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SMTP                                                          D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Standards Track                             May 6, 2007
Expires: November 7, 2007


                       Internet Mail Architecture
                      draft-crocker-email-arch-07

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Over its thirty-five 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 networked email specified little more than a simple split between
   the user world and the transmission world.  Core aspects of the
   service, such as the styles of mailbox address and basic message
   format, have remained remarkably constant.  However today's Internet
   Mail is marked by many independent operators, many different



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   components for providing service to users and many others for
   performing message transfer.  Public discussion of the service often
   lacks common terminology and a common frame of reference for these
   components and their activities.  Having a common reference model and
   terminology makes a basic difference when talking about problems with
   the service, changes in policy, or enhancement to the service's
   functionality.  This document offers an enhanced Internet Mail
   architecture that targets description of the existing service, in
   order to facilitate clearer and more efficient technical, operations
   and policy discussions about email.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Service Overview . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Document Conventions . . . . . . . . . . . . . . . . . . .  6
   2.  Responsible Actor Roles  . . . . . . . . . . . . . . . . . . .  6
     2.1.  User Actors  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Mail Handling Service (MHS) Actors . . . . . . . . . . . . 10
     2.3.  Administrative Actors  . . . . . . . . . . . . . . . . . . 13
   3.  Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.1.  Mailbox  . . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.2.  Domain Names . . . . . . . . . . . . . . . . . . . . . . . 16
     3.3.  Message Identifier . . . . . . . . . . . . . . . . . . . . 17
   4.  Services and Standards . . . . . . . . . . . . . . . . . . . . 18
     4.1.  Message Data . . . . . . . . . . . . . . . . . . . . . . . 21
     4.2.  User-Level Services  . . . . . . . . . . . . . . . . . . . 26
     4.3.  MHS-Level Services . . . . . . . . . . . . . . . . . . . . 28
   5.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     5.1.  Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 32
     5.2.  Re-Sending . . . . . . . . . . . . . . . . . . . . . . . . 34
     5.3.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 36
     5.4.  Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 37
     5.5.  Boundary Filter  . . . . . . . . . . . . . . . . . . . . . 39
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 39
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     8.1.  Normative  . . . . . . . . . . . . . . . . . . . . . . . . 39
     8.2.  Descriptive  . . . . . . . . . . . . . . . . . . . . . . . 42
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 42
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 43
   Intellectual Property and Copyright Statements . . . . . . . . . . 44







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1.  Introduction

   Over its thirty-five year history Internet Mail has undergone
   significant changes in scale and complexity, as it has become a
   global infrastructure service.  The changes have been evolutionary,
   rather than revolutionary, reflecting a strong desire to preserve its
   installed base of users and utility.  Today, Internet Mail is marked
   by many independent operators, many different components for
   providing service to users and many other components for performing
   message transfer.

   Public collaboration on email technical, operations and policy
   activities, including those responding to the challenges of email
   abuse, has brought in a much wider range of participants than email's
   technical community originally had.  In order to do work on a large,
   complex system, they need to share the same view of how it is put
   together, as well as what terms to use to refer to the pieces and
   their activities.  Otherwise, it is difficult to know exactly what
   another participant means.  It is these differences in each person's
   perspective that motivates this document, to describe the realities
   of the current system.  Internet mail is the subject of ongoing
   technical, operations and policy work, and the discussions often are
   hindered by different models of email service design and different
   meanings for the same terms.  This architecture document seeks to
   facilitate clearer and more efficient technical, operations and
   policy exchanges about email.

   This document offers an enhanced Internet Mail architecture to
   reflect the current service.  In particular it:



      *  Documents refinements to the email model

      *  Clarifies functional roles for the architectural components

      *  Clarifies identity-related issues, across the email service

      *  Defines terminology for architectural components and their
         interactions

1.1.  Background

   The first standardized architecture for networked 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



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   to one or more others, creating a virtual MUA-to-MUA exchange
   environment.

   As shown in Figure 1 this defines two logical "layers" of
   interoperability.  One is directly between Users.  The other is
   between the neighboring components, along the transfer path.  In
   addition, there is interoperability between the layers, first when a
   message is posted from the User to the MHS and later when it is
   delivered from the MHS to the User.

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

                Figure 1: Basic Internet Mail Service Model

   As it has evolved, the operational service has sub-divided each of
   these layers into more specialized modules.  Core aspects of the
   service, such as mailbox addressing and message format style, have
   remained remarkably constant.  So the original distinction between
   user-level concerns and transfer-level concerns is retained, but with
   an elaboration to each level of the architecture.  The term "Internet
   Mail" is used to refer to the entire collection of user and transfer
   components and services.

