SMTP                                                          D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Standards Track                       February 24, 2008
Expires: August 27, 2008

                       Internet Mail Architecture

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   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 distinguished by many independent operators, many different
   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

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   terminology facilitates discussion 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
     1.4.  Changes to Previous Version  . . . . . . . . . . . . . . .  6
   2.  Responsible Actor Roles  . . . . . . . . . . . . . . . . . . .  8
     2.1.  User Actors  . . . . . . . . . . . . . . . . . . . . . . .  9
     2.2.  Mail Handling Service (MHS) Actors . . . . . . . . . . . . 12
     2.3.  Administrative Actors  . . . . . . . . . . . . . . . . . . 15
   3.  Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     3.1.  Mailbox  . . . . . . . . . . . . . . . . . . . . . . . . . 18
     3.2.  Domain Names . . . . . . . . . . . . . . . . . . . . . . . 19
     3.3.  Message Identifier . . . . . . . . . . . . . . . . . . . . 19
   4.  Services and Standards . . . . . . . . . . . . . . . . . . . . 21
     4.1.  Message Data . . . . . . . . . . . . . . . . . . . . . . . 25
     4.2.  User-Level Services  . . . . . . . . . . . . . . . . . . . 30
     4.3.  MHS-Level Services . . . . . . . . . . . . . . . . . . . . 32
   5.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     5.1.  Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 36
     5.2.  Re-Sending . . . . . . . . . . . . . . . . . . . . . . . . 37
     5.3.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 39
     5.4.  Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 40
     5.5.  Boundary Filter  . . . . . . . . . . . . . . . . . . . . . 41
   6.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 42
     6.1.  Security Considerations  . . . . . . . . . . . . . . . . . 42
     6.2.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 42
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     7.1.  Normative  . . . . . . . . . . . . . . . . . . . . . . . . 42
     7.2.  Informative  . . . . . . . . . . . . . . . . . . . . . . . 44
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 45
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 47
   Intellectual Property and Copyright Statements . . . . . . . . . . 48

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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
   distinguished 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

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

   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.

   The operational service has evolved sub-divisions for 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
   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 Author's message, as discussed
   in Section 2.1.4.  In fact some uses of email consider the entire
   email service -- including Author 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].

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

                Figure 1: Basic Internet Mail Service Model

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 Author and Recipient

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

      *  No requirement for Author 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, 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 responsible for handling a message.  That is, all
   participants rely on the core services being universally supported
   and accessible, either directly or through gateways that translate
   between Internet Mail and email environments that conform to other
   standards.  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
   administrative arrangement often is required and can include access
   control, routing constraints and information query 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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

      Discussion venue:   Please direct discussion about this document
         to the IETF-SMTP mailing list <>.

1.4.  Changes to Previous Version

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   INSTRUCTIONS TO THE RFC EDITOR:   Remove this sub-section prior to

   Many small editing changes, for wordsmithing improvements to make
   details more consistent.  This section documents the nature and basis
   for changes with significant impact.

   Originator->Author:   The term "Originator" is used by RFC 2822 more
      broadly than just the From: field, which specifically defines who
      the author of the content is.  I believe this distinguishes two
      constructs, one for the content author and one for the first
      agency that handles the message, in terms of the transfer service.
      So the change from "Originator" to "Author" seems pretty
      straightforward.  The challenge is in using the term Originator,
      as defined in RFC 2822 and applying it to the system's

   Source->Originator:   This change is more of a challenge.  We need
      the "Originator" term and construct, but the architecture is
      already complex enough.  Hence, adding a new construct seems like
      a very poor resolution.  The document has used "Source" as an MHS
      term for the MSA set of functions.  While one could argue against
      re-labeling it as Originator, I believe this is a reasonable
      choice and likely to be comfortable for community use, since
      "Source" does not have an established history.

   Bounce->Return:   'bounce address' is not accurate, because the
      address is used for more than that, but it *is* as established
      term within portions of the broader email community.  I also
      believe the extensive discussion on this point, last year,
      justifies the change.

      The problem with saying "Bounce" is that is not merely
      linguistically impure, it is plain wrong and has already caused
      serious problems.  Witness SPF.  Frankly, we need to fix RFC2821,
      but that's a separate battle to fight and not one for this forum.

