructure, in a sequence of independent
   transmissions through some number of MTAs.  A very different task is



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   a User-level sequence of postings and deliveries, through Mediators.
   For such re-postings, a Mediator does share some functionality with
   basic MTA relaying, but it enjoys a degree of freedom with both
   addressing and content that is not available to MTAs.

   RFC2821.HELO or RFC2821.EHLO

      Set by: Source or Relay

      The MSA may specify its hosting domain identity for the SMTP HELO
      or EHLO command operation.

   RFC2821.MailFrom

      Set by: Source

      This is an end-to-end string that specifies an email address for
      receiving return control Bounces.  The name of this field is
      misleading, because it is not required to specify either the
      author or the agent responsible for submitting the message.
      Rather, the agent responsible for submission specifies the
      RFC2821.MailFrom address.  Ultimately the simple basis for
      deciding what address needs to be in the RFC2821.MailFrom is to
      determine what address needs to be informed about transmission-
      level problems (and, possibly, successes.)

   RFC2821.RcptTo

      Set by: Mediator

      This specifies the MUA mailbox address of a Recipient.  The string
      might not be visible in the message content header.  For example,
      the message destination address header fields, such as RFC2822.To,
      might specify a mailing list address, while the RFC2821.RcptTo
      address specifies a member of that list.

   RFC2821.Received

      Set by: Mediator

      An MSA may record a Received header field, to indicate initial
      submission trace information, including originating host and MSA
      host domain names and/or IP Addresses.

   The salient aspect of a Mediator, that distinguishes it from any
   other MUA creating an entirely new message, is that a Mediator
   preserves the integrity and tone of the original message, including
   the essential aspects of the original origination information.  The



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   Mediator might also add commentary.

   Examples of MUA message creation that are NOT performed by Mediators
   include:

   New message forwarding existing message:

      This action rather curiously provides a basic template for a class
      of Mediators.  However for it's typical occurrence it is not
      itself an example of a Mediator.  The new message is viewed as
      being from the Agent doing the forwarding, rather than being from
      the original Originator.

      A new message encapsulates the original message and is seen as
      strictly "from" the Mediator.  The Mediator might add commentary
      and certainly has the opportunity to modify the original message
      content.  The forwarded message is therefore independent of the
      original message exchange and creates a new message dialogue.
      However the final Recipient sees the contained message as from the
      original Originator.

   Reply:

      When a Recipient formulates a response to a message, the new
      message is not typically viewed as being a "forwarding" of the
      original.  It's focus is the new content; any inclusion of
      material from the original message is contextual and secondary.

   Annotator:

      The integrity of the original message is usually preserved, but
      one or more comments about the message are added in a manner that
      distinguishes commentary from original text.  The tone of the new
      message is that it is primarily commentary from a new Originator,
      similar to a Reply.

   The remainder of this section describes common examples of Mediators.

5.1  Aliasing

   A simple re-addressing facility that is available in most MDA
   implementations is called Aliasing.  It is performed just before
   delivering a message to the specified Recipient's mailbox.  Instead,
   the message is submitted back to the transfer service, for delivery
   to one or more alternate addresses.  Although implemented as part of
   the message delivery service, this facility is strictly a Recipient
   user function.  It resubmits the message, replacing the envelope
   address, on behalf of the mailbox address that was listed in the



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   envelope.

   What is most distinctive about this forwarding mechanism is how
   closely it compares to normal MTA store-and-forward Relaying.  In
   reality its only interesting difference is that it changes the
   RFC2821.RcptTo value.  Having the change be this small makes it easy
   to view aliasing as a part of the lower-level mail relaying activity.
   However the small change has a large semantic impact: The designated
   recipient has chosen a new recipient.  Hence, that original recipient
   must become responsible for any handling issues.

   An MDA that is re-posting a message to an alias typically changes
   only envelope information:

   RFC2822.TO, RFC2822.CC, RFC2822.BCC

      Set by: Originator

      These retain their original addresses.

