





Network Working Group                                         D. Crocker
Request for Comments: 5598                   Brandenburg InternetWorking
Category: Informational                                        July 2009


                       Internet Mail Architecture

Abstract

   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 need to 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.

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may




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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  History  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  The Role of This Architecture  . . . . . . . . . . . . . .  6
     1.3.  Document Conventions . . . . . . . . . . . . . . . . . . .  7
   2.  Responsible Actor Roles  . . . . . . . . . . . . . . . . . . .  7
     2.1.  User Actors  . . . . . . . . . . . . . . . . . . . . . . .  8
     2.2.  Message Handling Service (MHS) Actors  . . . . . . . . . . 11
     2.3.  Administrative Actors  . . . . . . . . . . . . . . . . . . 14
   3.  Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.1.  Mailbox  . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.2.  Scope of Email Address Use . . . . . . . . . . . . . . . . 18
     3.3.  Domain Names . . . . . . . . . . . . . . . . . . . . . . . 19
     3.4.  Message Identifier . . . . . . . . . . . . . . . . . . . . 19
   4.  Services and Standards . . . . . . . . . . . . . . . . . . . . 21
     4.1.  Message Data . . . . . . . . . . . . . . . . . . . . . . . 24
       4.1.4.  Identity References in a Message . . . . . . . . . . . 25
     4.2.  User-Level Services  . . . . . . . . . . . . . . . . . . . 29
     4.3.  MHS-Level Services . . . . . . . . . . . . . . . . . . . . 31
     4.4.  Transition Modes . . . . . . . . . . . . . . . . . . . . . 34
     4.5.  Implementation and Operation . . . . . . . . . . . . . . . 35
   5.  Mediators  . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     5.1.  Alias  . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     5.2.  ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 38
     5.3.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . . 39
     5.4.  Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 41
     5.5.  Boundary Filter  . . . . . . . . . . . . . . . . . . . . . 42
   6.  Considerations . . . . . . . . . . . . . . . . . . . . . . . . 42
     6.1.  Security Considerations  . . . . . . . . . . . . . . . . . 42
     6.2.  Internationalization . . . . . . . . . . . . . . . . . . . 43
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 45
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 47
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 50
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51











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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.

   The underlying technical standards for Internet Mail comprise a rich
   array of functional capabilities.  These specifications form the
   core:

      *  Simple Mail Transfer Protocol (SMTP) ([RFC0821], [RFC2821],
         [RFC5321]) moves a message through the Internet.

      *  Internet Mail Format (IMF) ([RFC0733], [RFC0822], [RFC2822],
         [RFC5322]) defines a message object.

      *  Multipurpose Internet Mail Extensions (MIME) [RFC2045] defines
         an enhancement to the message object that permits using
         multimedia attachments.

   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 need to 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.

   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
   terms.

   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:






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      *  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
         interactions

1.1.  History

   The first standardized architecture for networked email specified a
   simple split between the user world, in the form of Message User
   Agents (MUAs), and the transfer world, in the form of the Message
   Handling Service (MHS), which is composed of Message Transfer Agents
   (MTAs) [RFC1506].  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 architecture 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, remain
   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 2.1.4.  In fact, some uses of email consider the entire email
   service, including Author and Recipient, as a subordinate component.
   For these services, "end-to-end" refers to points outside the email
   service.  Examples are voicemail over email [RFC3801], EDI
   (Electronic Data Interchange) over email [RFC1767], and facsimile
   over email [RFC4142].







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

          Legend: === lines indicate primary (possibly indirect)
                      transfers or roles
                  ... lines indicate supporting transfers or roles

                Figure 1: Basic Internet Mail Service Model

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

      *  An email object

      *  Global addressing

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

      *  No requirement for prior arrangement between MTAs or between
         Authors and Recipients

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





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      *  No requirement for Author, Originator, or Recipients to be
         online at the same time

   The end-to-end portion of the service is the email object, called a
   "message".  Broadly, the message itself distinguishes 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.  The Role of This Architecture

   An Internet service is an integration of related capabilities among
   two or more participating nodes.  The capabilities are accomplished
   across the Internet by one or more protocols.  What connects a
   protocol to a service is an architecture.  An architecture specifies
   how the protocols implement the service by defining the logical
   components of a service and the relationships among them.  From that
   logical view, a service defines what is being done, an architecture
   defines where the pieces are (in relation to each other), and a
   protocol defines how particular capabilities are performed.