      Although not a verbatim use of "Reverse Path", the related term
      that seems to work publicly is "Return Address".  It is already
      established in the bricks-and-mortar postal world and seems to
      have some acceptance within parts of the email community.  (I've
      done a draft white paper on authentication for the Messaging Anti-
      Abuse Working Group and the membership had some debate about this
      vocabulary choice and converged on agreeing to it.)

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   Is the envelope part of the message?  I don't remember whether we
      resolved this.  For a variety of reasons, I believe the message
      includes its envelope, and am encouraged to find RFC2822upd says:

         In the context of electronic mail, messages are viewed as
         having an envelope and contents.  The envelope contains
         whatever information is needed to accomplish transmission and
         delivery.  (See [I-D.klensin-rfc2821bis] (Klensin, J., "Simple
         Mail Transfer Protocol," November 2007.) for a discussion of
         the envelope.)  The contents comprise the object to be
         delivered to the recipient.  This specification applies only to
         the format and some of the semantics of message contents.

      rfc2821bis says:

         SMTP transports a mail object.  A mail object contains an
         envelope and content.

      I think these justify having the term 'message' as including the

   Examples of 'new' messages:   Section Section 3.3.1contains a list of
      examples, discussing scenarios that might or might not be viewed
      as creating a "new" message, rather than retaining an existing
      one.  The list has been expanded.

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

      *  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.

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2.1.  User Actors

   Users are the sources and sinks of messages.  They can be humans,
   organizations 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:

      *  Authors

      *  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 figure depicts the flow of messages among Actors:

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

                 Figure 2: Relationships Among User Actors

2.1.1.  Author

   This is the user-level participant responsible for creating the
   message, its contents and its list of recipient addresses.  The MHS
   operates to send and deliver mail among Authors 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 message 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 Author.  An example
   of an automated form of reply is the Message Disposition Notification
   (MDN), which informs the Author about the Recipient's handling of the

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   message.  (See Section 4.1.)

2.1.3.  Return Handler

   The Return Handler -- also called "Bounce Handler" -- receives and
   services notifications generated by the MHS, as a result of 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 Return handling address (also known as a Return
   address) might have no visible characteristics in common with the
   address of the Author or Source.

2.1.4.  Mediator

   A Mediator receives, aggregates, reformulates and redistributes
   messages as part of a potentially-protracted, higher-level exchange
   among Users.  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 Author.

   The distinctive aspects of a Mediator are, therefore, above the MHS.
   A Mediator preserves the Author 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

   A Mediator's task can be complex and contingent, such as 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

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

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   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 2.2.3.

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 Author 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 figure depicts the relationships among transfer
   participants in Internet Mail.  It shows the Source as distinct from
   the Author, 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.

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   +------------+                           +-----------+
   |   Author   |         +--------+        | Recipient |
   +-----+------+   +....>| Return |        +-----------+
         |          .     +--------+              ^
         |          .         ^                   |
   ||    |          .         |  Mail Handling    |     ||
   ||    |          .         |  Service (MHS)    |     ||
         V          .         |                   |
     +---------+    .         ^              +----+---+
     |         |    .         |              |        |
     | Origin  +....+         +-<------------+  Dest  |
     |         |              |              |        |
     +----+----+              |              +--------+
          |                   |                   ^
          |  +-------------->-+-<-------------+   |
          V  |                |               |   |
     +-------+-+         +----+----+        +-+---+---+
     |  Relay  +-->...-->|  Relay  +------->|  Relay  |
     +---------+         +----+----+        +---------+
                         | Gateway +-->...

                 Figure 3: Relationships Among MHS Actors

2.2.1.  Originator

   The Originator 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 Originator 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 Originator operates with dual "allegiance".  It serves the Author
   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 Originator also has the responsibility for any post-submission,
   Author-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.

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2.2.2.  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 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 binary to
   text 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
   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
   Author and sending an error message to the Return address.  The
   potential for looping is avoided by having this message, itself,
   contain no Return address.

2.2.3.  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

   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 being 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

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

2.3.  Administrative Actors

   Actors often 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 discussion about designs and operations 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, routing mail between ADMDs

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   can be an issue, such as needing to route mail for partners over
   specially-trusted paths.

