SMTP                                                          D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Expires: July 27, 2005                                  January 26, 2005

                       Internet Mail Architecture

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on July 27, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   Over its thirty year history, Internet mail has undergone significant
   changes in scale and complexity.  The first standardized architecture
   for email specified a simple split between the user world and the
   transmission world, in the form of Mail User Agents (MUA) and Mail
   Transfer Agents (MTA).  Over time each of these has divided into
   multiple, specialized modules.  Public discussion and agreement about
   the nature of the changes to Internet mail has not kept pace, and
   abuses of the Internet mail service have brought these issues into

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   stark relief.  This draft offers clarifications and enhancements, to
   provide a more consistent base for community discussion of email
   service problems and proposed email service enhancements.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2 Document Changes . . . . . . . . . . . . . . . . . . . . . . .  5
   1.3 Discussion venue . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Email Actor Roles  . . . . . . . . . . . . . . . . . . . . . .  5
   2.1 User-Level Actors  . . . . . . . . . . . . . . . . . . . . . .  6
   2.2 Transfer-Level Actors  . . . . . . . . . . . . . . . . . . . .  8
   2.3 Administrative Actors  . . . . . . . . . . . . . . . . . . . . 11
   3.  Email Identities . . . . . . . . . . . . . . . . . . . . . . . 11
   3.1 Mailbox Addresses  . . . . . . . . . . . . . . . . . . . . . . 12
   3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . . 13
   3.3 Message Identifers . . . . . . . . . . . . . . . . . . . . . . 13
   3.4 Identity Referencing Convention  . . . . . . . . . . . . . . . 13
   4.  Protocols and Services . . . . . . . . . . . . . . . . . . . . 13
   4.1 Service Components . . . . . . . . . . . . . . . . . . . . . . 15
   4.2 Operational Configuration  . . . . . . . . . . . . . . . . . . 21
   4.3 Layers of Identity References  . . . . . . . . . . . . . . . . 21
   5.  Message Data . . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.1 Envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.2 Message Header Fields  . . . . . . . . . . . . . . . . . . . . 22
   5.3 Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   6.  Two Levels of Store-And-Forward  . . . . . . . . . . . . . . . 23
   6.1 MTA Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 23
   6.2 MUA Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 33
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
       Intellectual Property and Copyright Statements . . . . . . . . 34

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1.  Introduction

   Over its thirty year history, Internet mail has undergone significant
   changes in scale and complexity.  The first standardized architecture
   for email specified a simple split between the user world and the
   transmission world, in the form of Mail User Agents (MUA) and Mail
   Transfer Agents (MTA).  Over time each of these has sub-divided into
   more specialized modules.  However the basic style and use of names,
   addresses and message structure have remained remarkably constant.

   There are two, basic categories of participants in Internet Mail.
   Users are customers of the Mail Handling Service (MHS).  They
   represent the sources and sinks of that service.  The Mail Handling
   Service is responsible for accepting a message from one user and
   delivering to one or more others.

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

                  Figure 1: Basic Email Service Model

   Public discussion and agreement about terms of reference have not
   kept pace with the changes, and abuses of the Internet mail service
   have brought this into stark relief.  So, it is necessary to produce
   a revised architecture.  However it is important that the original
   distinction between user-level concerns and transfer-level concerns
   be retained.  This becomes challenging when the user-level exchange
   is, itself, a sequence, such as with group dialogue or organizational
   message flow, as occurs with a purchase approval process.  It is easy
   to confuse this user-level activity with the underlying mail
   transmission service exchanges.

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   For Internet mail, the term "end-to-end" usually refers to single
   posting and the set of deliveries resulting from a single transiting
   of the MHS.  However, note that specialized uses of email consider
   the entire email service -- including Originator and Recipient -- as
   a subordinate component.  For these services, "end-to-end" refers to
   points outside of the email service.  Examples are voicemail over
   email and EDI over email.

   The current draft seeks to:

   1.  Document changes that have taken place in refining the email

   2.  Clarify functional roles for the architectural components

   3.  Clarify identity-related issues, across the email service

   4.  Provide a common venue for further defining and citing modern
       Internet mail architecture

1.1  Service Overview

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

   1.  An email object

   2.  Global addressing

   3.  A connected sequence of point-to-point transfer mechanisms

   4.  No prior arrangement between originator and recipient

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

   The end-to-end portion of the service is the message.  Broadly the
   message, itself, is divided between handling control information and
   user message payload.

   A precept to the design of Internet mail is to permit user-to-user
   and MTA-to-MTA interoperability with no prior, direct administrative
   arrangement.  That is, all participants rely on having the core
   services be universally supported, either directly or through
   gateways that translate between Internet mail standards and other
   email conventions.

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   For localized environments (edge networks) prior, administrative
   arrangement can include access control, routing constraints and
   lookup service configuration.  In recent years one change to local
   environments is an increased requirement for authentication or, at
   least, accountability.  In these cases, the server performs explicit
   validation of the client's identity.

1.2  Document Changes

   The major changes from the previous version of this document are:

   Overall: Clarify roles and responsibilities

   Diagrams: Revised diagrams and tightened things up

   Distinct architectural 'sections': Added concept of ADMDs, as
      operational layer, separate from functional or architectural
      layer.  Added user "layer", as distinct from transfer.  Introduced

1.3  Discussion venue

   NOTE:  This document is the work of a single person, about a topic
      with considerable diversity of views.  It is certain to be
      incomplete and inaccurate.  Some errors simply need to be
      reported; they will get fixed.  Others need to be discussed by the
      community, because the real requirement is to develop common
      community views.  To this end, please treat the draft as a
      touchstone for public discussion.

