SMTP                                                          D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Intended status: Standards Track                        October 31, 2008
Expires: May 4, 2009

                       Internet Mail Architecture

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   Over its thirty-five year history, Internet Mail has changed
   significantly in scale and complexity, as it has become a global
   infrastructure service.  These changes have been evolutionary, rather
   than revolutionary, reflecting a strong desire to preserve both its
   installed base and its usefulness.  To collaborate productively on
   this large and complex system, all participants must work from a
   common view of it and use a common language to describe its
   components and the interactions among them.  But the many differences
   in perspective currently make it difficult to know exactly what
   another participant means.  To serve as the necessary common frame of
   reference, this document describes the enhanced Internet Mail
   architecture, reflecting the current service.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  History  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Document Conventions . . . . . . . . . . . . . . . . . . .  6
   2.  Responsible Actor Roles  . . . . . . . . . . . . . . . . . . .  7
     2.1.  User Actors  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Mail Handling Service (MHS) Actors . . . . . . . . . . . . 10
     2.3.  Administrative Actors  . . . . . . . . . . . . . . . . . . 13
   3.  Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     3.1.  Mailbox  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     3.2.  Scope of Email Address Use . . . . . . . . . . . . . . . . 17
     3.3.  Domain Names . . . . . . . . . . . . . . . . . . . . . . . 17
     3.4.  Message Identifier . . . . . . . . . . . . . . . . . . . . 18
   4.  Services and Standards . . . . . . . . . . . . . . . . . . . . 19
     4.1.  Message Data . . . . . . . . . . . . . . . . . . . . . . . 22
     4.2.  User-Level Services  . . . . . . . . . . . . . . . . . . . 27
     4.3.  MHS-Level Services . . . . . . . . . . . . . . . . . . . . 29
     4.4.  Transition Modes . . . . . . . . . . . . . . . . . . . . . 33
     4.5.  Implementation and Operation . . . . . . . . . . . . . . . 33
   5.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     5.1.  Alias  . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     5.2.  ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 36
     5.3.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 37
     5.4.  Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 39
     5.5.  Boundary Filter  . . . . . . . . . . . . . . . . . . . . . 40
   6.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 40
     6.1.  Security Considerations  . . . . . . . . . . . . . . . . . 40
     6.2.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 41
     6.3.  Internationalization . . . . . . . . . . . . . . . . . . . 41
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     7.1.  Normative  . . . . . . . . . . . . . . . . . . . . . . . . 42
     7.2.  Informative  . . . . . . . . . . . . . . . . . . . . . . . 44
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 45
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48
   Intellectual Property and Copyright Statements . . . . . . . . . . 49

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1.  Introduction

   Over its thirty-five year history, Internet Mail has changed
   significantly in scale and complexity, as it has become a global
   infrastructure service.  These changes have been evolutionary, rather
   than revolutionary, reflecting a strong desire to preserve both its
   installed base and its usefulness.  Today, Internet Mail is
   distinguished by many independent operators, many different
   components for providing service to users, as well as many different
   components that transfer messages.

   Public collaboration on technical, operations, and policy activities
   of email, including those that respond to the challenges of email
   abuse, has brought a much wider range of participants into the
   technical community.  To collaborate productively on this large and
   complex system, all participants must work from a common view of it
   and use a common language to describe its components and the
   interactions among them.  But the many differences in perspective
   currently make it difficult to know exactly what another participant

   It is the need to resolve these differences that motivates this
   document, which describes 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

   To serve as the necessary common frame of reference, this document
   describes the enhanced Internet Mail architecture, reflecting the
   current service.  The document focuses on:

      *  Capturing refinements to the email model

      *  Clarifying functional roles for the architectural components

      *  Clarifying identity-related issues, across the email service

      *  Defining terminology for architectural components and their

1.1.  History

   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 transfer world, in the form of the Mail Handling

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   Service (MHS), which is composed of Mail Transfer Agents (MTA).  The
   MHS accepts a message from one User and delivers it to one or more
   other users, 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 among
   the 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, although core aspects of the
   service, such as mailbox addressing and message format style,
   remaining remarkably constant.  The original distinction between the
   user level and transfer level remains, but with elaborations in each.
   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 that result from a single transit
   of the MHS.  A common exception is group dialogue that is mediated,
   through a Mailing List; in this case, two postings occur before
   intended Recipients receive an Author's message, as discussed in
   Section 2.1.3.  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 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

   End-to-end Internet Mail exchange is accomplished by using a
   standardized infrastructure with these components and

      *  An email object

      *  Global addressing

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

      *  No prior arrangement between MTAs or between Authors and

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

      *  No requirement for Author, Originator, or Recipients 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 control
   information, for handling, from the Author's content.

   A precept to the design of mail over the open Internet is permitting
   user-to-user and MTA-to-MTA interoperability without prior, direct
   arrangement between the independent administrative authorities
   responsible for handling a message.  All participants rely on having
   the core services universally supported and accessible, either
   directly or through Gateways that act as translators between Internet
   Mail and email environments conforming to other standards.  Given the
   importance of spontaneity and serendipity in interpersonal
   communications, not requiring such 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 configuration of the information
   query service.  Although recipient authentication has usually been
   required for message access since the beginning of Internet Mail, in
   recent years it also has been required for message submission.  In
   these cases, a server validates the client's identity, whether by
   explicit security protocols or by implicit infrastructure queries to
   identify "local" participants.

1.2.  Document Conventions

   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: header field in an email content
   header and <RFC2821.MailFrom> is the address in the SMTP "Mail From"

   When occurring without the RFC2822 qualifier, header field names are
   shown with a colon suffix.  For example, From:.

   References to labels for actors, functions or components have the
   first letter capitalized.

   Also, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   this document are to be interpreted as described in RFC 2119
   [RFC2119] [RFC2119].

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      RFC EDITOR:   Remove the following paragraph before publication.

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

2.  Responsible Actor Roles

   Internet Mail is a highly distributed service, with a variety of
   actors playing different roles.  These actors fall into three basic

      *  User

      *  Mail Handling Service (MHS)

      *  ADministrative Management Domain (ADMD)

   Although related to a technical architecture, the focus on actors
   concerns participant responsibilities, rather than functionality of
   modules.  For that reason, the labels used are different from those
   used in classic email architecture diagrams.