   For Internet Mail the term "end-to-end" usually refers to a single
   posting and the set of deliveries directly resulting from its single



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   transiting of the MHS.  A common exception is with group dialogue
   that is mediated via a mailing list, so that two postings occur
   before intended recipients receive an originator's message, as
   discussed in Section 2.1.3.  In fact 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 [RFC3801], EDI over email [RFC1767] and facsimile over email.
   [RFC4142]

1.2.  Service Overview

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



      *  An email object

      *  Global addressing

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

      *  No prior arrangement between Originator and Recipient

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

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

   The end-to-end portion of the service is the email object, called a
   message.  Broadly the message, itself, distinguishes between control
   information for handling, versus the author's message content.

   A precept to the design of mail over the open Internet is permitting
   user-to-user and MTA-to-MTA interoperability to take place with no
   prior, direct arrangement between the independent administrative
   authorities that are responsible for handling a message.  That is,
   all participants rely on having the core services be universally
   supported and accessible, either directly or through gateways that
   translate between Internet Mail standards and other email
   environments.  Given the importance of spontaneity and serendipity in
   the world of human communications, this lack of prearrangement
   between participants is a core benefit of Internet Mail and remains a
   core requirement for it.

   Within localized networks at the edge of the public Internet, prior



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   administrative arrangement often is required and 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 a server performs explicit validation of the client's
   identity.

1.3.  Document Conventions

   In this document, references to structured fields of a message use a
   two-part dotted notation.  The first part cites the document that
   contains the specification for the field and the second is the name
   of the field.  Hence <RFC2822.From> is the From field in an email
   content header and <RFC2821.MailFrom> is the address in the SMTP
   "Mail From" command.

   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 specified in RFC 2119 [RFC2119].

   Discussion venue:   Please direct discussion about this document to
      the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>.

   Changes:

      Added text to explain utility of having an architecture document.

      Added text explaining benefit of the ADMD construct.

      Added commentary on List-ID.

      Moved Bounce out of MHS in figure.

      Moved "generic" Identity field case-analysis text into common area
      after Table 1, reserving per-role text for per-role peculiarities.

      Extensive word-smithing and cleanup.


2.  Responsible Actor Roles

   Internet Mail is a highly distributed service, with a variety of
   actors serving different roles.  These divide into 3 basic types:



      *  User




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      *  Mail Handling Service (MHS)

      *  ADministrative Management Domain (ADMD)

   Although related to a technical architecture, the focus on Actors
   concerns participant responsibilities, rather than on functionality
   of modules.  Hence the labels used are different than for classic
   email architecture diagrams.

2.1.  User Actors

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



      *  Originators

      *  Recipients

      *  Mediators

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






















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   The following depicts the flow of messages among Actors:

   +------------+
   |            |<---------------------------+
   | Originator |<----------------+          |
   |            |<----+           |          |
   +-+---+----+-+     |           |          |
     |   |    |       |           |          |
     |   |    V       |           |          |
     |   |  +---------+-+         |          |
     |   |  | Recipient |         |          |
     |   |  +-----------+         |          |
     |   |                        |          |
     |   |       +--------+       |          |
     |   |       |        |       |          |
     |   V       V        |       |          |
     | +-----------+    +-+-------+-+        |
     | | Mediator  +--->| Recipient |        |
     | +-----------+    +-----------+        |
     |                                       |
     |       +-----------------------------+ |
     |       |                +----------+ | |
     |       |                |          | | |
     V       V                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 user-level Author role.

2.1.2.  Recipient

   The Recipient is a consumer of delivered content.  As described
   below, the MHS has a "Dest[ination]" role that correlates with the
   user-level Recipient role.

   A Recipient can 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



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   handling of the message.  (See Section 4.1.)

2.1.3.  Mediator

   A Mediator receives, aggregates, reformulates and redistributes
   messages as part of a potentially-protracted, higher-level exchange
   among Users.  Example Mediators include group dialogue, such as
   collaboration via mailing lists, 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 in
   very different ways.  Mediators are considered extensively in
   Section 5.

   When mail is delivered to a receiving mediator specified in the
   RFC2821.RcptTo command, the MHS handles it the same way as for any
   other Recipient.  That is, the MHS only sees posting and delivery
   sources and sinks and does not see (later) re-posting as a
   continuation of a process.  Hence when submitting messages, the
   Mediator is an Originator.