   RFC2821.RcptTo

      Set by: Mediator

      This field contains an alias address.

   RFC2821.MailFrom

      Set by: Mediator or original Source

      The agent responsible for submission to an alias address will
      often retain the original address to receive handling Bounces.
      The benefit of retaining the original MailFrom value is to ensure
      that the origination-side agent knows that there has been a
      delivery problem.  On the other hand, the responsibility for the
      problem usually lies with the Recipient, since the Alias mechanism
      is strictly under the Recipient's control.

   RFC2821.Received

      Set by: Mediator

      The agent should record Received information, to indicate the
      delivery to the original address and submission to the alias
      address.  The trace of Received header fields should therefore
      include everything from original posting through final delivery to
      the alias.




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5.2  ReSending

   Also called ReDirecting, ReSending differs from Forwarding by virtue
   of having the Mediator "splice" a message's addressing information,
   to connect the Originator of the original message and the Recipient
   of the new message.  This permits them to have direct exchange, using
   their normal MUA Reply functions.  Hence the new Recipient sees the
   message as being From the original Originator, even if the Mediator
   adds commentary.

   Identities specified in a resent message include

   RFC2822.From

      Set by: original Originator

      Names and email addresses for the original author(s) of the
      message content are retained.  The free-form (display-name)
      portion of the address might be modified to provide informal
      reference to the agent responsible for the redirection.

   RFC2822.Reply-To

      Set by: original Originator

      If this field is present in the original message, it is retained
      in the Resent message.

   RFC2822.Sender

      Set by: original Source

      This field is expected to contain the original Sender value.

   RFC2822.TO, RFC2822.CC, RFC2822.BCC

      Set by: original Originator

      These specify the original message Recipients.

   RFC2822.Resent-From

      Set by: Mediator

      The address of the original Recipient who is redirecting the
      message.  Otherwise, the same rules apply for the Resent-From
      field as for an original RFC2822.From field




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   RFC2822.Resent-Sender

      Set by: Mediator

      The address of the agent responsible for re-submitting the
      message.  For efficiency this field is often omitted if it
      contains the same address as RFC2822.Resent-From.  However this
      does not mean there is no Resend-Sender specified.  Rather, it
      means that that header field is virtual and that the address in
      the Resent-From field must be used.  Specification of the error
      return addresses (the Notification address, contained in
      RFC2821.MailFrom) is made by the Resent-Sender.  Typically the
      Bounce address is the same as the Resent-Sender address.  However
      some usage scenarios require it to be different.

   RFC2822.Resent-To, RFC2822.Resent-cc, RFC2822.Resent-bcc:

      Set by: Mediator

      The addresses of the new Recipients who will now be able to reply
      to the original author.

   RFC2821.MailFrom

      Set by: Mediator

      The agent responsible for re-submission (RFC2822.Resent-Sender) is
      also responsible for specifying the new MailFrom address.

   RFC2821.RcptTo

      Set by: Mediator

      This will contain the address of a new Recipient

   RFC2822.Received

      Set by: Mediator

      When resending a message, the submission agent may record a
      Received header field, to indicate the transition from original
      posting to resubmission.


5.3  Mailing Lists

   Mailing lists have explicit email addresses and they forward messages
   to a list of subscribed members.  The Mailing List Actor performs a



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   task that can be viewed as an elaboration of the ReDirector role.  In
   addition to sending the new message to a potentially large number of
   new Recipients, the Mediator can modify content, such as deleting
   attachments, formatting conversion, and adding list-specific
   comments.  In addition, archiving list messages is common.  Still,
   the message retains characteristics of being "from" the original
   Originator.

   Identities relevant to a mailing list processor, when submitting a
   message, include:

   RFC2919.List-id

      Set by: Mediator

      This provides a global mailing list naming framework that is
      independent of particular hosts.  Although [RFC2919] is a
      standards-track specification, it has not gained significant
      adoption.