   Discussion about this document should be directed to the:
   <> mailing list.  The IETF-SMTP mailing list
   <> is the most active,
   long-standing venue for discussing email architecture.  Although this
   list is primarily for discussing only the SMTP protocol, it is
   recommended that discussion of this draft take place on that mailing
   list.  This list tends to attend to end-to-end infrastructure and
   architecture issues more than other email-related mailing lists.

2.  Email Actor Roles

   Discussion of email architecture requires distinguishing different
   actors within the service, and being clear about the job each
   performs.  The best way to maintain the distinction between user
   activity and handling activities is to depict their details in
   separate diagrams.  Current Internet mail provides only a small set
   of capabilities for supporting different kinds of ongoing, user-level

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   Although related to a technical architecture, the focus of a
   discussions on Actors is on participant responsibilities, rather than
   functional modules.  Hence the labels used are different than for
   classic email architecture diagrams.  The figures depict the
   relationships among the Actors.  Actors often will be associated with
   entirely independent organizations from other Actors who are
   participating in the email service.

2.1  User-Level Actors

   Users are the sources and sinks of messages.  They may have an
   exchange that iterates and they may expand or contract the set of
   users participating in a set of exchanges.

   In Internet Mail there are three, basic types of user-level Actors:
   Originators, Recipients, and Mediators.  Fromhe t User-level
   perspective all mail transfer activities are performed by a
   monolithic, shared handling service.  Users are customers of this
   service.  The following depicts the relationships among them.

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

   The functions of these Actors are:

2.1.1  Originator

   Also called "Author", this is the user-level participant responsible

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   for creating original content and requesting its transmission.  The
   Mail Handling Service operates to send and deliver mail among
   Originators and Recipients.

2.1.2  Recipient

   The Recipient is a consumer of delivered content.

   A recipient may close the user-level communication loop by creating
   and submitting a new message that replies to an originator.  An
   automated, or semi-automated form of reply informs the Originator
   about the Recipient's disposition of the message.

2.1.3  Mediator

   A Mediator receives, aggregates, reformulates and distributes
   messages as part of a potentially-protracted, higher-level exchange
   among users.  A Mediator is viewed by the Mail Handling Service, when
   the Mediator's address is specified in the envelope.  When submitting
   messages, the Mediator is an Originator.  What is distinctive is that
   a Mediator preserves Originator information of the message(s) it
   reformulates, but makes meaningful changes to the content.  Hence the
   Mail Handling Service sees a new message, but Users receive a message
   that is interpreted as primarily being from the author of the
   original message.  The role of a Mediator permits distinct, active
   creativity, rather than being limited the more passive job of merely
   connecting together other participants.  Hence it is really the
   Mediator that is responsible for the new message.

   A Mediator's task may be complex, contingent and creative, such as by
   modifying and adding content or regulating which users may
   participate and when.  The popular example of this role is a group
   mailing list.  A sequence of mediators may even perform a series of
   formal steps, such as reviewing, modifying and approving a purchase

   Because a Mediator originates messages, it might also receive
   replies.  That is, a Mediator is a full-fledged User.

   Specialized Mediators include:

   Forwarder:  A new message encapsulates the original message and is
      seen as strictly "from" the Mediator.  However the Mediator might
      add commentary and certainly has the opportunity to modify the
      original message content.

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   Redirector:  Redirection differs from Forwarding by virtue of having
      the Mediator "splice" communication between the Originator of the
      original message and the Recipient of the new message.  Hence the
      new Recipient sees the message as being From the original

   Mailing List:  This Actor performs a task that can be viewed as an
      elaboration of the Redirector role.  In addition to sending the
      new message to a potentially large number of new Recipients,
      content might be modified, such as deletion of attachments,
      formatting conversion, and addition of list-specific comments.  In
      additional, archival of list messages is common.

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

   Adaptor:  {per Ned Freed}

   Security Filter:  Organizations often enforce security boundaries by
      having message subjected to analysis for conformance with the
      organization's safety policies.  Examples are detection of content
      classed as spam or a virus.  A Security Filter might alter the
      content, to render it safe, such as by removing content deemed
      unacceptable.  Typically these actions will result in the addition
      of content that records the actions.

2.2  Transfer-Level Actors

   The Mail Handling Service has the task of performing a single,
   end-to-end transfer on behalf of the originator and reaching the
   recipient address(es) specified in the envelope.  Protracted,
   iterative exchanges, such as those used for collaboration over time,
   are part of the User-level service, and are not part of this
   Transfer-level service.

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   The following depicts the relationships among transfer participants
   in Internet Mail.  It shows Source as distinct from the Originator,
   although it is common for them to be the same actor.  The figure also
   shows multiple Relays in the sequence.  It is legal to have only one,
   and for intra-organization mail services, this is common.