2.1.  User Actors

   Users are the sources and sinks of messages.  Users can be people,
   organizations, or processes.  They can have an exchange that
   iterates, and they can expand or contract the set of users that
   participate in a set of exchanges.  In Internet Mail, there are four
   types of Users:

      *  Authors

      *  Recipients

      *  Return Handlers

      *  Mediators

   Figure 2 shows the primary and secondary flows of messages among

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            ||  Author  ||<..................................<..
            ++=++=++=++=++                                     .
               || || ||     ++===========++                    .
               || || ++====>|| Recipient ||                    .
               || ||        ++=====+=====++                    .
               || ||               .                           .
               || ||               ..........................>.+
               || ||                                           .
               || ||               ...................         .
               || ||               .                 .         .
               || ||               V                 .         .
               || ||         +-----------+    ++=====+=====++  .
               || ++========>| Mediator  +===>|| Recipient ||  .
               ||            +-----+-----+    ++=====+=====++  .
               ||                  .                 .         .
               ||                  ..................+.......>.+
               ||                                              .
               ||    ..............+..................         .
               ||    .             .                 .         .
               \/    V             V                 '         .
            +-----------+    +-----------+    ++=====+=====++  .
            | Mediator  +===>| Mediator  +===>|| Recipient ||  .
            +-----+-----+    +-----+-----+    ++=====+=====++  .
                  .                .                 .         .

                 Figure 2: Relationships Among User Actors

   From the user 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.

   Whenever any MHS actor sends information to back to an Author or
   Originator in the sequence of handling a message, that actor is a

2.1.1.  Author

   The Author is responsible for creating the message, its contents, and
   its list of recipient addresses.  The MHS transfers the message from
   the Author and delivers it to the Recipients.  The MHS has an
   Originator role (Section 2.2.1) that correlates with the Author role.

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2.1.2.  Recipient

   The Recipient is a consumer of the delivered message.  The MHS has a
   Receiver role (Section 2.2.4)correlates with the Recipient role.
   This is labeled Recv in Figure 3.

   Any Recipient can close the user communication loop by creating and
   submitting a new message that replies to the 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
   message.  (See Section 4.1.)

   The Return Handler, also called "Bounce Handler," receives and
   services notifications that the MHS generates, as it transfers or
   delivers the message.  These notices can be about failures or
   completions and are sent to an address that is specified by the
   Originator<<initial def>> .  This Return handling address (also known
   as a Return address) might have no visible characteristics in common
   with the address of the Author or Originator.

2.1.3.  Mediator

   A Mediator receives, aggregates, reformulates, and redistributes
   messages among Authors and Recipients who are the principals in
   protracted exchanges.  This activity is easily confused with the
   underlying MHS transfer exchanges.  However, each serves very
   different purposes and operates in very different ways.

   When mail is delivered to the Mediator specified in the
   RFC2821.RcptTo command, the MHS handles it the same way as for any
   other Recipient.  The MHS sees each posting and delivery activity
   between sources and sinks as independent; it does not see subsequent
   re-posting as a continuation of a process.  Because the Mediator
   originates messages, it can receive replies.  Hence, when submitting
   messages, the Mediator is an Author.  So a Mediator really is a full-
   fledged User.  Mediators are considered extensively in Section 5.

   The distinctive aspects of a Mediator are outside the MHS.  A
   Mediator preserves the Author information of the message it
   reformulates and is permitted to make meaningful changes to the
   message content or envelope.  The MHS sees a new message, but users
   receive a message that they interpret as being from, or at least
   initiated by, the Author of the original message.  The role of a
   Mediator is not limited to merely connecting other participants; the
   Mediator is responsible for the new message.

   A Mediator's role is complex and contingent, for example, modifying
   and adding content or regulating which users are allowed to

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   participate and when.  The common example of this role is a group
   Mailing List.  In a more complex use, a sequence of Mediators could
   perform a sequence of formal steps, such as reviewing, modifying, and
   approving a purchase request.

   A Gateway is a particularly interesting form of Mediator.  It is a
   hybrid of User and Relay that connects heterogeneous mail services.
   Its purpose is to emulate a Relay.  For a detailed discussion, see
   Section 2.2.3. .

2.2.  Mail Handling Service (MHS) Actors

   The Mail Handling Service (MHS) performs a single end-to-end transfer
   on behalf of the Author to reach the Recipient addresses specified in
   the original RFC2821.RcptTo commands.  Exchanges that are either
   mediated or iterative and protracted, such as those used for
   collaboration over time are handled by the User actors, not by the
   MHS actors.

      Figure 3 shows the relationships among transfer participants in
   Internet Mail.  Although it shows the Originator (labeled Origin) as
   distinct from the Author and Receiver (labeled Recv) as distinct from
        Recipient,  each pair of roles usually has  the same actor.
      Transfers typically entail one or more Relays.  However direct
       delivery from the Originator to Receiver is possible.  Intra-
          organization mail services usually have only one Relay.

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           ++==========++                        ++===========++
           ||  Author  ||                        || Recipient ||
           ++====++====++   +--------+           ++===========++
                 ||         | Return |                  /\
                 ||         +-+------+                  ||
                 \/           .    ^                    ||
             +---------+      .    .                +---++---+
             |         |      .    .                |        |
         ||  |         |      .    .      MHS       |        |   ||
         ||  | Origin  +<......    .................+  Recv  |   ||
             |         |           ^                |        |
             +---++----+           .                +--------+
                 ||                .                    /\
                 ||  ..............+..................  ||
                 \/  .             .                 .  ||
             +-------+-+        +--+------+        +-+--++---+
             |  Relay  +=======>|  Relay  +=======>|  Relay  |
             +---------+        +----++---+        +---------+
                                | Gateway +-->...

                 Figure 3: Relationships Among MHS Actors

2.2.1.  Originator

   The Originator ensures that a message is valid for posting and then
   submits it to a Relay.  A message is valid if it conforms to both
   Internet Mail standards and local operational policies.  The
   Originator can simply review the message for conformance and reject
   it if it finds errors, or it can create some or all of the necessary
   information.  In effect, the Originator is responsible for the
   functions of the Mail Submission Agent.

   The Originator operates with dual allegiance.  It serves the Author
   and can be the same entity.  But 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 performs any post-submission, Author-related
   administrative tasks associated with message transfer and delivery.
   Notably, these tasks pertain to sending error and delivery notices,
   enforcing local policies, and dealing with messages from the Author
   that prove to be problematic for the Internet.  The Originator is

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   accountable for the message content, even when it is not responsible
   for it.  The Author creates the message, but the Originator handles
   any transmission issues with it.

2.2.2.  Relay

   The Relay performs MHS-level transfer-service routing and store-and-
   forward, by transmitting or retransmitting the message to its
   Recipients.  The Relay adds trace information [RFC2505] but does not
   modify the envelope information or the message content semantics.  It
   can modify message content representation, such as changing the form
   of transfer encoding from binary to text, but only as required to
   meet the capabilities of the next hop in the MHS.

   A Mail Handling Service (MHS) network consists of a set of Relays.
   This network is above any underlying packet-switching network that
   might be used and below any Gateways or other Mediators.