   The distinctive aspects of a Mediator are, therefore, above the MHS.
   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 can be complex and contingent, such as by modifying
   and adding content or regulating which users are allowed to
   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 can 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, and a detailed
      discussion is in Section 2.2.4.





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2.2.  Mail Handling Service (MHS) Actors

   The Mail Handling Service (MHS) has the task of performing a single,
   end-to-end transfer on behalf of the Originator and reaching the
   Recipient address(es) specified in the original RFC2821.RcptTo
   commands.  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.

   The following depicts the relationships among transfer participants
   in Internet Mail.  It shows the Source as distinct from the
   Originator, and Dest[ination] as distinct from Recipient, although it
   is common for each pair to be the same actor.  Transfers typically
   entail one or more Relays.  However direct delivery from the Source
   to Destination is possible.  For intra-organization mail services, it
   is common to have only one Relay.

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

                 Figure 3: Relationships Among MHS Actors







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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 can simply review the message for
   conformance and reject it if there are errors, or it can 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
   MHS, that is, the local ADministrative Management Domain (ADMD).

   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 can 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.

   NOTE:

      The choice of the label "Bounce" is unfortunate, due to its
      negative implication and narrow focus.  However it is the most
      popular term for the address.

2.2.3.  Relay

   A mail Relay performs email transfer-service routing and store-and-
   forward by (re-)transmitting the message on towards its Recipient(s).
   A Relay can add information to the envelope, such as with trace
   information.  However it does not modify existing envelope
   information or the message content semantics.  It can 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 (MHS) network.  This



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   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:



      *  User Mediators

      *  MHS Relays

      *  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 address.  The
   potential for looping is avoided by having this message, itself,
   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.
   One difference could have the concept of an email address be a
   hierarchical, machine-specific address, versus having it be a flat,
   global name space.  Another difference could be between text-only
   content, versus multi-media.  Hence the Relay function in a Gateway
   offers significant design challenges, to make the result be as
   seamless as possible.  The most 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 useful 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



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   Originator and Recipient can sustain a dialogue.  In particular can a
   Recipient's MUA automatically formulate a valid Reply that will reach
   the initial Originator?

2.3.  Administrative Actors

   Actors often will are associated with different organizations, each
   with its own administrative authority.  This operational
   independence, coupled with the need for interaction between groups,
   provides the motivation for distinguishing among ADministrative
   Management Domains (ADMD).  Each ADMD can have vastly different
   operating policies and trust-based decision-making.  An obvious
   example is the distinction between mail that is exchanged within a
   single organization, versus mail that is exchanged between
   independent organizations.  The rules for handling these two types of
   traffic tend to be quite different.  That difference requires
   defining the boundaries of each, and this requires the ADMD
   construct.

   Operation of Internet Mail services is apportioned to different
   providers (or operators).  Each can be an independent ADMD.  This
   independence of administrative decision-making defines boundaries
   that distinguish different portions of the Internet Mail service.
   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 ADMD potentially having a
   complex arrangement of functional components.  Figure 4 depicts
   relationships among ADMDs.  The benefit of having the ADMD construct
   is to facilitate discussions and designs that need to distinguish
   between "internal" issues and "external" ones.

   The architectural impact of needing to have boundaries between ADMD's
   is discussed in [Tussle].  Most significant is that the entities
   communicating across ADMD boundaries will typically have an added
   burden to enforce organizational policies concerning "external"
   communications.  At a more mundane level, the basis for routing mail
   between ADMDs is often an issue.

   Basic types of ADMDs include --



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





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      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 ADMDs, such as aggregation and
         filtering.

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

   +-------+                           +-------+    +-------+
   | ADMD1 |                           | ADMD3 |    | ADMD4 |
   | ----- |                           | ----- |    | ----- |
   |       |   +---------------------->|       |    |       |
   | User  |   |                       |-Edge--+--->|-User  |
   |  |    |   |    +---------+   +--->|       |    |       |
   |  V    |   |    |  ADMD2  |   |    +-------+    +-------+
   | Edge--+---+    |  -----  |   |
   |       |   |    |         |   |
   +-------+   +----|-Transit-+---+
                    |         |
                    +---------+

        Figure 4: ADministrative Management Domains (ADMD) Example

   Edge networks can 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 distinction 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 ADMD 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 can be implemented in
   different functional components, according to the needs of the ADMD.
   For example see [ID-spamops].

   User, Edge and Transit services can be offered by providers that
   operate component services or sets of services.  Further it is
   possible for one ADMD to host services for other ADMDs.