   +------------+                         +-----------+
   | Originator |                         | Recipient |
   +-----+------+                         +-----------+
         |                                      ^
         |         Mail Handling Service        |
   ||    |                                      |     ||
   ||    |                                      |     ||
         V                                      |
     +---------+    +--------+             +----+----+
     |         |    |        |<------------+         |
     | Source  +...>| Notice |             |  Dest   |
     |         |    |        |<---+        |         |
     +----+----+    +--------+    |        +---------+
          |                       |             ^
          V                       |             |
     +---------+             +----+----+   +----+----+
     |  Relay  +-->.......-->|  Relay  +-->|  Relay  |
     +---------+             +----+----+   +---------+
                             | Gateway +-->...

2.2.1  Source

   The Source role is responsible for ensuring that a message is valid
   for posting and then submitting it to a mail relay.  Validity
   includes conformance with Internet mail standards, as well as local
   operational policies.  Source may simply review the message for
   conformance, and reject it if there are errors, or it may create some
   or all of the necessary information.

   Source operates with dual allegiance.  It serves the Originator and
   often it is the same entity.  However its role in assuring validity
   means that it must represent the local operator of the Mail Handling

   Source also has the responsibility for any post-submission,
   originator-related administrative tasks associated with message

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   transmission and delivery.  Notably this pertains to error and
   delivery notices.  Hence, Source is best held accountable for the
   message content, even when they did not create any or most of it.

2.2.2  Notices Handler

   Transfer efforts might result in the generation of service reporting
   information about failures or completions.  These Transfer or
   Delivery notification messages are sent to an address that is
   specified by the Source.  A Notices handling address (also known as
   Bounce or Return address) might have no characteristics in common the
   with address of the Originator or Source.

2.2.3  Relay

   A mail relay performs email transfer-service routing and
   store-and-forward.  It adds envelope-related handling information and
   then (re-)transmits the message on towards its recipient(s).  A Relay
   does not modify the message contents.

   A basic transfer operation is between a client and a server Relay.  A
   set of Relays composes a Mail Handling Service network.  This is
   above any underlying packet-switching network that they might be

   Aborting message transfer results in having the Relay become an
   Originator and send an error message to the Notifications (Bounce)
   address.  (The potential for looping is avoided by having this
   message, itself, contain no Bounce address.

2.2.4  Gateway

   A Gateway is a special form of Relay that interconnects heterogeneous
   mail services.  Differences between the services can be as small as
   minor syntax variations, but usually encompass much more basic,
   semantic distinctions.  For example, the concept of an email address
   might be as different as a hierarchical, machine-specific address
   versus a flat, global name space.  Or between text-only and
   multi-media.  Hence, the Relay function of a gateway is the minor
   component.  The significant challenge is in the user-to-user
   functionality that matches syntax and semantics of independent email
   standards suites.

   The basic test of a gateway's adequacy is, of course, whether an
   originator can send a message to a recipient, without requiring any
   changes to the components in the originator's mail service or the
   recipient's mail service, other than adding the gateway.  To each of
   these otherwise independent services, the gateway will appear to be a

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   "native" participant.  However the ultimate test of a gateway's
   adequacy is whether the originator and recipient can sustain a
   dialogue.  In particular, can a recipient formulate a Reply?

2.3  Administrative Actors

   Operation of Internet mail services is apportioned to different
   providers (or operators) each is composed of an independent
   Administrative Domain.  Examples include an end-user operating their
   desktop client, a department operating a local relay, an IT
   department operating an enterprise relay, and an ISP operating a
   public, shared email service.  These can be configured into many
   combinations of administrative and operational relationships, with
   each Administrative Domain potentially having a complex arrangement
   of functional components.

   The interactions between functional components within an
   Administrative Domain are subject to the policies of that domain.
   Policies can cover such things as reliability, access control,
   accountability and content evaluation and may be implemented in
   different functional components, according to the needs of the
   Administrative Domain.

2.3.1  Provider

   Providers operate component services or sets of services.  It is
   possible for Providers to host services for other Providers.  Common
   examples are:

   Enterprise Service Providers: Operating an organization's internal
      data and/or mail operations.

   Internet Service Providers: Operating underlying data communication
      services that, in turn, are used by one or more Relays and Users.
      It is not their job to perform email functions, but to provide an
      environment in which those functions can be performed.

   Mail Service Providers: Operate email services, such as for
      end-users, or mailing lists.

   Operational pragmatics often dictate that Providers be involved in
   detailed administration and enforcement issues, to help insure the
   health of the overall Internet Mail Service.

3.  Email Identities

   Internet mail uses three forms of identity.  The most common is the
   mailbox address <addr-spec> [RFC2822].  The other two forms are the

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   <domain name> [RFC1034] and message identifier [RFC2822].

3.1  Mailbox Addresses

   An addr-spec has two distinct parts, divided by an at-sign ("@").
   The right-hand side contains a globally interpreted name for an
   administrative domain.  This domain name might refer to an entire
   organization, or to a collection of machines integrated into a
   homogeneous service, or to a single machine.  Domain names are
   defined and operated through the DNS [RFC1034], [RFC1035].

   The left-hand side of the at-sign contains a string that is globally
   opaque and is called the <local-part>.  It is to be interpreted only
   by the entity specified in the address's right-hand side.  All other
   entities must treat the local-part as a uninterpreted, literal string
   and must preserve all of its original details.  As such, its
   distribution is equivalent to sending a "cookie" that is only
   interpreted upon being returned to its originator.