   RFC2369.List-*

      Set by: Mediator

      [RFC2369] defines a collection of message header fields for use by
      mailing lists.  In effect, they supply list-specific parameters
      for common mailing list user operations.  The identifiers for
      these operations are for the list, itself, and the user-as-
      subscriber.

   RFC2822.From

      Set by: original Originator

      Names and email addresses for the original author(s) of the
      message content are specified.

   RFC2822.Reply-To

      Set by: original Originator or Mediator

      Mailing lists have introduced an ambiguity for the Reply-To field.
      Some List operations choose to force all replies to go to all list
      members.  They achieve this by placing the list address into the
      RFC2822.Reply-To field.  Hence, direct, "private" replies only to
      the original author cannot be achieved by using the MUA's typical
      "reply to author" function.  If the author created a Reply-To
      field, its information is lost.



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   RFC2822.Sender

      Set by: original Source or Mediator

      This will usually specify the address of the agent responsible for
      mailing list operations.  However, some mailing lists operate in a
      manner very similar to a simple MTA Relay, so that they preserve
      as much of the original handling information as possible,
      including the original RFC2822.Sender field.

   RFC2822.TO, RFC2822.CC

      Set by: original Originator

      These will usually contain the original list of Recipient
      addresses.

   RFC2821.MailFrom

      Set by: original Source or Mediator

      This may contain the original address to be notified of
      transmission issues, or the mailing list agent may set it to
      contain a new Notification address.  Typically, the value is set
      to a new address, so that mailing list members and posters are not
      burdened with transmission-related Bounces.

   RFC2821.RcptTo

      Set by: Mediator

      This contains the address of a mailing list member.

   RFC2821.Received

      Set by: Mediator

      An Mailing List Agent should record a Received header field, to
      indicate the transition from original posting to mailing list
      forwarding.  The Agent may choose to have the message retain the
      original set of Received header fields or may choose to remove
      them.  In the latter case, it should ensure that the original
      Received header fields are otherwise available, to ensure later
      accountability and diagnostic access to it.







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5.4  Gateways

   Gateways perform the basic routing and transfer work of message
   relaying, but they also make any message or address modifications
   that are needed to send the message into the next messaging
   environment.  When a Gateway connects two differing messaging
   services, its role is easy to identify and understand.  When it
   connects environments that have technical similarity, but may have
   significant administrative differences, it is easy to think that a
   Gateway is merely an MTA.  The critical distinction between an MTA
   and a Gateway is that the latter transforms addresses and/or message
   content, in order to map between the standards of two, different
   messaging services.  In virtually all cases, this mapping process
   results in some degree of semantic loss.  The challenge of Gateway
   design is to minimize this loss.

   A Gateway may set any identity field available to a regular MUA.
   Identities typically relevant to Gateways include:

   RFC2822.From

      Set by: original Originator

      Names and email addresses for the original author(s) of the
      message content are retained.  As for all original addressing
      information in the message, the Gateway may translate addresses in
      whatever way will allow them continue to be useful in the target
      environment.

   RFC2822.Reply-To

      Set by: original Originator

      The Gateway should retain this information, if it is originally
      present.  The ability to perform a successful reply by a Gatewayed
      Recipient is a typical test of Gateway functionality.

   RFC2822.Sender

      Set by: original Source or Mediator

      This may retain the original value or may be set to a new address

   RFC2822.TO, RFC2822.CC, RFC2822.BCC







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      Set by: original Recipient

      These usually retain their original addresses.

   RFC2821.MailFrom

      Set by: original Source or Mediator

      The agent responsible for gatewaying the message may choose to
      specify a new address to receive handling notices.

   RFC2822.Received

      Set by: Mediator

      The Gateway may record a Received header field, to indicate the
      transition from original posting to the new messaging environment.


5.5  Boundary Filter

   Organizations often enforce security boundaries by subjecting
   messages to analysis, for conformance with the organization's safety
   policies.  An example is detection of content classed as spam or a
   virus.  A Filter might alter the content, to render it safe, such as
   by removing content deemed unacceptable.  Typically these actions
   will result in the addition of content that records the actions.