   Basic types of ADMDs include --

      Edge:   Independent transfer services, in networks at the edge of
         the open 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 ADMDs, such as aggregation and

   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.  This further highlights the differences discussed in
   Section 2.2.2

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

                          Figure 4: 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.

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   The interactions between functional components within an ADMD are
   subject to the policies of that domain.  Policies can cover such
   things as:

   o  Reliability

   o  Access control

   o  Accountability

   o  Content evaluation and modification

   They can be implemented in different functional components, according
   to the needs of the ADMD.  For example see [RFC5068].

   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.

   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

      Mail Service Providers:

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

   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.

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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 associated with 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 not discussed further.

   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 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 Author.

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
   SHOULD NOT be interpreted by any domain except the one listed in the
   right-hand side of the addr-spec.  The exceptions are those
   specialized services conforming to standardized conventions, as noted

   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 authoring systems and Recipient
   Gateways, and they are invisible to the public email transfer
   infrastructure.  When an Author is explicitly sending via a Gateway
   out of the Internet, there are coding conventions for the local-part,
   so that the Author can formulate instructions for the Gateway.
   Standardized examples of this are the telephone numbering formats for

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   VPIM [RFC3801], such as "", and iFax
   [RFC3192], such as "FAX=+12027653000/".

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, an email address
   string has become the common identifier for representing online
   identity.  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.  For
   example, see: Section 4.1.4, Section 4.3.3, Section 5.

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
   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 System (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].  Any actor within the
   Originating ADMD can assign the Message-ID.  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

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   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

   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:

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

      *  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.

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

      *  If a message has offensive words deleted from it, then some
         recipients will consider it the same message, but some will

      *  If a message is translated into a different language, then some
         recipients will consider it the same message, but some will

      *  If a message is included in a digest of messages, it the digest
         constitutes a new message.

      *  If a message is forwarded by a recipient, what is forwarded is
         considered to be a new message.

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      *  If a message is "redirected", such as using RFC2822
         "Redirect-*" headers, 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 Message-ID SHOULD NOT be used for any function
   that has security implications.

3.3.2.  ENVID

   The ENVID (envelope identifier) is a tag that is primarily for use
   within Delivery Status Notifications (DSN), so that the Return
   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 ENVID may also be used for message tracking purposes [RFC3885].

   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 Return Address.

4.  Services and Standards

   Internet Mail's architecture distinguishes among six basic types of
   functionality, arranged to support a store-and-forward service
   architecture.  As shown in Figure 5 these types can have multiple
   instances, some of which represent specialized sub-roles.  This
   section considers the activities and relationships among these
   components, and the Internet Mail standards that apply to them.

      1.  Message

      2.  Mail User Agent (MUA)

             Originating MUA (oMUA)

             Receiving MUA (rMUA)

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      3.  Message Submission Agent (MSA)

             Author-focussed MSA functions (oMSA)

             MHS-focussed MSA functions (hMSA)

      4.  Message Transfer Agent (MTA)

      5.  Message Delivery Agent (MDA)

             Recipient-focused MDA functions (rMDA)

             MHS-focussed MDA functions (hMDA)

      6.  Message Store (MS)

          1.  Author MS (oMS)

                 oMS on a remote server (soMS)

                 oMS co-located with the oMUA (uoMS)

          2.  Recipient MS (rms)

                 rMS on a remote server (srMS)

                 rMS co-located with the rMUA (urMS)

   This section describes each functional component for Internet Mail,
   and the standards-based protocols 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.

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         A software system that primarily performs message relaying --
         and therefore is an MTA -- might also include MDA
         functionality.  That same MTA system might be able to interface
         with non-Internet email services and therefore qualify as a

         It is important not to confuse the engineering decisions 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                     ^
         .              | |                        +---------+  |
         . *******      | | .......................| Returns |  |
         . * oMS *<-----+ | .                      +---------+  |
         . *******        | .   *****************       ^       |
         .         +------V-.---*------------+  *       |       |
         .     MSA | +-------+  *   +------+ |  *       |       |
         .         | | oMSA  +--O-->| hMSA | |  *       |       |
         .         | +-------+  *   +--+---+ |  *       |       |
         .         +------------*------+-----+  *       |       |
   //==========\\               *      V {smtp  *       |       |
   || MESSAGE  ||               *   +------+    *  //===+===\\  |
   ||----------||           MHS *   | MTA  |    *  ||  dsn  ||  |
   || Envelope ||               *   +--+---+    *  \\=======//  |
   ||  SMTP    ||               *      V {smtp  *     ^   ^     |
   || Content  ||               *   +------+    *     |   | //==+==\\
   ||  RFC2822 ||               *   | MTA  +----*-----+   | || mdn ||
   ||  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 Author to its Recipients.  A message can be
   explicitly labeled as to its nature [RFC3458].