   As such, an architecture will more formally describe a service at a
   relatively high level.  A protocol that implements some portion of a
   service will conform to the architecture to a greater or lesser
   extent, depending on the pragmatic tradeoffs they make when trying to
   implement the architecture in the context of real-world constraints.
   Failure to precisely follow an architecture is not a failure of the
   protocol, nor is failure to precisely cast a protocol a failure of



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   the architecture.  Where a protocol varies from the architecture, it
   will of course be appropriate for it to explain the reason for the
   variance.  However, such variance is not a mark against a protocol:
   Happily, the IETF prefers running code to architectural purity.

   In this particular case, this architecture attempts to define the
   logical components of Internet email and does so post hoc, trying to
   capture the architectural principles that the current email protocols
   embody.  To different extents, email protocols will conform to this
   architecture more or less well.  Insofar as this architecture differs
   from those protocols, the reasons are generally well understood and
   are required for interoperation.  The differences are not a sign that
   protocols need to be fixed.  However, this architecture is a best
   attempt at a logical model of Internet email, and insofar as new
   protocol development varies from this architecture, it is necessary
   for designers to understand those differences and explain them
   carefully.

1.3.  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 part is the name of the
   field.  Hence <RFC5322.From> is the IMF From: header field in an
   email content header, and <RFC5321.MailFrom> is the address in the
   SMTP "Mail From" command.

   When occurring without the IMF (RFC 5322) 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.

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
   types:

      *  User

      *  Message Handling Service (MHS)

      *  ADministrative Management Domain (ADMD)







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   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 diagrams of email architecture.

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
   them.  As a pragmatic heuristic: User Actors can generate, modify, or
   look at the whole message.



























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

          Legend: === lines indicate primary (possibly indirect)
                      transfers or roles
                  ... lines indicate supporting transfers or roles

                 Figure 2: Relationships among User Actors

   From a User's perspective, all message-transfer activities are
   performed by a monolithic Message 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 back to an Author or
   Originator in the sequence of handling a message, that Actor is a
   User.

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) that 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.)

2.1.3.  Return Handler

   Also called "Bounce Handler", the Return Handler is a special form of
   Recipient tasked with servicing notifications generated by the MHS as
   it transfers or delivers the message.  (See Figure 3.)  These notices
   can be about failures or completions and are sent to an address that
   is specified by the Originator.  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.

2.1.4.  Mediator

   A Mediator receives, aggregates, reformulates, and redistributes
   messages among Authors and Recipients who are the principals in
   (potentially) 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
   RFC5321.RcptTo command for the original message, the MHS handles it
   the same way as for any other Recipient.  In particular, 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 a reformulated message, the
   Mediator is an Author, albeit an Author actually serving as an agent
   of one or more other Authors.  So a Mediator really is a full-fledged
   User.  Mediators are considered extensively in Section 5.

   A Mediator attempts to preserve the original Author's information in
   the message it reformulates but 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.



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   A Mediator's role is complex and contingent, for example, modifying
   and adding content or regulating which Users are allowed to
   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.  Message Handling Service (MHS) Actors

   The Message Handling Service (MHS) performs a single end-to-end
   transfer on behalf of the Author to reach the Recipient addresses
   specified in the original RFC5321.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.  As a pragmatic heuristic MHS Actors generate,
   modify, or look at only transfer data, rather than the entire
   message.

   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 +-->...                  |
          |                     +---------+                       |
          \-------------------------------------------------------/

         Legend: === and || lines indicate primary (possibly
                     indirect) transfers or roles
                 ... lines indicate supporting transfers or roles

                 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 also represents the local operator of the MHS, that is, the
   local ADministrative Management Domain (ADMD).




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   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
   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 Message Handling System (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.

   When a Relay stops attempting to transfer a message, it becomes an
   Author because it sends 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.




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   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
   namespace.  Another difference could be support for text-only content
   or multimedia.  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



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   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 that it facilitates discussion about
   designs, 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 between organizational
   partners over specially trusted paths.

   These are three basic types of ADMDs:

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

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

   Transit:    Mail Service Providers (MSPs) 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.


