   In other words, email scenarios can involve three distinct
   architectural layers, each providing its own type of data of store-
   and-forward service:

      *  User Mediators

      *  MHS Relays

      *  Packet Switches

   The bottom layer is the Internet's IP service.  The most basic email
   scenarios involve Relays and Switches.

   Aborting a message transfer makes the Relay an Author because it must
   send an error message to the Return address.  The potential for
   looping is avoided by omitting a Return address from this message.

2.2.3.  Gateway

   A Gateway is a hybrid of User and Relay that connects heterogeneous
   mail services.  Its purpose is to emulate a Relay and the closer it
   comes to this, the better.  A Gateway operates as a User when it
   needs the ability to modify message content.

   Differences between mail services can be as small as minor syntax
   variations, but they usually encompass significant, semantic
   distinctions.  One difference could be email addresses that are
   hierarchical and machine-specific rather than a flat, global

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   namespace.  Another difference could be support for text-only content
   or multi-media.  Hence the Relay function in a Gateway presents a
   significant design challenge, if the resulting performance is to be
   seen as nearly seamless.  The challenge is to ensure user-to-user
   functionality between the services, despite differences in their
   syntax and semantics.

   The basic test of Gateway design is 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 components in the Author's or
   Recipient's mail services other than adding the Gateway.  To each of
   these otherwise independent services, the Gateway appears to be a
   native participant.  But the ultimate test of Gateway design 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 Author?

2.2.4.  Receiver

   The Receiver performs final delivery or sends the message to an
   alternate address.  It can also perform filtering and other policy
   enforcement immediately before or after delivery.

2.3.  Administrative Actors

   Administrative actors can be associated with different organizations,
   each with its own administrative authority.  This operational
   independence, coupled with the need for interaction between groups,
   provides the motivation to distinguish among ADministrative
   Management Domains (ADMDs ).  Each ADMD can have vastly different
   operating policies and trust-based decision-making.  One obvious
   example is the distinction between mail that is exchanged within an
   organization and mail that is exchanged between independent
   organizations.  The rules for handling both 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 carried out by 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.  A
   department that operates a local Relay, an IT department that
   operates an enterprise Relay, and an ISP that operates a public
   shared email service can be configured into many combinations of
   administrative and operational relationships.  Each is a distinct
   ADMD, potentially having a complex arrangement of functional
   components.  Figure 4 depicts relationships among ADMDs.  The benefit
   of the ADMD construct is to facilitate discussion about designs,

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   policies and operations that need to distinguish between internal
   issues and external ones.

   The architectural impact of the need for boundaries between ADMDs is
   discussed in [Tussle].  Most significant is that the entities
   communicating across ADMD boundaries typically have the added burden
   of enforcing organizational policies concerning  external
   communications.  At a more mundane level, routing mail between ADMDs
   can be an issue, such as needing to route mail for partners over
   specially trusted paths.

   These are the basic types of ADMDs:

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

      Consumer:    This might be a type of Edge service, as is common
         for web-based email access.

      Transit:    Mail Service Providers (MSP) that offer value-added
         capabilities for Edge ADMDs, such as aggregation and filtering.

   The mail-level transit service is different from packet-level
   switching.  End-to-end packet transfers usually go through
   intermediate routers; email exchange across the open Internet can be
   directly between the Boundary MTAs of Edge ADMDs.  This distinction
   between direct and indirect interaction  highlights the differences
   discussed in Section 2.2.2
         +--------+     +---------+     +-------+     +-----------+
         |  ADMD1 |<===>|  ADMD2  |<===>| ADMD3 |<===>|   ADMD4   |
         |  ----- |     |  -----  |     | ----- |     |   -----   |
         |        |     |         |     |       |     |           |
         | Author |     |         |     |       |     |           |
         |   .    |     |         |     |       |     |           |
         |   V    |     |         |     |       |     |           |
         |  Edge..+....>|.Transit.+....>|-Edge..+....>|.Recipient |
         |        |     |         |     |       |     |           |
         +--------+     +---------+     +-------+     +-----------+

              Figure 4: Administrative Domain (ADMD) Example

   Edge networks can use proprietary email standards internally.
   However the distinction between Transit network and Edge network
   transfer services is significant because it highlights the need for
   concern over interaction and protection between independent
   administrations.  In particular, this distinction calls for

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   additional care in assessing the transitions of responsibility and
   the accountability and authorization relationships among participants
   in message transfer.

   The interactions of ADMD components are subject to the policies of
   that domain, which cover concerns such as these:

      *  Reliability

      *  Access control

      *  Accountability

      *  Content evaluation and modification

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

   Consumer, 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.

   These are common examples of ADMDs:

      Enterprise Service Providers:

         These ADMDs operate the internal data and/or the mail services
         within an organization.

      Internet Service Providers (ISP):

         These ADMDs operate the underlying data communication services,
         which are used by one or more Relay and User.  ISPs are not
         responsible for performing email functions, but they can
         provide an environment in which those functions can be

      Mail Service Providers:

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         These ADMDs operate email services, such as for consumers or
         client companies.

   Practical operational concerns demand that providers be involved in
   administration and enforcement issues.  This involvement can extend
   to operators of lower-level packet services.

3.  Identities

   Internet Mail uses three forms of identity: mailbox, domain name, and
   message-ID.  Each must 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, separated by an at-sign (@).  The right side
   is a globally interpreted domain name associated with an ADMD.
   Domain names are discussed in Section 3.3.  Formal Internet Mail
   addressing syntax can support source routes, to indicate the path
   through which a message ought to be sent.  The use of source routes
   is not common and has been deprecated in [RFC2821].

   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 domain
   name.  Except as noted later in this section all other entities MUST
   treat the <local-part> as an 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 creator.

   Some local-part values have been standardized, for contacting
   personnel at an organization.  These names cover common operations
   and business functions.  [RFC2142]

   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 side of the <addr-spec>.  The exceptions are those specialized
   services that conform to public, standardized conventions, as noted

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   A few types of addresses elaborate on basic email addressing, with a
   standardized, global schema for the <local-part>, Include are
   conventions between authoring systems and Gateways.  They are
   invisible to the public email transfer infrastructure.  When an
   Author is explicitly sending through a Gateway out of the Internet,
   coding conventions for the <local-part> allow the Author to formulate
   instructions for the Gateway.  Standardized examples of such
   conventions are the telephone numbering formats for VPIM [RFC3801],
   such as:


   and iFax [RFC3192], such as:


3.2.  Scope of Email Address Use

   Email addresses are being used far beyond their original role in
   email transfer and delivery.  In practical terms, an email address
   string has become the common identifier for representing online
   identity.  Hence, it is essential to be clear about both the nature
   and role of an identity string in a particular context and the entity
   responsible for setting that string.  For example, see Section 4.1.4,
   Section 4.3.3 and Section 5.