3.1.1  Global Standards for Local-Part

   It is common for sites to have local structuring conventions for the
   left-hand side (local-part) of an addr-spec.  This permits
   sub-addressing, such as for distinguishing different discussion
   groups by the same participant.  However it must be stressed that
   these conventions are strictly private to the user's organization and
   must not be interpreted by any domain except the one listed in the
   right-hand side of the add-spec.

   A small class of addresses have an elaboration on basic email
   addressing, with a standardized, global schema for the local-part.
   These are conventions between originating end-systems and recipient
   gateways, and they are invisible to the public email transfer
   infrastructure.  When an originator is explicitly sending via a
   gateway out of the Internet, there are coding conventions for the
   local-part, so that the originator can formulate instructions for the
   gateway.  Standardized examples of this are the telephone numbering
   formats for VPIM [RFC2421], such as "",
   and iFax [RFC2304], such as "FAX=+12027653000/".

3.1.2  Scope of Email Address Use

   Email addresses are being used far beyond their original email
   transfer and delivery role.  In practical terms, email strings have
   become a common form of user identity on the Internet.  What is
   essential, then, is to be clear about the nature and role of an
   identity string in a particular context and to be clear about the

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   entity responsible for setting that string.

3.2  Domain Names

   A domain name is a global reference to an Internet resource, such as
   a host, a service or a network.  A name usually maps to one or more
   IP Addresses.  A domain name can be administered to refer to
   individual users, but this is not common practice.  The name is
   structure as a hierarchical sequence of sub-names, separated by dots

   When not part of a mailbox address, a domain name is used in Internet
   mail to refer to a node that took action upon the message, such as
   providing the administrative scope for a message identifier, or
   performing transfer processing.

3.3  Message Identifers

   Message identifiers have two distinct parts, divided by an at-sign
   ("@").  The right-hand side contains a globally interpreted name for
   the administrative domain assigning the identifier.  The left-hand
   side of the at-sign contains a string that is globally opaque and
   serves to uniquely identify the message within the domain referenced
   on the right-hand side.  The duration of uniqueness for the message
   identifier is undefined.

   The identifier may be assigned by the user or by any component of the
   system along the path.  Although Internet mail standards provide for
   a single identifier, more than one is sometimes assigned.

3.4  Identity Referencing Convention

   In this document, fields references to identities are labeled in a
   two-part, dotted notation.  The first part cites the document
   defining the identity and the second defines the name of the
   identity.  Hence, <RFC2822.From> is the From field in an email
   content header, and <RFC2821.MailFrom> is the address in the SMTP
   "Mail From" command.

4.  Protocols and Services

   NOTE:  A discussion about any interesting system architecture is
      often complicated by confusion between architecture versus
      implementation.  An architecture defines the conceptual functions
      of a service, divided into discrete conceptual modules.  An
      implementation of that architecture may combine or separate
      architectural components, as needed for a particular operational
      environment.  It is important not to confuse the engineering

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      decisions that are made to implement a product, with the
      architectural abstractions used to define conceptual functions.

   Modern Internet email architecture distinguishes four types of
   functional components, arranged to support a store-and-forward
   service architecture:

         .............+ oMUA |<------------------------------+
         .            +--+---+                               |
         .               |      { smtp,submission            |
         .               V                                   |
         .            +------+                               |
         .            | MSA  |<--------------------+         |
         .            +--+---+                     |         |
         .               |      { smtp             |         |
         .               V                         |         |
         .            +------+                +====+====+    |
         .            | MTA  |                ||  dsn  ||    |
   +============+     +--+---+                +=========+    |
   ||  MESSAGE ||        .      { smtp           ^   ^       |
   ||          ||        .                       |   |       |
   ||(envelope)||        .                       |   |       |
   ||          ||        V                       |   |       |
   ||  RFC2822 ||     +------+                   |   |   +===+===+
   ||          ||     | MTA  +-------------------+   |   || mdn ||
   ||  MIME    ||     +--+---+                       |   +=======+
   +============+        |      { local, smtp, lmtp  |       |
         .               V                           |       |
         .            +------+                       |       |
         .            |      +-----------------------+       |
         .            | MDA  |                               |
         .            |      |<--------------------+         |
         .            +-+--+-+                     |         |
         .   local }    |  |                       |         |
         .              V  |                       |         |
         .        +------+ |                  +====+====+    |
         .        | MS-1 | |                  || sieve ||    |
         .        +-+--+-+ |                  +=========+    |
         .          |  |   |   { pop, imap         ^         |
         .          |  V   V                       |         |
         .          | +------+                     |         |
         .          | | MS-2 |                     |         |
         .          | +--+---+                     |         |
         .          |    |     { pop, imap, local  |         |
         .          V    V                         |         |
         .         +------+                        |         |
         .........>| rMUA +------------------------+---------+

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   Software implementations of these architectural components often
   compress them, such as having the same software do MSA, MTA and MDA
   functions.  However the requirements for each of these components of
   the service are becoming more extensive.  So, their separation is
   increasingly common.

4.1  Service Components

4.1.1  Mail User Agent (MUA)

   A Mail User Agent (MUA) works on behalf of end-users and end-user
   applications.  It is their "representative" within the email service.

   At the origination side of the service, the oMUA is used to create a
   message and perform initial "submission" into the transfer
   infrastructure, via a Mail Submission Agent (MSA).  It may also
   perform any creation- and posting-time archival.  An MUA outbox is
   part of the origination-side MUA.