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         +--------+     +---------+     +-------+     +-----------+
         |  ADMD1 |<===>|  ADMD2  |<===>| ADMD3 |<===>|   ADMD4   |
         |  ----- |     |  -----  |     | ----- |     |   -----   |
         |        |     |         |     |       |     |           |
         | Author |     |         |     |       |     | Recipient |
         |   .    |     |         |     |       |     |     ^     |
         |   V    |     |         |     |       |     |     .     |
         |  Edge..+....>|.Transit.+....>|-Edge..+....>|..Consumer |
         |        |     |         |     |       |     |           |
         +--------+     +---------+     +-------+     +-----------+

         Legend: === lines indicate primary (possibly indirect)
                     transfers or roles
                 ... lines indicate supporting transfers or roles

              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
   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.








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   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 performed.

   Mail Service Providers:

      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

   The forms of identity used by Internet Mail are: mailbox, domain
   name, message-ID, and ENVID (envelope identifier).  Each is globally
   unique.

3.1.  Mailbox

      "A mailbox receives mail.  It is a conceptual entity that does not
      necessarily pertain to file storage."  [RFC5322]

   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 [RFC5321].

   The portion to the left of the at-sign contains a string that is
   globally opaque and is called the <local-part>.  It is interpreted
   only by the entity specified by the address's domain name.  Except as
   noted later in this section, all other entities treat the
   <local-part> as an uninterpreted literal string and preserve all




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   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
   are not 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
   below.

   Basic email addressing defines the <local-part> as being globally
   opaque.  However, there are some uses of email that add a
   standardized, global schema to the value, such as between an Author
   and a Gateway.  The <local-part> details remain invisible to the
   public email transfer infrastructure, but provide addressing and
   handling instructions for further processing by the Gateway.
   Standardized examples of these conventions are the telephone
   numbering formats for the Voice Profile for Internet Mail (VPIM)
   [RFC3801], such as:

                       +16137637582@vpim.example.com,

   and iFax ([RFC3192], [RFC4143] such as:

                FAX=+12027653000/T33S=1387@ifax.example.com.

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 Sections
   4.1.4, 4.3.3, and 5.








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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 an individual User, but this is not common practice.  The
   name is structured as a hierarchical sequence of labels, separated by
   dots (.), with the top of the hierarchy being on the right end of the
   sequence.  There can be many names in the sequence -- that is, the
   depth of the hierarchy can be substantial.  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.

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

   IMF provides for, at most, a single Message-ID:.  The Message-ID: for
   a single message, which is a user-level IMF tag, has a variety of
   uses including threading, aiding identification of duplicates, and
   DSN (Delivery Status Notification) tracking.  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: is 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.  [RFC5322] states that "a message identifier pertains to



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   exactly one version 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:

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

   o  If a message has minor additions to the content, such as a Mailing
      List tag at the beginning of the RFC5322.Subject header field, or
      some Mailing List administrative information added to the end of
      the primary body part text, it is probably the same message.

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

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

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

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

   o  If a message is forwarded by a Recipient, what is forwarded is a
      new message.

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

   The absence of both objective, precise criteria for regenerating 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: is not intended to be used for any function
   that has security implications.






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3.4.2.  ENVID

   The ENVID (envelope identifier) can be used for message-tracking
   purposes ([RFC3885], [RFC3464]) 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 reuse 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
   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.

      Message

      Message 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)





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      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  ||                *   +------+    *     .   . //==+==\\
    ||  imf     ||                *   | MTA  +....*......   . || mdn ||
    ||  mime    ||                *   +--+---+    *         . \\=====//
    \\==========//                * smtp}| {local *         .     ^
          .           MDA         *      | {lmtp  *         .     .
          .      +----------------+------V-----+  *         .     .
          .      | +----------+   *   +------+ |  *         .     .
          .      | |          |   *   |      | +..*..........     .
          .      | |   rMDA   |<-(D)--+ hMDA | |  *               .
          .      | |          |   *   |      | |<.*........       .
          .      | +-+------+-+   *   +------+ |  *       .       .
          .      +------+---------*------------+  *       .       .
          .  smtp,local}|         *****************       .       .
          .             V                                 .       .
          .          +-----+                         //===+===\\  .
          .          | rMS |                         || sieve ||  .
          .          +--+--+                         \\=======//  .
          .             |{imap,pop,local                  ^       .
          .             V                                 .       .
          .       ++==========++                          .       .
          .       ||          ||                          .       .
          .......>||   rMUA   ++...........................       .
                  ||          ++...................................
                  ++==========++

    Legend: --- lines indicate primary (possibly indirect)
                transfers or roles
            === boxes indicate data objects



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            ... lines indicate supporting transfers or roles
            *** lines indicate aggregated service

                     Figure 5: Protocols and Services

4.1.  Message Data

   The purpose of the Message Handling System (MHS) is to exchange an
   IMF message object among participants [RFC5322].  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 multimedia subordinate
   objects, called "body-parts" or, popularly, "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
      RFC5321.MailFrom address.  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.