3.3.  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 can encompass an
   organization, a collection of machines integrated into a homogeneous
   service, or 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 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 to the host that took action upon the
   message, such as providing the administrative scope for a message
   identifier or performing transfer processing.

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3.4.  Message Identifier

   There are two standardized tags for identifying messages: Message-ID:
   and ENVID.  A Message-ID: pertains to content, and an ENVID pertains
   to transfer.

3.4.1.  Message-ID

   Internet Mail standards provide for, at most, a single Message-ID:.
   The Message-ID: for a single message, which is a user-level tag, has
   a variety of uses including threading, aiding identification of
   duplicates, and DSN tracking.  [RFC2822].  The Originator assigns the
   Message-ID:.  The Recipient's ADMD is the intended consumer of the
   Message-ID:, although any actor along the transfer path can use it.

   Message-ID: MUST be globally unique.  Its format is similar to that
   of a mailbox, with two distinct parts, separated by an at-sign (@).
   Typically, the right side specifies the ADMD or host that assigns the
   identifier, and the left side contains a string that is globally
   opaque and serves to uniquely identify the message within the domain
   referenced on the right side.  The duration of uniqueness for the
   message identifier is undefined.

   When a message is revised in any way, the decision whether to assign
   a new Message-ID: requires a subjective assessment to determine
   whether the editorial content has been changed enough to constitute a
   new message.  [RFC2822] states that "a message identifier pertains to
   exactly one instantiation of a particular message; subsequent
   revisions to the message each receive new message identifiers."  Yet
   experience suggests that some flexibility is needed.  An impossible
   test is whether the recipient will consider the new message to be
   equivalent to the old one.  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.

   Here are some guidelines and examples:

      *  If a message is changed only in form, such as character-
         encoding, it 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 text, it is probably the same

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

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

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

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

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

      *  If a message is "redirected", such as using RFC2822 "Resent-*"
         header fields, some recipients will consider it the same
         message, but some will not.

   The absence of both objective, precise criteria for re-generating a
   Message-ID: and 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.  For that
   reason, the Message-ID: SHOULD NOT be used for any function that has
   security implications.

3.4.2.  ENVID

   The ENVID (envelope identifier) can be used for message-tracking
   purposes [RFC3885] concerning a single posting/delivery transfer.
   The ENVID labels a single transit of the MHS by a specific message.
   So, the ENVID is used for one message posting, until that message is
   delivered.  A re-posting of the message, such as by a Mediator, does
   not re-use that ENVID, but can use a new one, even though the message
   might legitimately retain its original Message-ID:.

   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

   The Internet Mail architecture comprises six basic types of

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   functionality, which are arranged to support a store-and-forward
   service.  As shown in Figure 5, each type can have multiple
   instances, some of which represent specialized roles.  This section
   considers the activities and relationships among these components,
   and the Internet Mail standards that apply to them.


         Mail User Agent (MUA)

            Author MUA (aMUA)

            Recipient MUA (rMUA)

         Message Submission Agent (MSA)

            Author-focused MSA functions (aMSA)

            MHS-focused MSA functions (hMSA)

         Message Transfer Agent (MTA)

         Message Delivery Agent (MDA)

            Recipient-focused MDA functions (rMDA)

            MHS-focused MDA functions (hMDA)

         Message Store (MS)

            Author MS (aMS)

            Recipient MS (rMS)

     This figure shows function modules and the standardized protocols
                            used between them.

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                     ||        ||                             +-------+
          ...........++  aMUA  ||<............................+ Disp  |
          .          ||        ||                             +-------+
          .          ++=+==+===++                                 ^
          .  local,imap}|  |{smtp,submission                      .
          .  +-----+    |  |                          +--------+  .
          .  | aMS |<---+  | ........................>| Return |  .
          .  +-----+       | .                        +--------+  .
          .                | .    *****************       ^       .
          .          +-----V-.----*------------+  *       .       .
          .      MSA | +-------+  *   +------+ |  *       .       .
          .          | | aMSA  +-(S)->| hMSA | |  *       .       .
          .          | +-------+  *   +--+---+ |  *       .       .
          V          +------------*------+-----+  *       .       .
    //==========\\                *      V {smtp  *       .       .
    || MESSAGE  ||                *   +------+    *  //===+===\\  .
    ||----------||            MHS *   | MTA  |    *  ||  dsn  ||  .
    || Envelope ||                *   +--+---+    *  \\=======//  .
    ||  SMTP    ||                *      V {smtp  *     ^   ^     .
    || Content  ||                *   +------+    *     .   . //==+==\\
    ||  RFC2822 ||                *   | MTA  +....*......   . || mdn ||
    ||  MIME    ||                *   +--+---+    *         . \\=====//
    \\==========//                * smtp}| {local *         .     ^
          .           MDA         *      | {lmtp  *         .     .
          .      +----------------+------V-----+  *         .     .
          .      | +----------+   *   +------+ |  *         .     .
          .      | |          |   *   |      | +..*..........     .
          .      | |   rMDA   |<-(D)--+ hMDA | |  *               .
          .      | |          |   *   |      | |<.*........       .
          .      | +-+------+-+   *   +------+ |  *       .       .
          .      +------+---------*------------+  *       .       .
          .             |         *****************       .       .
          .             V{smtp,imap,pop,local             .       .
          .          +-----+                         //===+===\\  .
          .          | rMS |                         || sieve ||  .
          .          +--+--+                         \\=======//  .
          .             |{imap,pop,local                  ^       .
          .             V                                 .       .
          .       ++==========++                          .       .
          .       ||          ||                          .       .
          .......>||   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].  All of its
   underlying mechanisms serve to deliver 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 handling trace information and structured fields
   that are part of the Author's message content.  The body can be
   unstructured lines of text or a 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, notably:

      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.  An MDA and MTA are shown as sources
         of DSNs in Figure 5, and the destination is shown as Returns.
         DSNs provide information about message transit, such as
         transfer errors or successful delivery.  [RFC3461]

      Message Disposition Notification (MDN):

         A Message Disposition Notification (MDN) is a message that
         provides information about post-delivery 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 addresses.  The mailbox for this is shown as
         Disp in Figure 5.