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

   An MUA may, itself, have a distributed architecture, such as
   implementing a "thin" user interface module on a limited end-user
   device, with the bulk of the MUA functionality operated remotely on a
   more capable server.  An example of such an architecture might use
   IMAP [RFC3501] for most of the interactions between an MUA client and
   an MUA server.

   A special class of MUA performs message re-posting, as discussed in
   the <Mediator> section.

   Identity fields set by the MUA include:


      Actor: Originator

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

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      Actor: Originator

      If a message recipient sends a reply message that would otherwise
      use RFC2822.From field address(es) contained in the original
      message, then they are instead to use the address(es) in the
      RFC2822.Reply-To field.  In other words, this field is a direct
      override of the From field, for responses from recipients.


      Actor: Source

      This specifies the address responsible for submission into the
      transfer service.  For efficiency, this field should be omitted if
      it contains the same address as RFC2822.From.  However this does
      not mean there is no Sender specified.  Rather, it means that that
      header field is virtual and that the address in the From field
      must be used.  Specification of the error return addresses -- the
      "notifications" (or "bounces") address, contained in
      RFC2821.MailFrom -- is made by the RFC2822.Sender.  Typically the
      notifications address is the same as the Sender address.  However
      some usage scenarios require it to be different.

   RFC2822.To, RFC2822.CC

      Actor: Recipient

      These specify MUA recipient addresses.  The distinction between To
      and CC is subjective.  Generally, a To addressee is considered
      primary and is expected to take action on the message.  A CC
      addressee typically receives a copy only for their information.


      Actor: Recipient

      A message might be copied to an addressee whose participation is
      not to be disclosed to the RFC2822.To or RFC2822.CC recipients.
      The BCC header field indicates a message copy to such a recipient.
      Typically, the field lists no addresses or only lists the address
      of the single recipient receiving the copy.  This usually ensures
      that even other BCC recipients do not know of each other.  An MUA
      will typically make separate postings for TO and CC recipients,
      versus BCC recipients.  The former will see no indication that any
      BCCs were sent, whereas the latter have a BCC field present.  It
      might be empty, contain a comment, or contain one or more BCC

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      addresses, depending upon the preferences or the Originator.

4.1.2  Mail Submission Agent (MSA)

   A Mail Submission Agent (MSA) accepts the message submission from the
   oMUA and enforces the policies of the hosting network and the
   requirements of Internet standards.  Enforcement might be passive,
   involving review and approval or rejection, or it might be active,
   involving direct modification of the message.  An MSA implements a
   server function to MUAs and a client function to MTAs (or MDAs).

   Examples of MSA-styled functions, in the world of paper mail, might
   range across the very different capabilities of administrative
   assistants, postal drop boxes, and post office front-counter

   The MUA/MSA interface can be implemented within single host and use
   private conventions for their interactions.  Historically,
   standards-based MUA/MSA interactions have used SMTP [RFC2821].
   However a recent alternative is SUBMISSION [RFC2476].  Although
   SUBMISSION derives from SMTP, it operates on a separate TCP port, and
   will typically impose distinct requirements, such as access

   Identities set by the MSA include:

   RFC2821.HELO or RFC2821.EHLO

      Actor: Source

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


      Actor: Source

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

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      Actor: Recipient

      This specifies the MUA inbox address of a recipient.  The string
      might not be visible in the message content header.  For example,
      the message destination address header fields, such as RFC2822.To,
      might specify a mailing list address, while the RFC2821.Rcpt-To
      address specifies a member of that list.


      Actor: Source

      An MSA may record a Received header field, to indicate initial
      submission trace information, including originating host and MSA
      host domain names and/or IP Addresses.

4.1.3  Mail Transfer Agent (MTA)

   An <MTA> relays a message to another other MTA or to an <MDA>, in a
   point-to-point exchange.  Relaying is performed by a sequence of
   MTAs, until the message reaches its destination MDA.  Hence an MTA
   implements both client and server MTA functionality.

   The basic functionality of an MTA is similar to that of a packet
   switch or IP router.  That is, it does email store-and-forward email,
   with a routing decision determining where the next-hop destination
   shall be.  The primary "routing" mechanism for Internet mail is the
   DNS MX record [RFC1035].  As with most "link layer" mechanisms
   Internet mail's SMTP supports a basic level of reliability, by virtue
   of providing for retransmission after al transfer failure.  However
   the degree of persistence by an MTA can be highly variable.

   However email objects are typically much larger than the payload of a
   packet or datagram, and the end-to-end latencies are typically much
   higher.  Contrary to typical packet switches (and Instant Messaging
   services) Internet mail MTAs typically store messages in a manner
   that allows recovery across services interruptions, such as host
   system shutdown.

   Internet mail primarily uses SMTP [RFC2821], [RFC0821] to effect
   point-to-point transfers between peer MTAs.  Other transfer
   mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]

   An important characteristic of MTA-MTA communications, over the open
   Internet, is that they do not require prior arrangement between the

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   independent administrations operating the different MTAs.  Given the
   importance of spontaneity and serendipity in the world of human
   communications, this lack of prearrangement, between the
   participants, is a core benefit of Internet mail and remains a core
   requirement for it.

   Identities set by the MTA include:


      Actor: Relay

      The MTA may specify its hosting domain identity for the SMTP HELO
      or EHLO command operation.


      Actor: Source

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


      Actor: Relay

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

4.1.4  Mail Delivery Agent (MDA)

   The <MDA> delivers email to the recipient's inbox.