   A message comprises a transit handling envelope and the message
   content.  The envelope contains information used by the MHS.  The
   content is divided into a structured header and the body.  The header
   comprises transit trace information and end-user structured fields.
   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],

   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
         Returns in Figure 5.  DSNs provide information about message
         transit, such as transmission errors or successful delivery

      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.

      Message Filtering (SIEVE):

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         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
         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

   Internet Mail has a fragmented framework for transit-related
   "handling" information.  Information that is directly used by the MHS
   is called the "envelope".  It directs handling activities by the
   transfer service as is carried in transfer service commands.  That
   is, the envelope exists in the transfer protocol SMTP [RFC2821].

   Trace information records handling activity and is recorded in the
   message Header.  [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

4.1.4.  Identity References in a Message

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

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

                            Layered Identities

   The most common address-related fields are:

   RFC2822.From:   Set by - Author

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

   RFC2822.Reply-To:   Set by - Author

      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:   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.

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      Specification of the notifications Return addresses -- contained
      in RFC2821.MailFrom -- is made by the RFC2822.Sender.  Typically
      the Return address is the same as the Sender address.  However
      some usage scenarios require it to be different.

   RFC2822.To/.CC:   Set by - Author

      These specify MUA Recipient addresses.  However some or all of the
      addresses in these fields might not be present in the
      RFC2821.RcptTo commands.

      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 - Author

      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 single
      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 Author.

   RFC2821.HELO/.EHLO:   Set by - Source

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

   RFC3461.ENVID:   Set by - Source

      The MSA can specify an opaque string, to be included in a DSN, as
      a means of assisting the Return address recipient in identifying
      the message that produced a DSN, or message tracking.

   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 Actor responsible for submitting the

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      message.  Rather, the Actor 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 - Author

      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.

   RFC2919.List-Id:   Set by - Mediator Author

      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 Author

      [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

   RFC0791.SourceAddr:   Set by - The Client SMTP sending host
      immediately preceding the current receiving SMTP server.

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      [RFC0791] defines the basic unit of data transfer for the
      Internet, the IP Datagram.  It contains a "Source Address" field
      that specifies the IP Address for the host (interface) from which
      the datagram was sent.  This information is set and provided by
      the IP layer, and is therefore independent of mail-level
      mechanisms.  As such, it is often taken to be authoritative,
      although it is possible to provide false addresses.

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).

   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.

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      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.To, RFC2822.CC


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

   As discussed in [RFC1733] the operational relationship among 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.

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      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.

   Identities relevant to the MSA include:


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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

   Identities relevant to the MTA include:

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      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:   Set by - Author Source or Mediator Source

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

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      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 Author 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

      RFC3461.ENVID  Set by - Author Source or Mediator Source

      RFC2821.MailFrom:   Set by - Author Source or Mediator Source

      RFC2821.RcptTo:   Set by - Mediator Author

      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
   might also add commentary.

   Examples of MUA message creation 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 Actor doing the forwarding, rather than being

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         from the original Author.

         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 Author.


         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.


         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
         Author, 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
   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

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   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/.CC/.BCC:   Set by - Author

         These retain their original addresses.

      RFC2821.RcptTo:   Set by - Mediator Author

         This field contains an alias address.

      RFC2821.MailFrom:   Set by - Author Source or Mediator Source

         The Actor responsible for submission to an alias address will
         often retain the original address to receive handling Returns.
         The benefit of retaining the original MailFrom value is to
         ensure that the origination-side Actor 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 Dest

         The Actor 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 Author 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 Author, even
   if the Mediator adds commentary.

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   Identities specified in a resent message include

      RFC2822.From:   Set by - original Author

         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 Actor responsible for the redirection.