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   Common ADMD 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
         necessarily their job to perform email functions, but they can,
         instead, 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: mailbox, domain name and
   message-id.  Each is required to be globally unique.

3.1.  Mailbox

      "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 is a globally interpreted domain name that is part of an ADMD.
   Domain Names are discussed in Section 3.2.  Formal Internet Mail
   addressing syntax can support source routes, to indicate the path
   through which a message should be sent.  Although legal, the use of
   source routes is not part of the modern Internet Mail service and it
   is ignored in the rest of this document.

   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



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   interpreted only by the entity specified by the address's right-hand
   side domain name.  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
   Web browser "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
   used by the same participant.  However it is worth stressing 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, and those specialized services
   conforming to standardized conventions, as noted in the next
   paragraph.

   There are a few types of addresses that have 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 [RFC3801], such as
   "+16137637582@vpim.example.com", and iFax [RFC3192], 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 domain name usually maps to one or
   more IP Addresses.  Conceptually the name might encompass an entire
   organization, 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



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   name is structured as a hierarchical sequence of sub-names, separated
   by dots ("."), with the top of the hierarchy being on the right-end
   of the sequence.  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
   Mail to refer to the ADMD or the host that took action upon the
   message, such as providing the administrative scope for a message
   identifier, or performing transfer processing.

3.3.  Message Identifier

   There are two standardized tags, for identifying messages: Message-ID
   and ENVID.

3.3.1.  Message-ID

   The Message-ID is a user-level tag, primarily used for threading and
   for eliminating duplicates.  [RFC2822].  It is associated with the
   RFC2822.From field, although any actor within the originating ADMD
   might assign it.  The recipient's ADMD is the intended consumer of
   the Message-ID, although any actor along the transfer path might use
   it.  Internet Mail standards provide for a single Message-ID; however
   more than one is sometimes assigned.

   Like a mailbox address, a Message-ID has two distinct parts, divided
   by an at-sign ("@").  The right-hand side is globally interpreted and
   specifies the ADMD or host assigning the identifier.  The left-hand
   side 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.

   When a message is revised in any way, the question of whether to
   assign a new Message-ID requires a subjective assessment, deciding
   whether the editorial content has been changed enough to constitute a
   new message.  [RFC2822] says "a message identifier pertains to
   exactly one instantiation of a particular message; subsequent
   revisions to the message each receive new message identifiers."
   However real-world experience dictates some flexibility.  An
   impossible test is whether the recipient will consider the new
   message to be equivalent to the old.  For most components of Internet
   Mail, there is no way to predict a specific recipient's preferences
   on this matter.  Both creating and failing to create a new Message-ID
   have their downsides.

   The best that can be offered, here, are some guidelines and examples:




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   o  If a message is changed only in terms of form, such as character-
      encoding, it clearly is still the same message.

   o  If a message has minor additions to the content, such as a mailing
      list tag at the beginning of the RFC2822.Subject header field, or
      some mailing list administrative information added to the end of
      the primary body-part's text, then it probably is still the same
      message.

   o  If a message has viruses deleted from it, it probably is still the
      same message.

   o  If a message has offensive words deleted from it, then some
      recipients will consider it the same message, but some will not.

   o  If a message is translated into a different language, then some
      recipients will consider it the same message, but some will not.

   The absence of objective, precise criteria for Message-ID re-
   generation, along with the absence of strong protection associated
   with the string, means that the presence of an ID can permit an
   assessment that is marginally better than a heuristic, but the ID
   certainly has no value on its own for strict formal reference or
   comparison.  Hence it is not appropriate to use the Message-ID for
   any process that might be called "security".

3.3.2.  ENVID

   The ENVID (envelope identifier) is a tag that is primarily for use
   within Delivery Status Notifications (DSN), so that the Bounce
   Address (RFC2821.MailFrom) recipient can correlate the DSN with a
   particular message.  [RFC3461] The ENVID is therefore used from one
   message posting, until the directly-resulting message deliveries.  It
   does not survive re-postings.

   The format of an ENVID is free-form.  Although its creator might
   choose to impose structure on the string, none is imposed by Internet
   standards.  By implication, the scope of the string is defined by the
   domain name of the Bounce Address.


4.  Services and Standards

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





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      *  Message

      *  Mail User Agent (MUA)

      *  Message Submission Agent (MSA)

      *  Message Transfer Agent (MTA)

      *  Message Delivery Agent (MDA)

      *  Message Store (MS)

   This section describes each functional component for Internet Mail,
   and the standards-based protocols that are associated with their
   operation.

   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 can 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.