      Message Filtering (SIEVE):

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         Sieve is a scripting language used to specify conditions for
         differential handling of mail, typically at the time of
         delivery [RFC5228].  Scripts can be conveyed in a variety of
         ways, as a MIME part.  Figure 5 shows a Sieve script  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. the Figure 5 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 used directly by the MHS is called
   the "envelope."  It directs handling activities by the transfer
   service and is carried in transfer service commands.  That is, the
   envelope exists in the transfer protocol SMTP.  [RFC2821]

   Trace information, such as  RFC2822.Received, is recorded in the
   message header and is not subsequently altered.  [RFC2822]

4.1.2.  Header Fields

   Header fields are attribute name/value pairs that cover an extensible
   range of email service parameters, structured user content, and user
   transaction meta-information.  The core set of header fields is
   defined in [RFC2822], [RFC0822].  It is common practice to extend
   this set for different applications.  Procedures for registering
   header fields are defined in [RFC3864].  An extensive set of existing
   header field registrations is provided in [RFC4021].

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

4.1.3.  Body

   The body of a message might be lines of ASCII text or a
   hierarchically structured  composition of multi-media body-part
   attachments, using MIME.  [RFC2045], [RFC2046], [RFC2047], [RFC4288],

4.1.4.  Identity References in a Message

   Table 1 lists the core identifiers present in a message during

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

                        Table 1: Layered Identities

   These are the most common address-related fields:

      RFC2822.From:   Set by - Author

         Names and addresses for authors of the message content are
         listed in the From: field.

      RFC2822.Reply-To:   Set by - Author

         If a Recipient sends a reply message that would otherwise use
         the RFC2822.From field addresses in the original message, the
         addresses in the RFC2822.Reply-To field are used instead.  In
         other words, this field overrides the From: field for responses
         from Recipients.

      RFC2822.Sender:   Set by - Originator

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         This field specifies the address responsible for submitting the
         message to the transfer service.  This field can be omitted if
         it contains the same address as RFC2822.From.  However,
         omitting this field does not mean that no Sender is specified;
         it means that that header field is virtual and that the address
         in the From: field MUST be used.

         Specification of the notifications Return addresses, which are
         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 fields 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 as a courtesy.

      RFC2822.BCC:   Set by - Author

         A copy of the message might be sent 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.  Use of this field is discussed in [RFC2822].

      RFC2821.HELO/.EHLO:   Set by - Originator, MSA, MTA

         Any SMTP client -- including Originator, MSA, or MTA -- can
         specify its hosting domain identity for the SMTP HELO or EHLO
         command operation.

      RFC3461.ENVID:   Set by - Originator

         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

      RFC2821.MailFrom:   Set by - Originator

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

      RFC2821.RcptTo:   Set by - Author, Final MTA, MDA.

         This field 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.ORCPT:   Set by - Author.

         This is an optional parameter to the RCPT command, indicating
         the original address to which the current RCPT TO address
         corresponds, after a mapping was performed during transit.  An
         ORCPT is the only reliable way to correlate a DSN from a multi-
         recipient message transfer with the intended recipient.

      RFC2821.Received:   Set by - Originator, Relay, Mediator, Dest

         This field contains trace information, including originating
         host, Relays, Mediators, and MSA host domain names and/or IP

      RFC2821.Return-Path:   Set by - Originator

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

      RFC2919.List-Id:   Set by - Mediator Author

         This field 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.  The result is rarely a true domain name, listed
         in the domain name service,  although it can be.

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      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.  [RFC2369]

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

         [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, which makes it independent of mail-
         level mechanisms.  As such, it is often taken to be
         authoritative, although it is possible to provide false

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 MHS
   architecture that is, in turn, above the Internet Transport layer.
   Because the motivation for email, and much of its use, is for
   interaction among people, the nature and details of these protocol
   exchanges often are determined by the needs of interpersonal and
   group communication.  To accommodate the idiosyncratic behavior
   inherent in such communication, only subjective guidelines, rather
   than strict rules, can be offered for some aspects of system
   behavior.  Mailing Lists provide particularly salient examples.

4.2.1.  Mail User Agent (MUA)

   A Mail User Agent (MUA) works on behalf of User actors and User
   applications.  It is their representative within the email service.

   The Author MUA (aMUA) 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 (aMS).  An MUA aMS can organize
   messages in many different ways.  A common model uses aggregations,
   called "folders".  This model allows 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 transfer (Sent).  But none of these folders
   is required.  For example, IMAP allows drafts to be stored in any

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   folder; so no Drafts folder is present.

   The Recipient MUA (rMUA) works on behalf of the Recipient to process
   received mail.  This processing includes generating user-level
   disposition 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 itself can have a
         distributed implementation, such as a "thin" user interface
         module on a constrained device such as a smartphone, with most
         of the MUA functionality running 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.  An 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.

   An MUA can be automated, on behalf of a user who is not present at
   the time the MUA is active.  One example is a bulk sending service
   that has a timed-initiation feature.  These services are not to be
   confused with a mailing list Mediator, since there is no incoming
   message triggering the activity of the automated service.

   A popular and problematic MUA is an automatic responder, such as one
   that sends out-of-office notices.  This behavior might be confused
   with that of a Mediator, but this MUA is generating a new message.
   Automatic responders can annoy users of mailing lists unless they
   follow [RFC3834]. ****** The recommendations in RFC 3834 are an
   important consequence of the addressing architecture of Internet Mail
   so they do help illustrate the architecture. *****

   These identity fields are relevant to a typical MUA:




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         RFC2822.To, RFC2822.CC


4.2.2.  Message Store (MS)

   An MUA can employ a long-term Message Store (MS).  Figure 5 depicts
   an Author's MS (aMS) and a Recipient's MS (rMS).  An MS can be
   located on a remote server or on the same machine as the MUA.

   An MS acquires messages from an MDA either by a local mechanism or by
   using POP or IMAP.  The MUA accesses the MS either by a local
   mechanism or by using POP or IMAP.  Using POP for message access,
   rather than bulk transfer, is rare, awkward, and largely non-

4.3.  MHS-Level Services

4.3.1.  Mail Submission Agent (MSA)

   A Mail Submission Agent (MSA) accepts the message submitted by the
   aMUA and enforces the policies of the hosting ADMD and the
   requirements of Internet standards.  An MSA represents an unusual
   functional dichotomy.  It represents the interests of the Author
   (aMUA) during message posting, to facilitate posting success; it also
   represents the interests of the MHS.  In the architecture, these
   responsibilities are modeled, as shown in Figure 5, by dividing the
   MSA into two sub-components, aMSA and hMSA, respectively.  Transfer
   of responsibility for a single message, from an Author's environment
   to the MHS, is called "posting".  In Figure 5 it is marked as the (S)
   transition, within the MSA.

