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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

Crocker Informational [Page 1] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 2] RFC 5598 Email Architecture July 2009

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:

Crocker Informational [Page 3] RFC 5598 Email Architecture July 2009

  • 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].

Crocker Informational [Page 4] RFC 5598 Email Architecture July 2009

                                       +--------+
                    ++================>|  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

Crocker Informational [Page 5] RFC 5598 Email Architecture July 2009

  • 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

Crocker Informational [Page 6] RFC 5598 Email Architecture July 2009

 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)

Crocker Informational [Page 7] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 8] RFC 5598 Email Architecture July 2009

         ++==========++
         ||  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.

Crocker Informational [Page 9] RFC 5598 Email Architecture July 2009

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.

Crocker Informational [Page 10] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 11] RFC 5598 Email Architecture July 2009

         ++==========++                        ++===========++
         ||  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).

Crocker Informational [Page 12] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 13] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 14] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 15] RFC 5598 Email Architecture July 2009

       +--------+     +---------+     +-------+     +-----------+
       |  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.

Crocker Informational [Page 16] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 17] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 18] RFC 5598 Email Architecture July 2009

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

Crocker Informational [Page 19] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 20] RFC 5598 Email Architecture July 2009

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)

Crocker Informational [Page 21] RFC 5598 Email Architecture July 2009

    Message Store (MS)
       Author MS (aMS)
       Recipient MS (rMS)
 This figure shows function modules and the standardized protocols
 used between them.

Crocker Informational [Page 22] RFC 5598 Email Architecture July 2009

                   ++========++
                   ||        ||                             +-------+
        ...........++  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

Crocker Informational [Page 23] RFC 5598 Email Architecture July 2009

          ... 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.

Crocker Informational [Page 24] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 25] RFC 5598 Email Architecture July 2009

 +----------------------+----------------+---------------------------+
 | 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.

Crocker Informational [Page 26] RFC 5598 Email Architecture July 2009

 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.

Crocker Informational [Page 27] RFC 5598 Email Architecture July 2009

 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].

Crocker Informational [Page 28] RFC 5598 Email Architecture July 2009

    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

Crocker Informational [Page 29] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 30] RFC 5598 Email Architecture July 2009

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.

Crocker Informational [Page 31] RFC 5598 Email Architecture July 2009

 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].

Crocker Informational [Page 32] RFC 5598 Email Architecture July 2009

 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).

Crocker Informational [Page 33] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 34] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 35] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 36] RFC 5598 Email Architecture July 2009

       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:

Crocker Informational [Page 37] RFC 5598 Email Architecture July 2009

    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.

Crocker Informational [Page 38] RFC 5598 Email Architecture July 2009

    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.

Crocker Informational [Page 39] RFC 5598 Email Architecture July 2009

    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.

Crocker Informational [Page 40] RFC 5598 Email Architecture July 2009

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.

Crocker Informational [Page 41] RFC 5598 Email Architecture July 2009

    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].

Crocker Informational [Page 42] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 43] RFC 5598 Email Architecture July 2009

    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.

Crocker Informational [Page 44] RFC 5598 Email Architecture July 2009

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.

Crocker Informational [Page 45] RFC 5598 Email Architecture July 2009

 [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.

Crocker Informational [Page 46] RFC 5598 Email Architecture July 2009

 [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.

Crocker Informational [Page 47] RFC 5598 Email Architecture July 2009

 [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.

Crocker Informational [Page 48] RFC 5598 Email Architecture July 2009

 [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.

Crocker Informational [Page 49] RFC 5598 Email Architecture July 2009

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.

Crocker Informational [Page 50] RFC 5598 Email Architecture July 2009

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

Crocker Informational [Page 51] RFC 5598 Email Architecture July 2009

 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

Crocker Informational [Page 52] RFC 5598 Email Architecture July 2009

    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

Crocker Informational [Page 53] RFC 5598 Email Architecture July 2009

Author's Address

 Dave Crocker
 Brandenburg InternetWorking
 675 Spruce Drive
 Sunnyvale, CA  94086
 USA
 Phone: +1.408.246.8253
 EMail: dcrocker@bbiw.net

Crocker Informational [Page 54]

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