      RFC2822.Reply-To:   Set by - original Author

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

      RFC2822.Sender:   Set by - Author Source or Mediator Source.

      RFC2822.To/.CC/.BCC:   Set by - original Author

         These specify the original message Recipients.

      RFC2822.Resent-From:   Set by - Mediator Author

         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 Actor 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-To/-CC/-BCC:   Set by: Mediator Author

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

      RFC2821.MailFrom:   Set by - Mediator Source

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

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      RFC2821.RcptTo:   Set by - Mediator Author

         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.

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, converting the format, and adding list-specific
   comments.  In addition, archiving list messages is common.  Still the
   message retains characteristics of being "from" the original Author.

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

      RFC2919.List-Id:   Set by - Mediator Author

      RFC2369.List-*:   Set by - Mediator Author

      RFC2822.From:   Set by - original Author

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

      RFC2822.Reply-To:   Set by - original Author or Mediator Author

      RFC2822.Sender:   Set by - Author Source or Mediator Source

         This will usually specify the address of the Actor 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/.CC  Set by - original Author

         These usually contain the original list of Recipient addresses.

      RFC2821.MailFrom  Set by - Author Source or Mediator Source

         This can contain the original address to be notified of
         transmission issues, or the mailing list Actor 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 Returns.

      RFC2821.RcptTo  Set by - Mediator Author

         This contains the address of a mailing list member.

      RFC2821.Received  Set by - Mediator Dest

         A Mailing List Actor can record a Received header field, to
         indicate the transition from original posting to mailing list
         forwarding.  The Actor 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 content, structure, address, or
   attribute modifications 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
   latter can make substantive changes to a message, 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

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

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      RFC2822.From:   Set by - original Author

         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 Author

         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 - Author Source or Mediator Source

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

      RFC2822.To/.CC/.BCC  Set by - original Recipient

         These usually retain their original addresses.

      RFC2821.MailFrom  Set by - Author Source or Mediator Source

         The Actor 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 the original posting environment 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.

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6.  Considerations

6.1.  Security Considerations

   This document does not specify any new Internet Mail functionality.
   Consequently it is not intended to introduce any security

   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.

6.2.  IANA Considerations

   This document has no actions for IANA.

7.  References

7.1.  Normative

   [RFC0791]  Postel, J., "Internet Protocol", 1981 September.

   [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.

   [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.

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   [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.

   [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.

   [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

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              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.

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

7.2.  Informative

   [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.

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

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

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

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

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

   [RFC3885]  Allman, E. and T. Hansen, "SMTP Service Extension for
              Message Tracking", RFC 3885, September 2004.

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

   [RFC5068]  Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
              E. Allman, "Email Submission Operations: Access and
              Accountability Requirements", RFC 5068, BCP 134, Nov 2007.

   [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 [RFC5068].
   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 Eric Allman, Nathaniel
   Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch,
   Ned Freed, Eric Hall, Tony Hansen, Willemien Hoogendoorn, Brad
   Knowles, John Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David
   MacQuigg, Alexey Melnikov, der Mouse, S. Moonesamy, Chris Newman,
   Daryl Odnert, Rahmat M. Samik-Ibrahim, Marshall Rose, Hector Santos,
   Jochen Topf, Greg Vaudreuil.

   Diligent proof-reading was performed by Bruce Lilly.


         Administrative  15
         Author  10
         Edge  16
         Gateway  14
         Mediator  11
         Originator  13
         Recipient  10
         Relay  14
         Return Handler  11
         Transit  16
         User  16

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         MHS  12
      Administrative Actors  15
      Author  10

      Discussion of document  6

      Edge Actor  16
      end-to-end  4

      Gateway  12, 14

      Internet Mail  4

      Mail  4
      Mail exchange  5
      Mail Handling Service  3
      Mail Handling System  12
      Mail Transfer Agent  3
      Mail User Agent  3
      MDN  10
      Mediator  11
      Message Disposition Notification  10
      MHS  3, 12
         Actors  12
      MTA  3
      MUA  3

      Originator  13

      Recipient  10
      Relay  14
      Return Handler  11

      Transit Actor  16

      UA  3
      User Actor  16
      User Agent  3

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Author's Address

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Drive
   Sunnyvale, CA  94086

   Phone: +1.408.246.8253

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
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   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
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   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use of
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   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at

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