   The hMSA takes 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 MSA performs final
   message preparation for submission and effects the transfer of
   responsibility to the MHS, via the hMSA.  The amount of preparation
   depends upon the local implementations.  Examples of oMSA tasks
   include adding header fields, such as Date: and Message-ID:, and
   modifying portions of the message from local notations to Internet
   standards, such as expanding an address to its formal RFC2822

   Historically, standards-based MUA/MSA message postings have used
   SMTP.  [RFC2821] The standard currently preferred is SUBMISSION.
   [RFC4409] Although SUBMISSION derives from SMTP, it uses a separate
   TCP port and imposes distinct requirements, such as access

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   These identities are relevant to the MSA:







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
   Recipients.  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.  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
   change addresses in the envelope or reformulate the editorial
   content.  A change in data form, such as to MIME Content-Transfer-
   Encoding, is within the purview of an MTA, but removal or replacement
   of body content is not.  An MTA also adds trace information.

      NOTE:    Within a destination ADMD, email relaying modules can
         make a variety of changes to the message, prior to delivery.
         In such cases, these modules are acting as Gateways, rather
         than MTAs.

   Internet Mail uses SMTP [RFC2821], [RFC0821] primarily 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, the Internet Mail SMTP supports a
   basic level of reliability, by virtue of providing for retransmission
   after a temporary transfer failure.  Unlike typical packet switches
   (and Instant Messaging services), Internet Mail MTAs are expected to
   store messages in a manner that allows recovery across service
   interruptions, such as host system shutdown.  The degree of such

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   robustness and persistence by an MTA can vary.  The base SMTP
   specification provides a framework for protocol response codes.  An
   extensible enhancement to this framework is defined in [RFC5248]

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

   MTAs can perform any of these well-established roles:

      Boundary MTA:   An MTA that is part of an ADMD and interacts with
         MTAs in other ADMDs.  This is also called a Border MTA.  There
         can be different Boundary MTAs, according to the direction of

         Outbound MTA:   An MTA that relays messages to other ADMDs.

         Inbound MTA:   An MTA that receives inbound SMTP messages from
            MTA  Relays in other ADMDs, for example, an MTA running on
            the host listed as the target of an MX record.

      Final MTA:   The MTA that transfers a message to the MDA.

   These identities are relevant to the MTA:





      RFC2822.Received:   Set by - Relay Server


4.3.3.  Mail Delivery Agent (MDA)

   A transfer of responsibility from the MHS to a Recipient's
   environment (mailbox) is called "delivery."  In the architecture, as
   depicted in Figure 5, delivery takes place within a Mail Delivery

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   Agent (MDA) and is shown as the (D) transition from the MHS-oriented
   MDA component (hMDA) to the Recipient-oriented MDA component (rMDA).

   An MDA can provide distinctive, address-based functionality, made
   possible by its detailed information about the properties of the
   destination address.  This information 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 is required to know where to deliver the

   Like 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 as shows as "(D)"
   in Figure 5.  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 that the Recipient specifies.

   Transfer into the MDA is accomplished by a normal MTA transfer
   mechanism.  Transfer from an MDA to an MS uses an access protocol,
   such as POP or IMAP.

      NOTE:    The term "delivery" can refer to the formal, MHS function
         specified here or to the first time a message is displayed to a
         Recipient.  A simple, practical test for whether the MHS-based
         definition applies is whether a DSN can be generated.

   These identities are relevant to the MDA:

      RFC2821.Return-Path:   Set by - Author Originator or Mediator

         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.

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4.4.  Transition Modes

   From the origination site to the point of delivery, Internet Mail
   usually follows a "push" model.  That is, the actor that holds the
   message initiates transfer to the next venue, typically with SMTP
   [RFC2821] or LMTP [RFC2033].  With a "pull" model, the actor that
   holds the message waits for the actor in the next venue to initiate a
   request for transfer.  Standardized mechanisms for pull-based MHS
   transfer are ETRN [RFC1985] and ODMR [RFC2645].

   After delivery, the Recipient's MUA (or MS) can gain access by having
   the message pushed to it or by having the receiver of access pull the
   message, such as by using POP [RFC1939] and IMAP [RFC3501].

4.5.  Implementation and Operation

   A discussion of any interesting system architecture often bogs down
   when architecture and implementation are confused.  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.  For example, a software system
   that primarily performs message relaying  is an MTA, yet it might
   also include MDA functionality.  That same MTA system might be able
   to interface with non-Internet email services and thus perform both
   as an MTA and as a Gateway.

   Similarly, implemented modules might be configured to form
   elaborations of the architecture.  An interesting example is a
   distributed MS.  One portion might be a remote server and another
   might be local to the MUA.  As discussed in [RFC1733], there are
   three operational relationships among such MSs:

      Online:   The MS is remote, and messages are accessible only when
         the MUA is attached to the MS so that the MUA will re-fetch 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 they are connected.  When they
         are disconnected, mail can arrive at the rMS and the user can
         make changes to the uMS.  The two stores are re-synchronized
         when they are reconnected.

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5.  Mediators

   Basic message transfer from Author to 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 sequence of postings and deliveries
   through Mediators.  A Mediator forwards a message, through a re-
   posting process.  The Mediator shares some functionality with basic
   MTA relaying, but has greater flexibility in both addressing and
   content than is available to MTAs.

   This is the core set of message information that is commonly set by
   all types of Mediators:

      RFC2821.HELO/.EHLO:   Set by - Mediator Originator

      RFC3461.ENVID:   Set by - Mediator Originator

      RFC2821.RcptTo:   Set by - Mediator Author

      RFC2821.Received:   Set by - Mediator Dest

         The Mediator 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 include
         everything from original posting, through relaying, to final

   The aspect of a Mediator that distinguishes it from any other MUA
   creating a 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 messages that a Mediator does not create include:

      New message that forwards an existing message:

         Although this action provides a basic template for a class of
         Mediators, its typical occurrence is not, itself, an example of
         a Mediator.  The new message is viewed as being from the actor
         that is doing the forwarding, rather than from the original

         A new message encapsulates the original message and is seen as
         from the new Originator.  This Mediator Originator might add
         commentary and can modify the original message content.

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         Because the forwarded message is a component of the message
         sent by the new Originator, the new message creates a new
         dialogue.  However the final Recipient still sees the contained
         message as from the original Author.


         When a Recipient responds to the Author of a message, the new
         message is not typically viewed as a forwarding of the
         original.  Its focus is the new content, although it might
         contain all or part of the material from the original message.
         The earlier material is merely contextual and secondary.  This
         includes automated replies, such as vacation out-of-office
         notices, as discussed in Section 4.2.1.


         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 primary
         purpose of the new message is to provide commentary from a new
         Author, similar to a Reply.

   The remainder of this section describes common examples of

5.1.  Alias

   One function of an MDA is to determine the internal location of a
   mailbox in order to perform delivery.  An Alias is a simple re-
   addressing facility that provides one or more new Internet Mail
   addresses, rather than a single, internal one; the message continues
   through the transfer service, for delivery to one or more alternate
   addresses.  Although typically implemented as part of an MDA, this
   facility is a Recipient function.  It resubmits the message, although
   all handling information except the envelope recipient
   (rfc2821.RcptTo) address is retained.  In particular, the Return
   address (rfc2821.MailFrom) is unchanged.