   The following figure shows function modules and the standardized
   protocols used between them.  Additional protocols and configurations
   are possible.  Boxes defined by asterisks (*) represent functions
   that often are distributed among two or more systems.











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                     +------+                              +-------+
         ............+ oMUA |..............................| Disp  |
         .           +--+-+-+                              +-------+
         .   local,imap}| |{smtp,submission                     ^
         .              | |                        +---------+  |
         . *******      | | .......................| Bounces |  |
         . * oMS *<-----+ | .                      +---------+  |
         . *******        | .   *****************       ^       |
         .         +------V-.---*------------+  *       |       |
         .     MSA | +-------+  *   +------+ |  *       |       |
         .         | | oMSA  +--O-->| hMSA | |  *       |       |
         .         | +-------+  *   +--+---+ |  *       |       |
         .         +------------*------+-----+  *       |       |
   /+==========+\               *      V {smtp  *       |       |
   || MESSAGE  ||               *   +------+    *  /+===+===+\  |
   ||----------||           MHS *   | MTA  |    *  ||  dsn  ||  |
   || Envelope ||               *   +--+---+    *  \+=======+/  |
   ||  SMTP    ||               *      V {smtp  *     ^   ^     |
   ||  RFC2822 ||               *   +------+    *     |   | /+==+==+\
   || Content  ||               *   | MTA  +----*-----+   | || mdn ||
   ||  RFC2822 ||               *   +--+---+    *         | \+=====+/
   ||  MIME    ||               * smtp}| {local *         |     |
   \+==========+/   MDA         *      | {lmtp  *         |     |
         .         +------------+------V-----+  *         |     |
         .         | +------+   *   +------+ |  *         |     |
         .         | |      |   *   |      | +--*---------+     |
         .         | | rMDA |<--O---+ hMDA | |  *               |
         .         | |      |   *   |      | |<-*-------+       |
         .         | +-+----+   *   +------+ |  *       |       |
         .         +---+--+-----*------------+  *       |       |
         .             |  |     *****************       |       |
         .     pop} +--+  +---+                         |       |
         .    imap} |         | {local                  |       |
         .  ******************V********                 |       |
         .  *       |       +------+  * rMS        /+===+===+\  |
         .  *       |       | srMS |  *            || sieve ||  |
         .  *       V       +--+-+-+  *            \+=======+/  |
         .  *  +------+   pop} | |    *                 ^       |
         .  *  | urMS |<-------+ |    *                 |       |
         .  *  +--+---+  imap}   |    *                 |       |
         .  ***************************                 |       |
         .  local}|  +------+       |{pop,imap          |       |
         .        +->|      |<------+                   |       |
         ...........>| rMUA +---------------------------+       |
                     |      +-----------------------------------+
                     +------+

                     Figure 5: Protocols and Services



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4.1.  Message Data

   The purpose of the Mail Handling Service (MHS) is to exchange a
   message object among participants. , [RFC2822] [RFC0822] Hence all of
   its underlying mechanisms are merely in the service of getting that
   message from its Originator to its Recipients.  A message can 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 MHS, or generated by it.  The content is divided into a
   structured header and the body.  The body may be unstructured simple
   lines of text, or it may be a MIME tree of multi-media subordinate
   objects, called body-parts, or attachments.  [RFC2045], [RFC2046],
   [RFC2047], [RFC4288], [RFC4289], [RFC2049].

   In addition, Internet Mail has a few conventions for special control
   data:



      Delivery Status Notification (DSN):

         A Delivery Status Notification (DSN) is a message that can be
         generated by the MHS (MSA, MTA or MDA) and sent to the
         RFC2821.MailFrom address.  The mailbox for this is shown as
         Bounces in Figure 5.  It provides information about message
         transit, such as transmission errors or successful delivery.
         [RFC3461]

      Message Disposition Notification (MDN):

         A Message Disposition Notification (MDN) is a message that
         provides information about user-level, Recipient-side message
         processing, such as indicating that the message has been
         displayed [RFC3798] or the form of content that can be
         supported.  [RFC3297] It can be generated by an rMUA and is
         sent to the Disposition-Notification-To address(es).  The
         mailbox for this is shown as Disp in Figure 5.  It

      Message Filtering (SIEVE):

         SIEVE is a scripting language that permits specifying
         conditions for differential handling of mail, typically at the
         time of delivery.  [RFC3028] It can be conveyed in a variety of
         ways, as a MIME part.  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



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         more of them might be subject to Sieve directives, especially
         within a single ADMD.  Hence the Figure shows only one
         relationship, for (relative) simplicity.