   A Mail Delivery Agent (MDA) can provide distinctive, address-based
   functionality, made possible by its detailed knowledge of the
   properties of the destination address.  This knowledge might also be
   present elsewhere in the recipient's Administrative Domain, such as
   at an organizational border gateway.  However it is required for the
   MDA, if only because the MDA must know where to store the message.
   This knowledge is used to achieve differential handling of messages.

   Using Internet protocols, delivery is effected with POP [RFC1939] or
   IMAP [RFC3501].  When coupled with an internal, local mechanism, SMTP
   permits "push" delivery to the recipient system, at the initative of
   the upstream email service.  POP is used for "pull" delivery at the
   initiative of the recipient system.  Notably, SMTP and POP effect a

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   transfer of message control from the email service to the recipient
   host.  In contrast, IMAP provides on-going, interactive access to a
   message store, and does not effect a transfer of message control to
   the end-user host.  Instead, control stays with the message store
   host that is being access by the user.

   Identities set by the MDA include:

   RFC2821.HELO or RFC2821.EHLO

      Actor: Relay

      The MDA may specify its hosting domain identity for the SMTP HELO
      or EHLO command operation.


      Actors: Source, Relay, Dest

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

4.1.5  Message Store

   An MUA's uses a long-term Message Store (MS).  A rich set of choices
   for the use of that store derives from permitting more than one to be
   associated with a single user, demonstrated as MS-1 and MS-2 in the
   Figure.  MS-1 is shown as being remote from the MUA and MS-2 as being
   local.  Further the relationship between two message store may vary.
   Between the MDA and the MUA, these choices are supported by a wide
   variety of protocol options.

   The operational relationship among two MSs can be:

   Online:  Only a remote MS is used, with messages being accessible
      only when the MUA is attached to the MS, and the MUA repeatedly
      fetches all or part of a message, from one session to the next.

   Offline:  The MS is local to the user, and messages are moved from
      any remote store, rather than (also) being retained there.

   Disconnected:  A remote MS and a local MS synchronize all or parts of
      their contents, while connected.  The user may make changes while
      disconnected, and the two stores are re-synchronized upon

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4.2  Operational Configuration

   Mail service components can be arranged into numerous organizational
   structures, each with independent software and administration.  One
   common arrangement is to distinguish:

   1.  an open, core, global email transfer infrastructure

   2.  independent transfer services in networks at the edge of the core

   3.  end-user services

   Edge networks may use proprietary email standards.  However the
   distinction between "public" network and edge network transfer
   services is primarily significant because it highlights the need for
   concern over interaction and protection between independent
   administrations.  In particular, this distinctions calls for
   additional care in assessing transitions of responsibility, as well
   as the accountability and authorization relationships among
   participants in email transfer.

   On the other hand, real-world operations of Internet mail
   environments do impose boundaries such as access control at
   organizational firewalls to the Internet.  It should be noted that
   the current Internet Mail architecture offers no special constructs
   for these configuration choices.  The current design of Internet mail
   is for a seamless, end-to-end store-and-forward sequence.  It is
   possible that the architectural enhancement will not require new
   protocols, but rather will require clarification of best practises,
   as exemplified by a recent effort [ID-spamops]

4.3  Layers of Identity References

   For a message in transit, the core identity fields combine into:

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     | Layer                 | Field       | Set By              |
     | Message Content       | MIME Header | Originator          |
     | Message header fields | From        | Originator          |
     |                       | Sender      | Source              |
     |                       | Reply-To    | Originator          |
     |                       | To, CC, BCC | Originator          |
     |                       | Received    | Source, Relay, Dest |
     |                       | Return-Path | MDA, from MailFrom  |
     | SMTP                  | HELO        | Latest Relay Client |
     |                       | MailFrom    | Source              |
     |                       | RCPT-TO     | Originator          |
     | IP                    | IP Address  | Latest Relay Client |

5.  Message Data

5.1  Envelope

   Information that is directly used or produced by the email transfer
   service is called the "envelope".  It controls and records handling
   activities by the transfer service.  Internet mail has a fragmented
   framework for handling this "handling" information.  The envelope
   exists partly in the transfer protocol SMTP [RFC2821] and partly in
   the message object [RFC2822].

   Direct envelope addressing information, as well as optional transfer
   directives, are carried in-band by MTAs.  All other envelope
   information, such as trace records, is carried within the content
   header fields.  Upon delivery, SMTP-level envelope information is
   typically encoded within additional content header fields, such as
   Return-Path and Received (From and For).

5.2  Message Header Fields

   Header fields are attribute/value pairs covering an extensible range
   of email service, user content and user transaction meta-information.
   The core set of header fields is defined in [RFC2822], [RFC0822].  It
   is common to extend this set, for different applications.  A complete
   set of registered header fields is being developed through

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

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5.3  Body

   The body of a message might simply be lines of ASCII text or it might
   be structured into a composition of multi-media, body-part
   attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],
   and [RFC2049].  It should be noted that MIME structures each
   body-part into a recursive set of MIME header field meta-data and
   MIME Content sections.