   What is distinctive about this forwarding mechanism is how closely it
   resembles normal MTA store-and-forward relaying.  Its only
   significant difference is that it changes the RFC2821.RcptTo value.
   Because this change is so small, aliasing can be viewed as a part of
   the lower-level mail relaying activity.  However, this small change
   has a large semantic impact: The designated recipient has chosen a
   new recipient.

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      NOTE:    When the replacement list includes more than one address,
         the alias is increasingly likely to have delivery problems.
         Any problem reports go to the original Author, not the
         administrator of the alias entry. This makes it more difficult
         to resolve the problem, because the original Author has no
         knowledge of the Alias mechanism.

   Alias typically changes only envelope information:

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

         These fields retain their original addresses.

      RFC2821.MailFrom:   Set by - Author

         The benefit of retaining the original MailFrom value is to
         ensure that an actor related to the originating ADMD knows
         there has been a delivery problem.  On the other hand, the
         responsibility for handling problems, when transiting from the
         original recipient mailbox to the alias mailbox usually lies
         with that original Recipient, because the Alias mechanism is
         strictly under that Recipient's control.  Retaining the
         original MailFrom address prevents this.

5.2.  ReSender

   Also called the ReDirector, the ReSender's actions differ from
   forwarding because the Mediator "splices" a message's addressing
   information to connect the Author of the original message with the
   Recipient of the new message.  This connection permits them to have
   direct exchange, using their normal MUA Reply functions, while also
   recording full reference information about the Recipient who served
   as a Mediator.  Hence, the new Recipient sees the message as being
   from the original Author, even if the Mediator adds commentary.

   These identities are relevant to a resent message:

      RFC2822.From:   Set by - original Author

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         Names and addresses for the original Author  of the message
         content are retained.  The free-form (display-name) portion of
         the address might be modified to provide informal reference to
         the ReSender.

      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's Originator or Mediator

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

         These fields specify the original message Recipients.

      RFC2822.Resent-From:   Set by - Mediator Author

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

      RFC2822.Resent-Sender:   Set by - Mediator Originator

         The address of the actor responsible for resubmitting the
         message.  As with RFC2822.Sender, this field can be omitted
         when it contains the same address as RFC2822.Resent-From.

      RFC2822.Resent-To/-CC/-BCC:   Set by: Mediator Author

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

      RFC2821.MailFrom:   Set by - Mediator Originator

         The actor responsible for resubmission (RFC2822.Resent-Sender)
         is also responsible for specifying the new MailFrom address.

5.3.  Mailing Lists

   A Mailing List receives messages as an explicit addressee and then
   re-posts them to a list of subscribed members.  The Mailing List
   performs a task that can be viewed as an elaboration of the ReSender.
   In addition to sending the new message to a potentially large number
   of new Recipients, the Mailing List can modify content, for example,
   by deleting attachments, converting the format, and adding list-
   specific comments.  Mailing Lists also archive messages posted by

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   Authors.  Still the message retains characteristics of being from the
   original Author.

   These identities are relevant to a mailing list processor, when
   submitting a message:

      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 of the
         message content are retained.

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

         Although problematic, it is common for a Mailing List to assign
         its own addresses to the Reply-To: header field of messages
         that it posts.  This assignment is intended to ensure that
         replies go to all list members, rather than to only the
         original Author.  As a User actor, a Mailing List is the Author
         of the new message and can legitimately set the Reply-To:
         value.  As a Mediator attempting to represent the message on
         behalf of its original Author, creating or modifying a
         Reply-To: field can be viewed as violating that Author's
         intent.  Modifying the field to include the list address can
         send to the entire list replies that are meant only for the
         original Author.  When the Mailing List does not set the field,
         a reply meant for the entire list can instead go only to the
         original Author.  At best, either choice is a matter of group
         culture for the particular list.

      RFC2822.Sender:   Set by - Author Originator or Mediator

         This field usually specifies the address of the actor
         responsible for Mailing List operations.  Mailing Lists that
         operate in a manner similar to a simple MTA Relay preserve as
         much of the original handling information as possible,
         including the original RFC2822.Sender field.  (Note that this
         mode of operation causes the Mailing List to behave much like
         an Alias, with a possible difference in number of new

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

         These fields usually contain the original list of Recipient

      RFC2821.MailFrom:   Set by - Mediator Originator

         Because a Mailing List can modify the content of a message in
         any way, it is responsible for that content; that is, it is an
         Author.  As such, the Return Address is specified by the
         Mailing List.  Although it is plausible for the Mailing List to
         re-use the Return Address employed by the original Originator,
         notifications sent to that address after a message has been
         processed by a Mailing List could be problematic.

5.4.  Gateways

   A Gateway performs the basic routing and transfer work of message
   relaying, but it also is permitted to modify content, structure,
   address, or attributes as needed to send the message into a messaging
   environment that operates under 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 follow similar technical
   standards, but significantly different administrative policies, it is
   easy to view a Gateway as merely an MTA.

   The critical distinction between an MTA and a Gateway is that a
   Gateway can make substantive changes to a message to map between the
   standards.  In virtually all cases, this mapping results in some
   degree of semantic loss.  The challenge of Gateway design is to
   minimize this loss.  Standardized gateways to Internet Mail are
   facsimile [RFC4143], voicemail [RFC3801], and MMS [RFC4356]

   A Gateway can set any identity field available to an MUA.  These
   identities are typically relevant to Gateways:

      RFC2822.From:   Set by - original Author

         Names and addresses for the original Author of the message
         content are retained.  As for all original addressing
         information in the message, the Gateway can translate addresses
         as required to continue to be useful in the target environment.

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      RFC2822.Reply-To:   Set by - original Author

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

      RFC2822.Sender:   Set by - Author Originator or Mediator

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

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

         These fields usually retain their original addresses.

      RFC2821.MailFrom:   Set by - Author Originator or Mediator

         The actor responsible for handling the message can specify a
         new address to receive handling notices.

5.5.  Boundary Filter

   To enforce security boundaries, organizations can subject messages to
   analysis, for conformance with its 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 add content to the
   message that records the actions.

6.  Considerations

6.1.  Security Considerations

   This document describes the existing Internet Mail architecture.  It
   introduces no new capabilities.  The security considerations of this
   deployed architecture are documented extensively in the technical
   specifications referenced by this document.  These specifications
   cover classic security topics, such as authentication and privacy.
   For example, email transfer protocols can use standardized mechanisms
   for operation over authenticated and/or encrypted links, and message
   content has similar protection standards available.  Examples of such
   mechanisms include SMTP-TLS [RFC3207], SMTP-Auth [RFC2554], OpenPGP
   [RFC4880], and S/MIME [RFC3851].