4.1.1.  Envelope

   Information that is directly used by, or produced by, the MHS is
   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
   message object header fields.  Upon delivery, some SMTP-level
   envelope information is typically encoded within additional message
   object header fields, such as Return-Path.  [RFC2821],[RFC2822]

4.1.2.  Header Fields

   Header fields are attribute name/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 header fields are defined
   in [RFC4021].  An extensive set of existing header field
   registrations is provided in [RFC3864].

   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 hierarchically structured into a composition of multi-media body-
   part attachments, using MIME.  [RFC2045], [RFC2046], [RFC2047],
   [RFC4288], [RFC2049] MIME structures each body-part into a recursive
   set of MIME header field meta-data and MIME Content sections.



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4.1.4.  Identity References in a Message

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

       +-----------------------+-------------+---------------------+
       | 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 |
       +-----------------------+-------------+---------------------+

                            Layered Identities

   The most common address-related fields are:

   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) that are 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







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      Set by: Source

      This specifies the address responsible for submitting the message
      into the transfer service.  For efficiency this field can 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.  However some or all of the
      addresses in these fields might not be present in the
      RFC2821.RcptTo commands, due to handling process that might
      transfer from the former to the latter.

      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

      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 of the Originator.






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   RFC2821.HELO/.EHLO

      Set by: Source

      The MSA can 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 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 mailbox, while the RFC2821.RcptTo
      address specifies a member of that list.

   RFC2821.Received

      Set by: Source, Relay, Mediator, Dest

      This indicates trace information, including originating host,
      relays, Mediators, and MSA host domain names and/or IP Addresses.

   RFC2821.Return-Path

      Set by: Source

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







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   RFC2919.List-Id

      Set by: Mediator Originator

      This provides a globally unique mailing list naming framework that
      is independent of particular hosts.  [RFC2919]

      The identifier is in the form of a domain name; however the string
      usually is constructed by combining the two parts of an email
      address and the result rarely is a true domain name, listed in the
      domain name service -- although it can be.

   RFC2369.List-*

      Set by: Mediator Originator

      [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.
      [RFC2369]

4.2.  User-Level Services

   Interactions at the user level entail protocol exchanges, distinct
   from those that occur at lower layers of the Internet Mail
   architecture, which is above the Internet Transport layer.  Because
   the motivation for email, and much of its use, is for interaction
   among humans, the nature and details of these protocol exchanges
   often are determined by the needs of human and group communication.
   In terms of efforts to specify behaviors, one effect of this is to
   require subjective guidelines, rather than strict rules, for some
   aspects of system behavior.  Mailing Lists provide particularly
   salient examples of this.

4.2.1.  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.

   The Origination-side MUA (oMUA) creates a message and performs
   initial "submission" into the transfer infrastructure, via a Mail
   Submission Agent (MSA).  It can also perform any creation- and
   posting-time archival in its Message Store (oMS).  An MUA's oMS will
   typically include a folder for messages under development (Drafts), a
   folder for messages waiting to be sent (Queued or Unsent) and a
   folder for messages that have been successfully posted for
   transmission (Sent).



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

   NOTE:   Although not shown in Figure 5, an MUA can, 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
      standardized approach for such scenarios is defined by [RFC4550].

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

   Identity fields relevant to a typical end-user MUA include:



      RFC2822.From

      RFC2822.Reply-To

      RFC2822.Sender

      RFC2822.To, RFC2822.CC

      RFC2822.BCC

4.2.2.  Message Store (MS)

   An MUA can employ a long-term Message Store (MS).  Figure 5 depicts
   an Origination-side Ms (oMS) and a Recipient-side MS (rMS).  There is
   a rich set of choices for configuring a store, because any MS may
   comprise a distributed set of component stores.  In Figure 5, the rMS
   demonstrates this by showing an rMS that is located on a remote
   server (srMS) and an rMS that is on the same machine as the MUA
   (urMS).  The relationship between two message stores, themselves, can
   vary.

   As discussed in [RFC1733] the operational relationship among MSs can
   be --







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      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 completely moved
         from any remote store, rather than (also) being retained there.

      Disconnected:

         An rMS and a uMS are kept synchronized, for all or part of
         their contents, while there is a connection between them.
         While they are disconnected, mail can continue to arrive at the
         rMS and the user may continue to make changes to the uMS.  Upon
         reconnection, the two stores are re-synchronized.