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   Message Filtering (SIEVE):

      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, such as
      a MIME part in a message.  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.  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 [RFC5321].

   Trace information, such as RFC5322.Received, is recorded in the
   message header and is not subsequently altered [RFC5322].

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 [RFC5322].  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 multimedia body part
   attachments using MIME ([RFC2045], [RFC2046], [RFC2047], [RFC4288],
   and [RFC2049]).

4.1.4.  Identity References in a Message

   Table 1 lists the core identifiers present in a message during
   transit.




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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          | Originator                |
   | IP                   | Source Address | Latest Relay Client       |
   +----------------------+----------------+---------------------------+

   Legend:
      Layer - The part of the email architecture that uses the
      identifier.

      Field - The protocol construct that contains the identifier.

      Set By - The Actor role responsible for specifying the identifier
      value (and this can be different from the Actor that performs the
      fill-in function for the protocol construct).

                        Table 1: Layered Identities

   These are the most common address-related fields:

   RFC5322.From:  Set by - Author

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









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   RFC5322.Reply-To:  Set by - Author

      If a Recipient sends a reply message that would otherwise use the
      RFC5322.From field addresses in the original message, the
      addresses in the RFC5322.Reply-To field are used instead.  In
      other words, this field overrides the From: field for responses
      from Recipients.

   RFC5322.Sender:  Set by - Originator

      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 RFC5322.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 is to be used.

      Specification of the notifications Return Addresses, which are
      contained in RFC5321.MailFrom, is made by the RFC5322.Sender.
      Typically, the Return address is the same as the Sender address.
      However, some usage scenarios require it to be different.

   RFC5322.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
      RFC5321.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.

   RFC5322.BCC:  Set by - Author

      A copy of the message might be sent to an addressee whose
      participation is not to be disclosed to the RFC5322.To or
      RFC5322.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 [RFC5322].

   RFC5321.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.





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   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 tracking.

   RFC5321.MailFrom:  Set by - Originator

      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 RFC5321.MailFrom address.  Ultimately,
      the simple basis for deciding which address needs to be in the
      RFC5321.MailFrom field is to determine which address is to be
      informed about transfer-level problems (and possibly successes).

   RFC5321.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 IMF destination address header fields, such as
      RFC5322.To, might specify a Mailing List mailbox, while the
      RFC5321.RcptTo address specifies a member of that list.

   RFC5321.ORCPT:   Set by - Originator.

      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.

   RFC5321.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 Addresses.

   RFC5321.Return-Path:  Set by - Originator

      The MDA records the RFC5321.MailFrom address into the
      RFC5321.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].



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      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.

   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 addresses.

4.2.  User-Level Services

   Interactions at the user level entail protocol exchanges, distinct
   from those that occur at lower layers of the Internet Mail 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.  Message User Agent (MUA)

   A Message 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
   archiving in its Message Store (aMS).  An MUA aMS can organize
   messages in many different ways.  A common model uses aggregations,
   called "folders"; in IMAP they are called "mailboxes".  This model



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   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 folder, so no Drafts folder needs to be 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 a 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 identity fields are relevant to a typical MUA:

      RFC5322.From

      RFC5322.Reply-To

      RFC5322.Sender

      RFC5322.To, RFC5322.CC

      RFC5322.BCC




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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 proactively by a local
   mechanism or even by a standardized mechanism such as SMTP(!), or
   reactively by using POP or IMAP.  The MUA accesses the MS either by a
   local mechanism or by using POP or IMAP.  Using POP for individual
   message accesses, rather than for bulk transfer, is relatively rare
   and inefficient.

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 aMSA 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 IMF
   representation.

   Historically, standards-based MUA/MSA message postings have used SMTP
   [RFC5321].  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 authorization.







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   These identities are relevant to the MSA:

      RFC5321.HELO/.EHLO

      RFC3461.ENVID

      RFC5321.MailFrom

      RFC5321.RcptTo

      RFC5321.Received

      RFC0791.SourceAddr

4.3.2.  Message Transfer Agent (MTA)

   A Message 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
   [RFC2505].