   The core of the Internet Mail architecture does not impose any

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   security requirements or functions on the end-to-end or hop-by-hop
   components.  For example, it does not require participant
   authentication and does not attempt to prevent data disclosure.

   Particular message attributes might expose specific security
   considerations.  For example, the blind carbon copy feature of the
   architecture invites disclosure concerns, as discussed in section 7.2
   of [RFC2821] and section 5 of [RFC2822].  Transport of text or non-
   text content in this architecture has security considerations that
   are discussed in [RFC2822], [RFC2045], [RFC2046], and [RFC4288] as
   well as the security considerations present in the IANA media types
   registry for the respective types.

   Agents that automatically respond to email raise significant security
   considerations, as discussed in [RFC3834].  Gateway behaviors affect
   end-to-end security services, as discussed in [RFC2480].  Security
   considerations for boundary filters are discussed in [RFC5228].

   See section 7.1 of [RFC2821] for a discussion of the topic of
   origination validation.  As mentioned in Section 4.1.4, it is common
   practice for components of this architecture to use the
   [RFC0791].SourceAddr to make policy decisions [RFC2505], although the
   address can be "spoofed".  It is possible to use it without
   authorization.  SMTP and Submission authentication [RFC2554],
   [RFC4409] provide more secure alternatives.

   The discussion of trust boundaries, ADMDs, actors, roles, and
   responsibilities in this document highlights the relevance and
   potential complexity of security factors for operation of an Internet
   mail service.  The core design of Internet Mail to encourage open and
   casual exchange of messages has met with scaling challenges, as the
   population of email participants has grown to include those with
   problematic practices.  For example, spam, as defined in [RFC2505],
   is a by-product of this architecture.  A number of standards track or
   BCP documents on the subject have been issued.  [RFC2505], [RFC5068],

6.2.  IANA Considerations

   This document has no actions for IANA.

6.3.  Internationalization

   Because its origins date back to the use of ASCII, Internet Mail has
   had an ongoing challenge to support the wide range of necessary
   international data representations.  For a discussion of this topic,
   see [MAIL-I18N].

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7.  References

7.1.  Normative

   [RFC0791]  Postel, J., "Internet Protocol", RFC 791, 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.

   [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,

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              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.

   [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.

   [RFC3834]  Moore, K., "Recommendations for Automatic Responses to
              Electronic Mail", RFC 3834, August 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

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              Diverse Service Environments (Lemonade) Profile",
              June 2006.

   [RFC5228]  Showalter, T., "Sieve: A Mail Filtering Language",
              RFC 5228.

   [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", RFC 5248, June 2008.

7.2.  Informative

              Internet Mail Consortium, "Using International Characters
              in Internet Mail", IMC IMCR-010, August 1998.

   [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.

   [RFC1985]  De       Winter, J., "SMTP Service Extension for Remote
              Message Queue Starting", August 1996.

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

   [RFC2142]  Crocker, D., "Mailbox Names for Common services, Roles and
              Functions", RFC 2142, May 1997.

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

   [RFC2480]  Freed, N., "Gateways and MIME Security Multiparts",
              RFC 2480, January 1999.

   [RFC2505]  Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs",
              RFC 2505, February 1999.

   [RFC2554]  Myers, J., "SMTP Service Extension for Authentication",
              RFC 2554, March 1999.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over

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              Transport Layer Security", RFC 3207, February 2002.

   [RFC3685]  Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
              Extensions", RFC 3685, February 2004.

   [RFC3801]  Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
              Mail - version 2 (VPIMv2)", RFC 3801, June 2004.

   [RFC3851]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004.

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

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

   [RFC4143]  Toyoda, K. and D. Crocker, "Facsimile Using Internet Mail
              (IFAX) Service of ENUM", RFC 4143, November 2005.

   [RFC4356]  Gellens, R., "Mapping Between the Multimedia Messaging
              Service (MMS) and Internet Mail", RFC 4356, January 2006.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

   [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 an early version of [RFC5068].
   Discussion of the Originator 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, as did
   Chris Newman for the final versions, while also serving as cognizant
   Area Director for the document.  Tony Hansen served as document
   shepherd, through the IETF process.

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   Later reviews and suggestions were provided by Eric Allman, Nathaniel
   Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch,
   Ned Freed, Eric Hall, Willemien Hoogendoorn, Brad Knowles, John
   Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg,
   Alexey Melnikov, der Mouse, S. Moonesamy, Daryl Odnert, Rahmat M.
   Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, Greg

   Diligent early proof-reading was performed by Bruce Lilly.  Diligent
   technical editing was provided by Susan Hunziker


      accountability  11
      accountable  12-13
         Administrative  13
         Author  8
         Consumer  14
         Edge  14
         Gateway  12
         Originator  11
         Recipient  9
         Return Handler  9
         Transit  14
         MHS  10
      ADMD  11, 13-14, 18, 23, 29, 36
      Administrative Actors  13
      Administrative Management Domain  11
      aMSA  29
      Author  8, 10
      author  33

      body-parts  22
      bounce handler  9
      boundary  14

      Consumer Actor  14
      content  10, 12-13, 18, 22, 30

      delivery  4, 9-10, 12-13, 17, 22-23, 33, 35-36

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      Discussion of document  7

      Edge Actor  14
      end-to-end  4
      envelope  9, 12, 19, 22-23, 30, 35-36
      ETRN  33

      Gateway  10, 12

      header  22
      hMSA  29

      Internet Mail  4

      LMTP  33
      local-part  16

      Mail  4
      Mail      User Agent  4
      Mail From  35
      Mail Handling Service  4, 10
      Mail Submission Agent  11
      Mail Transfer Agent  4
      mailbox  35
      MDA  35
      MDN  9
      message  6, 22
      Message Disposition Notification  9
      MHS  4, 9-12, 19-20, 22-23
         Actors  10
      MSA  11, 29
      MTA  4, 14
         boundary  14
      MUA  4, 13, 28-29

      ODMR  33
      Originator  10-11

      posting  4, 9, 11, 19, 28-29, 33, 36
      pull  33

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      push  33

      RcptTo  10
      Receiver  10
      Recipient  9-10, 35
      recipient  33
      relay  10
      responsibility  29
      responsible  12-13
      Return address  35
      Return Handler  9
      role  9, 17
         Author  8
         Originator  11
         Recipient  9

      SIEVE  22
      SMTP  33

      transfer  10, 12-13
      Transit Actor  14
      transition  29

      UA  4
      User Agent  4

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

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