6.  Two Levels of Store-And-Forward

   Basic email transfer is accomplished with an asynchronous
   store-and-forward communication infrastructure.  This means that
   moving a message from an originator to a recipient involves a
   sequence of independent transmissions through some number of
   intermediaries, called MTAs.  A very different task is the user-level
   process of re-posting a message through a new submission process,
   after final delivery for an earlier transfer sequence.  Such
   MUA-based re-posting shares some functionality with basic MTA
   relaying, but it enjoys a degree of freedom with both addressing and
   content that is not available to MTAs.

   The primary "routing" mechanism for Internet mail is the DNS MX
   record [RFC1035].  It is an advertisement, by a recipient domain, of
   hosts that are able to relay mail to it, within the portion of the
   Internet served by this instance of the DNS.

6.1  MTA Relaying

   MTAs relay mail.  They are like packet-switches and IP routers.
   Their job is to make routing assessments and to move the message
   payload data closer to the recipient.  It is not their job to
   reformulate the payload or to change addresses in the envelope or the

6.2  MUA Forwarding

   As discussed in <Mediator> section, forwarding is performed by MUAs
   that take a received message and submit it back to the transfer
   service, for delivery to one or more different addresses.  A
   forwarded message may appear identical to a relayed message, such as
   for Alias forwarders, or it may have minimal similarity, as with a

6.2.1  MUA Basic Forwarding

   The simplest type of forwarding involves creating an entirely new
   message, with new content, that includes the original message between

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   Originator-1 and Recipient-1.  However this forwarded communication
   is between Recipient-1 (who could also be called Originator-2) and a
   new recipient, Recipient-2.  The forwarded message is therefore
   independent of the original message exchange and creates a new
   message dialogue.

6.2.2  MUA Re-Sending

   A recipient may wish to declare that an alternate addressee should
   take on responsibility for a message, or otherwise become involved in
   the original communication.  They do this through a user-level
   forwarding function, called re-sending.  The act of re-sending, or
   re-directing, splices a communication between Originator-1 and
   Recipient-1, to become a communication between Originator-1 and new
   Recipient-2.  In this case, the content of the new message is the old
   message, including preservation of the essential aspects of the
   original message's origination information.

   Identities specified in a resent message include


      Actor: Originator

      Names and email addresses for the original author(s) of the
      message content are retained.  The free-form (display-name)
      portion of the address might be modified to provide informal
      reference to the agent responsible for the redirection.


      Actor: Originator

      If this field is present in the original message, it should be
      retained in the Re-sent message.


      Actor: Source

      This field is expected to contain the original Sender value.

   RFC2822.TO, RFC2822.CC, RFC2822.BCC

      Actor: Recipient

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      These specify the original message recipients.


      Actor: Mediating Originator

      The address of the original recipient who is redirecting the
      message.  Otherwise, the same rules apply for the Resent-From
      field as for an original RFC2822.From field


      Actor: Mediating Source

      The address of the agent responsible for re-submitting the
      message.  For efficiency, this field should be omitted if it
      contains the same address as RFC2822.Resent-From.  However this
      does not mean there is no Resend-Sender specified.  Rather, it
      means that that header field is virtual and that the address in
      the Resent-From field must be used.  Specification of the error
      return addresses (the "bounces" address, contained in
      RFC2821.MailFrom) is made by the Resent-Sender.  Typically the
      bounce address is the same as the Resent-Sender address.  However
      some usage scenarios require it to be different.

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

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


      Actor: Mediating Source

      The agent responsible for re-submission (RFC2822.Resent-Sender) is
      also responsible for specifying the new RFC2821.MailFrom address.


      Actor: Recipient

      This will contain the address of a new recipient


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      Actor: Mediating Source

      When re-sending a message, the submission agent may record a
      Received header field, to indicate the transition from original
      posting to resubmission.

6.2.3  MUA Reply

   When a recipient formulates a response to a message, the new message
   is not typically viewed as being a "forwarding" of the original.

6.2.4  MUA Gateways

   Gateways perform the basic routing and transfer work of message
   relaying, but they also make any message or address modifications
   that are needed to send the message into the next messaging
   environment.  When a gateway connects two differing messaging
   services, its role is easy to identify and understand.  When it
   connects environments that have technical similarity, but may have
   significant administrative differences, it is easy to think that a
   gateway is merely an MTA.  The critical distinguish between an MTA
   and a gateway is that the latter modifies addresses and/or message

   A gateway may set any identity field available to a regular MUA.
   Identities typically set by gateways include:


      Actor: Originator

      Names and email addresses for the original author(s) of the
      message content are retained.  As for all original addressing
      information in the message, the gateway may translate addresses in
      whatever way will allow them continue to be useful in the target


      Actor: Originator

      The gateway should retain this information, if it is originally
      present.  The ability to perform a successful reply by a gatewayed
      recipient is a typical test of gateway functionality.

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      Actor: Source

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

   RFC2822.TO, RFC2822.CC, RFC2822.BCC

      Actor: Recipient

      These usually retain their original addresses.


      Actor: Source

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


      Actors - Source, Relay, Dest

      The gateway may record a Received header field, to indicate the
      transition from original posting to the new messaging environment.

6.2.5  MUA Alias Handling

   A simple re-addressing facility that is available in most MDA
   implementations is called Aliasing.  It is performed just before
   placing a message into the specified recipient's inbox.  Instead, the
   message is submitted back to the transfer service, for delivery to
   one or more alternate addresses.  Although implemented as part of the
   message delivery service, this facility is strictly a recipient user
   function.  In effect it resubmits the message to a new address, on
   behalf of the listed recipient.