   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 ([RFC5321], [RFC2821], [RFC0821]) primarily
   to effect point-to-point transfers between peer MTAs.  Other transfer
   mechanisms include Batch SMTP [RFC2442] and On-Demand Mail Relay
   (ODMR) SMTP [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 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].



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   Although quite basic, the dominant 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 other.

   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 mail-flow.

                  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:

      RFC5321.HELO/.EHLO

      RFC3461.ENVID

      RFC5321.MailFrom

      RFC5321.RcptTo

      RFC5322.Received:  Set by - Relay Server

      RFC0791.SourceAddr

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






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   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
   message.

   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 and is shown as
   "(D)" in Figure 5.  The MHS portion (hMDA) primarily functions as a
   server SMTP engine.  A common additional role is to redirect 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:

      RFC5321.Return-Path:  Set by - Author Originator or Mediator
         Originator

         The MDA records the RFC5321.MailFrom address into the
         RFC5321.Return-Path field.

      RFC5322.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.

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
   [RFC5321] or the Local Mail Transfer Protocol (LMTP) [RFC2033].  With
   a "pull" model, the Actor that holds the message waits for the Actor



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   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.

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



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   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:

      RFC5321.HELO/.EHLO:  Set by - Mediator Originator

      RFC3461.ENVID:  Set by - Mediator Originator

      RFC5321.RcptTo:  Set by - Mediator Author

      RFC5321.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
         delivery.

   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
         Author.
         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.
         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.

      Reply:

         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



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         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.

      Annotation:

         The integrity of the original message is usually preserved, but
         one or more comments about the message are added in a manner
         that distinguishes commentary from original text.  The 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 Mediators.

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
   (rfc5321.RcptTo) address is retained.  In particular, the Return
   Address (rfc5321.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 RFC5321.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.

   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.

   Including the core set of message information listed at the beginning
   of this section, Alias typically changes:






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      RFC5322.To/.CC/.BCC:  Set by - Author

         These fields retain their original addresses.

      RFC5321.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.

   Including the core set of message information listed at the beginning
   of this section, these identities are relevant to a resent message:

      RFC5322.From:  Set by - original Author

         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 an informal reference
         to the ReSender.

      RFC5322.Reply-To:  Set by - original Author

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

      RFC5322.Sender:  Set by - Author's Originator or Mediator
         Originator

      RFC5322.To/.CC/.BCC:  Set by - original Author

         These fields specify the original message Recipients.




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      RFC5322.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 RFC5322.From field.

      RFC5322.Resent-Sender:  Set by - Mediator Originator

         The address of the Actor responsible for resubmitting the
         message.  As with RFC5322.Sender, this field can be omitted
         when it contains the same address as RFC5322.Resent-From.

      RFC5322.Resent-To/-CC/-BCC:  Set by - Mediator Author

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

      RFC5321.MailFrom:  Set by - Mediator Originator

         The Actor responsible for resubmission (RFC5322.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
   Authors.  Still the message retains characteristics of being from the
   original Author.

   Including the core set of message information listed at the beginning
   of this section, 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

      RFC5322.From:  Set by - original Author

         Names and email addresses for the original Author of the
         message content are retained.





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      RFC5322.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.  When the Reply-To is modified in this way, a reply
         that is meant only for the original Author will instead go to
         the entire list.  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.

      RFC5322.Sender:  Set by - Author Originator or Mediator Originator

         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 RFC5322.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
         addressees.)

      RFC5322.To/.CC:  Set by - original Author

         These fields usually contain the original list of Recipient
         addresses.

      RFC5321.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
         reuse 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.








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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 the Multimedia
   Messaging Service (MMS) [RFC4356].

   A Gateway can set any identity field available to an MUA.  Including
   the core set of message information listed at the beginning of this
   section, these identities are typically relevant to Gateways:

      RFC5322.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.

      RFC5322.Reply-To:  Set by - original Author

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

      RFC5322.Sender:  Set by - Author Originator or Mediator Originator

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

      RFC5322.To/.CC/.BCC:  Set by - original Recipient

         These fields usually retain their original addresses.






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      RFC5321.MailFrom:  Set by - Author Originator or Mediator
         Originator

         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 [RFC4954], OpenPGP
   [RFC4880], and S/MIME [RFC3851].

   The core of the Internet Mail architecture does not impose any
   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 [RFC5321] and Section 5 of [RFC5322].  Transport of text or non-
   text content in this architecture has security considerations that
   are discussed in [RFC5322], [RFC2045], [RFC2046], and [RFC4288];
   also, security considerations are present for some of the media types
   registered with IANA.