4.3.  MHS-Level Services

4.3.1.  Mail Submission Agent (MSA)

   A Mail Submission Agent (MSA) accepts the message submission from the
   oMUA and enforces the policies of the hosting ADMD and the
   requirements of Internet standards.  An MSA represents an unusual
   functional dichotomy.  A portion of its task is to represent MUA
   (uMSA) interests during message posting, to facilitate posting
   success, and another portion is to represent MHS (hMSA) interests.
   This is best modeled, as shown in Figure 5, with two sub-components,
   one for the oMUA (oMSA) and one for the MHS (hMSA)

   The hMSA's function is to take transit responsibility for a message
   that conforms to the relevant Internet standards and to local site
   policies.  It rejects messages that are not in conformance.  The
   oMSA's is to perform final message preparation for submission and to
   effect the transfer of responsibility to the MHS, via the hMSA.  The
   amount of preparation will depend upon the local implementations.
   Examples of oMSA tasks could be to add header fields, such as Date:
   and Message-ID, to modify portions of the message from local
   notations to Internet standards, such as expanding an address to its
   formal RFC2822 representation.

   Historically, standards-based MUA/MSA interactions have used SMTP
   [RFC2821].  A recent alternative is SUBMISSION [RFC4409].  Although
   SUBMISSION derives from SMTP, it uses a separate TCP port and imposes
   distinct requirements, such as access authorization.




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   Identities relevant to the MSA include:



      RFC2821.HELO/.EHLO

      RFC2821.MailFrom

      RFC2821.RcptTo

      RFC2821.Received

4.3.2.  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 a destination MDA.  Hence an MTA implements both
   client and server MTA functionality.  It does not make changes to
   addresses in the envelope or reformulate the editorial content.
   Hence a change in data form, such as to the MIME Content-Transfer-
   Encoding, is within the purview of an MTA, whereas removal or
   replacement of body content is not.  Also it can 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
   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.

   Identities relevant to the MTA include:





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      RFC2821.HELO/.EHLO

      RFC2821.MailFrom

      RFC2821.RcptTo

      RFC2822.Received

         Set by: Relay Server

4.3.3.  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 ADMD, such as at an organizational border
   (Boundary) Relay.  However it is required for the MDA, if only
   because the MDA must know where to deliver the message.

   As with an MSA, an MDA serves two roles, as depicted in Figure 5.
   Formal transfer of responsibility, called "delivery" is effected
   between the two components that embody these roles.  The MHS portion
   (hMDA) primarily functions as a server SMTP engine.  A common
   additional role is to re-direct the message to an alternative
   address, as specified by the recipient addressee's preferences.  The
   job of the recipient portion of the MDA (rMDA) is to perform any
   delivery-actions are desired by the recipient.

   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







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         Set by: Originator Source or Mediator Source

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

      RFC2822.Received

         Set by: MDA server

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


5.  Mediators

   Basic email transfer from an Originator to the specified Recipients
   is accomplished by using an asynchronous, store-and-forward
   communication infrastructure, in a sequence of independent
   transmissions through some number of MTAs.  A very different task is
   a User-level sequence of postings and deliveries, through Mediators.
   A Mediator forwards a message, through a re-posting process.  The
   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/.EHLO

         Set by: Mediator Source

      RFC2821.MailFrom

         Set by: Originator Source or Mediator Source

      RFC2821.RcptTo

         Set by: Mediator Originator

      RFC2821.Received

         Set by: Mediator Dest

   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 its origination information.  The Mediator



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   might also add commentary.

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

   New message that forwards an existing message:

      This action rather curiously provides a basic template for a class
      of Mediators.  However for its 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 back to the original
      message's author, the new message is not typically viewed as being
      a "forwarding" of the original.  Its focus is the new content,
      although it might contain all or part of the material in the
      original message.  Therefore the earlier material is merely
      contextual and secondary.

   Annotation:

      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

   Aliasing is a simple re-addressing facility that is available in most
   MDA implementations.  It is performed just before placing a message
   into the specified Recipient's mailbox.  Instead the message is
   submitted back to the transfer service, for delivery to one or more
   alternate addresses.  Although typically implemented as part of an
   MDA, this facility is strictly a Recipient user function.  It



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   resubmits the message, replacing the envelope address, on behalf of
   the mailbox address that was listed in the envelope.

   What is most distinctive about this forwarding mechanism is how
   closely it compares to normal MTA store-and-forward Relaying.  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
   SHOULD become responsible for any handling issues.  This change would
   be reflected by replacing the message's RFC2821.MailFrom address to
   be one within the scope of the ADMD doing the aliasing.

   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 Originator

         This field contains an alias address.