   What is most distinctive about this forwarding mechanism is how
   closely it compares to normal MTA store-and-forward.  In reality its
   only interesting difference is that it changes the RFC2821.RCPT-TO
   value.  Notably it does not typically change the RFC2821.Mailfrom

   An MDA that is re-posting a message to an alias typically changes
   only envelope information:

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   RFC2822.TO, RFC2822.CC, RFC2822.BCC

      Actor: Recipient

      These retain their original addresses.


      Actor: Recipient

      This field contains an alias address.


      Actor: Mediating Source

      The agent responsible for submission to an alias address will
      usually retain the original address to receive handling
      notifications.  The benefit of retaining the original MailFrom
      value is to ensure that the origination-side agent knows of that
      there has been a delivery problem.  On the other hand, the
      responsibility for the problem usually lies with the recipient,
      since the Alias mechanism is strictly under the recipient's


      Actor: Mediating Recipient

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

6.2.6  MUA Mailing Lists

   Mailing lists have explicit email addresses and they forward messages
   to a list of subscribed members.  Mailing list processing is a
   user-level activity, outside of the core email transfer service.  The
   mailing list address is, therefore, associated with a distinct
   user-level entity that can perform arbitrary actions upon the
   original message, before submitting it to the mailing list
   membership.  Hence, mailing lists are similar to gateways.

   Identities set by a mailing list processor, when submitting a
   message, include:

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      Actor: Mediating Originator

      This provides a global mailing list naming framework that is
      independent of particular hosts.  Although [RFC2919] is a
      standards-track specification, it has not gained significant


      Actor: Mediating Recipient

      [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


      Actor: Originator

      Names and email addresses for the original author(s) of the
      message content are specified.


      Actor: Originator

      Mailing lists have introduced an ambiguity for the Reply-To field.
      Some List operations choose to force all replies to go to all list
      members.  They achieve this by placing the list address into the
      RFC2822.Reply-To field.  Hence, direct, "private" replies only to
      the original author cannot be achieved by using the MUA's typical
      "reply to author" function.  If the author created a Reply-To
      field, its information is lost.


      Actor: Source

      This will usually specify the address of the agent responsible for
      mailing list operations.  However, some mailing lists operate in a
      manner very similar to a simple MTA relay, so that they preserve
      as much of the original handling information as possible,
      including the original RFC2822.Sender field.

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   RFC2822.TO, RFC2822.CC

      Actor: Mediating Recipient

      These will usually contain the original list of recipient


      Actor: Mediating Source

      This may contain the original address to be notified of
      transmission issues, or the mailing list agent may set it to
      contain a new notification address.  Typically, the value is set
      to a new address, so that mailing list members and posters are not
      burdened with transmission-related notifications.


      Actor: Recipient

      This contains the address of a mailing list member.


      Actor: Mediating Recipient

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

7.  Security Considerations

   This document does not specify any new Internet mail functionality.
   Consequently it should introduce no new security considerations.

   However its discussion of the roles and responsibilities for
   different mail service modules, and the information they create,
   highlights the considerable security considerations that must be
   present when implementing any component of the Internet mail service.

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8  References

              "Registration of mail and MIME header fields",
              draft-klyne-hdrreg-mail-04.txt (work in progress), Apr

              Hutzler, C., Crocker, D., Resnick, P., Sanderson, R. and
              E. Allman, "Email Submission Between Independent
              Networks", draft-spamops-00 (work in progress), March

   [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.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

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

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2048]  Freed, N., Klensin, J. and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail

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              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.

   [RFC2298]  Fajman, R., "An Extensible Message Format for Message
              Disposition Notifications", RFC 2298, March 1998.

   [RFC2304]  Allocchio, C., "Minimal FAX address format in Internet
              Mail", RFC 2304, March 1998.

   [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.

   [RFC2421]  Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
              Mail - version 2", RFC 2421, September 1998.

   [RFC2423]  Vaudreuil, G. and G. Parsons, "VPIM Voice Message MIME
              Sub-type Registration", RFC 2423, September 1998.

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

   [RFC2476]  Gellens, R. and J. Klensin, "Message Submission", RFC
              2476, December 1998.

   [RFC2645]  "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
              Addresses", RFC 2465, August 1999.

   [RFC2782]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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

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

   [RFC2919]  Chandhok, R. and G. Wenger, "List-Id: A Structured Field
              and Namespace for the Identification of Mailing Lists",
              RFC 2919, March 2001.

   [RFC3028]  Showalter, T., "Sieve: A Mail Filtering Language", RFC
              3028, January 2001.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service

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              Extension for Delivery Status Notifications (DSNs)", RFC
              3461, January 2003.

   [RFC3501]  Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 3501, March 2003.

Author's Address

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Drive
   Sunnyvale, CA  94086

   Phone: +1.408.246.8253

Appendix A.  Acknowledgements

   The Email Architecture section derives from draft-hutzler-spamops
   [ID-spamops].  The text has been further elaborated.

   Discussion of the Source actor role was greatly clarified during
   discussions in the IETF's Marid working group.

   Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
   insight on the framework and details of early drafts.  Additional
   review and suggestions were provided by Nathaniel Borenstein, Ed
   Bradford, Cyrus Daboo, Tony Finch, Ned Freed, Eric Hall, Bruce Lilly,
   Eric Hall, Chris Newman, Jochen Topf.

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