   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].



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   See Section 7.1 of [RFC5321] 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 ([RFC4409],
   [RFC4954]) 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 (see [RFC2505],
   [RFC5068], and [RFC5235]).

6.2.  Internationalization

   The core Internet email standards are based on the use of US-ASCII --
   that is, SMTP [RFC5321] and IMF [RFC5322], as well as their
   predecessors.  They describe the transport and composition of
   messages as composed strictly of US-ASCII 7-bit encoded characters.
   The standards have been incrementally enhanced to allow for
   characters outside of this limited set, while retaining mechanisms
   for backwards-compatibility.  Specifically:

   o  The MIME specifications ([RFC2045], [RFC2046], [RFC2047],
      [RFC2049]) allow for the use of coded character sets and
      character-encoding schemes ("charsets" in MIME terminology) other
      than US-ASCII.  MIME's [RFC2046] allows the textual content of a
      message to have a label affixed that specifies the charset used in
      that content.  Equally, MIME's [RFC2047] allows the textual
      content of certain header fields in a message to be similarly
      labeled.  However, since messages might be transported over SMTP
      implementations only capable of transporting 7-bit encoded
      characters, MIME's [RFC2045] also provides for "content transfer
      encoding" so that characters of other charsets can be re-encoded
      as an overlay to US-ASCII.

   o  MIME's [RFC2045] allows for the textual content of a message to be
      in an 8-bit character-encoding scheme.  In order to transport
      these without re-encoding them, the SMTP specification supports an
      option [RFC1652] that permits the transport of such textual





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      content.  However, the [RFC1652] option does not address the use
      of 8-bit content in message header fields, and therefore [RFC2047]
      encoding is still required for those.

   o  A series of experimental protocols on Email Address
      Internationalization (EAI) have been released that extend SMTP and
      IMF to allow for 8-bit encoded characters to appear in addresses
      and other information throughout the header fields of messages.
      [RFC5335] specifies the format of such message header fields
      (which encode the characters in UTF-8), and [RFC5336] specifies an
      SMTP option for the transport of these messages.

   o  MIME's [RFC2045] and [RFC2046] allow for the transport of true
      multimedia material; such material enables internationalization
      because it is not restricted to any particular language or locale.

   o  The formats for Delivery Status Notifications (DSNs -- [RFC3462],
      [RFC3463], [RFC3464]) and Message Disposition Notifications (MDNs
      -- [RFC3798]) include both a structured and unstructured
      representation of the notification.  In the event that the
      unstructured representation is in the wrong language or is
      otherwise unsuitable for use, this allows an MUA to construct its
      own appropriately localized representation of notification for
      display to the User.

   o  POP and IMAP have no difficulties with handling MIME messages,
      including ones containing 8bit, and therefore are not a source of
      internationalization issues.

   Hence, the use of UTF-8 is fully established in existing Internet
   Mail.  However, support for long-standing encoding forms is retained
   and is still used.



















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7.  References

7.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [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.

   [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]  Gellens, R., "ON-DEMAND MAIL RELAY (ODMR) SMTP with
              Dynamic IP Addresses", RFC 2645, August 1999.

   [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 3192, October 2001.



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   [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.

   [RFC3462]  Vaudreuil, G., "The Multipart/Report Content Type for the
              Reporting of Mail System Administrative Messages",
              RFC 3462, January 2003.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, 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", BCP 90, RFC 3864,
              September 2004.

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

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

   [RFC4289]  Freed, N. and J. Klensin, "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. and A. Melnikov, "Internet Email to Support
              Diverse Service Environments (Lemonade) Profile",
              RFC 4550, June 2006.




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   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

   [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", BCP 138, RFC 5248, June 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

7.2.  Informative References

   [RFC0733]  Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
              "Standard for the format of ARPA network text messages",
              RFC 733, November 1977.

   [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.

   [RFC1506]  Houttuin, J., "A Tutorial on Gatewaying between X.400 and
              Internet Mail", RFC 1506, August 1993.

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

   [RFC1733]  Crispin, M., "Distributed Electronic Mail Models in
              IMAP4", RFC 1733, 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", RFC 1985, 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]  Freed, N., Newman, D., and Hoy, M., "The Batch SMTP Media
              Type", RFC 2442, November 1998.