6.  Security Considerations

   This document does not specify any new Internet Mail functionality.
   Consequently it should introduce no new security considerations.

   However its discussion of the roles and responsibilities for
   different mail service modules, and the information they create,
   highlights the considerable security considerations that must be
   present when implementing any component of the Internet Mail service.
   In addition, email transfer protocols can operate over authenticated
   and/or encrypted links, and message content can be authenticated or
   encrypted.

7.  References

7.1  References - Normative

   [ID-hdr-reg]
              "Registration of mail and MIME header fields",
              draft-klyne-hdrreg-mail-04.txt (work in progress),



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              Apr 2004.

   [RFC0821]  Postel, J., "Simple Mail Transfer Protocol", STD 10,
              RFC 821, August 1982.

   [RFC0822]  Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2033]  Myers, J., "Local Mail Transfer Protocol", RFC 2033,
              October 1996.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, November 1996.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2298]  Fajman, R., "An Extensible Message Format for Message
              Disposition Notifications", RFC 2298, March 1998.

   [RFC2304]  Allocchio, C., "Minimal FAX address format in Internet
              Mail", RFC 2304, March 1998.



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   [RFC2369]  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.

   [RFC2421]  Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
              Mail - version 2", RFC 2421, September 1998.

   [RFC2423]  Vaudreuil, G. and G. Parsons, "VPIM Voice Message MIME
              Sub-type Registration", RFC 2423, September 1998.

   [RFC2442]  "The Batch SMTP Media Type", RFC 2442, November 1998.

   [RFC2476]  Gellens, R. and J. Klensin, "Message Submission",
              RFC 2476, December 1998.

   [RFC2645]  "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
              Addresses", RFC 2465, August 1999.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC2919]  Chandhok, R. and G. Wenger, "List-Id: A Structured Field
              and Namespace for the Identification of Mailing Lists",
              RFC 2919, March 2001.

   [RFC3028]  Showalter, T., "Sieve: A Mail Filtering Language",
              RFC 3028, January 2001.

   [RFC3297]  Klyne, G., Iwazaki, R., and D. Crocker, "Content
              Negotiation for Messaging Services based on Email",
              RFC 3297, July 2002.

   [RFC3458]  Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
              Context for Internet Mail", RFC 3458, January 2003.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, January 2003.

   [RFC3501]  Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 3501, March 2003.

   [RFC4021]  Klyne, G. and J. Palme, "Registration of Mail and MIME
              Header Fields", RFC 4021, March 2005.




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7.2  Reference - Descriptive

   [ID-ffpim]
              Crocker, D. and G. Klyne, "Full-mode Fax Profile for
              Internet Mail: FFPIM", March 2004.

   [ID-spamops]
              Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
              E. Allman, "Email Submission Between Independent
              Networks", draft-spamops-00 (work in progress),
              March 2004.

   [RFC1767]  Crocker, D., "MIME Encapsulation of EDI Objects",
              RFC 1767, March 1995.

   [Tussle]   Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
              "Tussle in Cyberspace: Defining Tomorrow‚ÇÖs Internet",
              ACM SIGCOMM, 2002.


Author's Address

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Drive
   Sunnyvale, CA  94086
   USA

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net

Appendix A.  Acknowledgements

   This work derives from a section in draft-hutzler-spamops [ID-
   spamops].  Discussion of the Source actor role was greatly clarified
   during discussions in the IETF's Marid working group.

   Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
   insight on the framework and details of the original drafts.

   Later reviews and suggestions were provided by Nathaniel Borenstein,
   Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ned Freed,
   Eric Hall, Brad Knowles, Bruce Lilly, Mark E. Mallett, David
   MacQuigg, Chris Newman, Daryl Odnert, Rahmat M. Samik-Ibrahim, Hector
   Santos, Jochen Topf, Willemien Hoogendoorn.






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