      RFC2821.MailFrom

         Set by: Originator Source or Mediator 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







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         Set by: Mediator Dest

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

5.2.  Re-Sending

   Also called Re-Directing, Re-Sending 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: Originator Source or Mediator Source

      RFC2822.TO, RFC2822.CC, RFC2822.BCC

         Set by: original Originator






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         These specify the original message Recipients.

      RFC2822.Resent-From

         Set by: Mediator Originator

         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

      RFC2822.Resent-Sender

         Set by: Mediator Source

         The address of the agent responsible for re-submitting the
         message.  As with RFC2822.Sender, this field is often omitted
         when it would merely contain the same address as
         RFC2822.Resent-From.

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

         Set by: Mediator Originator

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

      RFC2821.MailFrom

         Set by: Mediator Source

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

      RFC2821.RcptTo

         Set by: Mediator Originator

         This will contain the address of a new Recipient

      RFC2822.Received

         Set by: Mediator Dest

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





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5.3.  Mailing Lists

   Mailing lists have explicit email addresses and they re-post messages
   to a list of subscribed members.  The Mailing List Actor performs a
   task that can be viewed as an elaboration of the Re-Director 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 Originator

      RFC2369.List-*

         Set by: Mediator Originator

      RFC2822.From

         Set by: original Originator

         Names and email addresses for the original author(s) of the
         message content are specified -- or, rather, retained.

      RFC2822.Reply-To

         Set by: original Originator or Mediator Originator

      RFC2822.Sender

         Set by: Originator Source or Mediator Source

         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.






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      RFC2822.TO, RFC2822.CC

         Set by: original Originator

         These usually contains the original list of Recipient
         addresses.

      RFC2821.MailFrom

         Set by: Originator Source or Mediator Source

         This can contain the original address to be notified of
         transmission issues, or the mailing list agent can 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 Originator

         This contains the address of a mailing list member.

      RFC2821.Received

         Set by: Mediator Dest

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

5.4.  Gateways

   A Gateway performs the basic routing and transfer work of message
   relaying, but it also may make any message or address modifications
   that are needed to send the message into a messaging environment that
   operates according to different standards or potentially incompatible
   policies.  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 can 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



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   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 can 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 can 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: Originator Source or Mediator Source

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

      RFC2822.TO, RFC2822.CC, RFC2822.BCC

         Set by: original Recipient

         These usually retain their original addresses.

      RFC2821.MailFrom

         Set by: Originator Source or Mediator Source






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

      RFC2822.Received

         Set by: Mediator Dest

         The Gateway can 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 is not intended to introduce any security
   considerations.

   However its discussion of the roles and responsibilities for
   different mail service modules, and the information they create,
   highlights the considerable degree to which security issues are
   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 or authorship can be
   authenticated or encrypted.


7.  IANA Considerations

   This document has no actions for IANA.


8.  References

8.1.  Normative

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




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   [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.

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [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.

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

   [RFC2645]  "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
              Addresses", RFC 2645, August 1999.

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



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   [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.

   [RFC3192]  Allocchio, C., "Minimal FAX address format in Internet
              Mail", RFC 2304, October 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.

   [RFC3798]  Hansen, T. and G. Vaudreuil, "Message Disposition
              Notification", RFC 3798, May 2004.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", RFC 3864,
              September 2004.

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

   [RFC4288]  Freed, N., Klensin, J., and J. Postel, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 4288, December 2005.

   [RFC4289]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 4289, December 2005.

   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              RFC 4409, April 2006.




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   [RFC4550]  Maes, S., , S., and Isode Ltd., "Internet Email to Support
              Diverse Service Environments (Lemonade) Profile",
              June 2006.

8.2.  Descriptive

   [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.

   [RFC1733]  Crispin, M., "Distributed Electronic Models in IMAP4",
              December 1994.

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

   [RFC3801]  Vaudreuil, G. and G. Parsons, "", RFC 3801, June 2004.

   [RFC4142]  Crocker, D. and G. Klyne, "Full-mode Fax Profile for
              Internet Mail: FFPIM", December 2005.

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


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, John Leslie, Bruce Lilly, Mark E. Mallett,
   David MacQuigg, Chris Newman, Daryl Odnert, Rahmat M. Samik-Ibrahim,
   Marshall Rose, Hector Santos, Jochen Topf, Willemien Hoogendoorn,
   Valdis Kletnieks.

   Diligent proof-reading was performed by Bruce Lilly.






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Author's Address

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

   Phone: +1.408.246.8253
   Email: dcrocker@bbiw.net









































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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

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

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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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