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

   [RFC2505]  Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs",
              BCP 30, RFC 2505, February 1999.

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

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

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

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

   [RFC3851]  Ramsdell, B., "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)", RFC 4142, November 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.

   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

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



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   [RFC5235]  Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
              Extensions", RFC 5235, January 2008.

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

   [RFC5336]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email Addresses", RFC 5336, September 2008.

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







































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Appendix A.  Acknowledgments

   This work began in 2004 and has evolved through numerous rounds of
   community review; it derives from a section in an early version of
   [RFC5068].  Over its 5 years of development, the document has gone
   through 14 incremental versions, with vigorous community review that
   produced many substantive changes.  Review was performed in the IETF
   and other email technical venues.  Although not a formal activity of
   the IETF, issues with the document's contents were resolved using the
   classic style of IETF community open, group decision-making.  The
   document is already cited in other work, such as in IMAP and Sieve
   specifications and in academic classwork.  The step of standardizing
   is useful to provide a solid and stable reference to the Internet's
   now-complex email service.

   Details 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.

   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
   Vaudreuil, Patrick Cain, Paul Hoffman, Vijay Gurbani, and Hans
   Lachman.

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

   The final stages of development for this document were guided by a
   design team comprising Alexey Melnikov, Pete Resnick, Carl S.
   Gutekunst, Jeff Macdonald, Randall Gellens, Tony Hansen, and Tony
   Finch.  Pete Resnick developed the final version of the section on
   internationalization.










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Index

   7
      7-bit  44

   A
      accountability  12
      accountable  13-14
      Actor
         Administrative  14
         Author  10
         Consumer  15
         Edge  15
         Gateway  13
         Originator  12
         Recipient  10
         Return Handler  10
         Transit  15
      actor  7, 19, 26, 28-29, 35-36, 38-40, 42-43, 49
      Actors
         MHS  11
      addr-spec  17
      address
         addr-spec  17
         local-part  18
      ADMD  12, 14-15, 19, 25, 31, 37
      Administrative Actors  14
      Administrative Management Domain  12
      aMSA  31
      Author  10-11
      author  35

   B
      body parts  24
      bounce handler  10
      boundary  15

   C
      charset  44
      Consumer Actor  15
      content  11, 13-14, 20, 24, 32

   D
      delivery  4, 10-11, 13-14, 18, 24-25, 35, 37-38
      Discussion of document  7
      domain name  17, 21, 28
      DSN  44




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   E
      EAI  44
      Edge Actor  15
      encoding  44
      end-to-end  4-6, 11, 15, 28

      envelope  10, 13, 21, 24-25, 32, 37
      ETRN  35

   G
      Gateway  11, 13
      gateway  6, 12-13, 18, 25, 32

   H
      header  24
      hMSA  31

   I
      identifier  18-19, 21, 25, 29
      IMAP  24, 31, 34-35, 44
      IMF  19, 24, 44
      Internet Mail  4

   L
      left-hand side  18
      LMTP  24, 35
      local-part  18

   M
      Mail  4
      Mail From  37
      Mail Submission Agent  12
      mailbox  17, 19, 24, 28, 30, 33, 37-38
      MDA  24, 37
      MDN  10, 24, 44
      message  6, 24
      Message Disposition Notification  10
      Message Handling Service  4
      Message Handling System  11
      Message Transfer Agent  4
      Message User Agent  4
      MHS  4, 10-13, 21-22, 24-25
         Actors  11
      MIME  24, 44
      MS  24
      MSA  12, 24, 31
      MTA  4, 15
         boundary  15



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      MUA  4, 14, 24, 30-31

   O
      ODMR  35
      operations  3, 15, 18, 29, 40
      Originator  10-12

   P
      POP  24, 31, 34-35, 44
      posting  4, 10, 12, 21, 30-31, 35, 37
      pull  35
      push  35

   R
      RcptTo  11
      Receiver  11
      Recipient  10-11, 37
      recipient  35
      relay  11
      responsibility  31
      responsible  13-14
      Return Address  37
      Return Handler  10
      role  10, 18
         Author  10
         Originator  12
         Recipient  10

   S
      SIEVE  24-25
      SMTP  24, 35, 44

   T
      transfer  11, 13-14
      Transit Actor  15
      transition  31

   U
      UA  4
      User Agent  4











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Author's Address

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

   Phone: +1.408.246.8253
   EMail: dcrocker@bbiw.net









































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