©À
SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking
Expires: September 29, 2005 March 28, 2005
Internet Mail Architecture
draft-crocker-email-arch-04
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Over its thirty-four year history, Internet Mail has undergone
significant changes in scale and complexity, as it has become a
global infrastructure service. The first standardized architecture
for email specified a simple split between the user world, in the
form of Mail User Agents (MUA), and the transmission world, in the
form of the Mail Handling Service (MHS) composed of Mail Transfer
Agents (MTA). Core aspects of the service, such as address and
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message style, have remained remarkably constant. Today, Internet
Mail is marked by many independent operators, many different
components for providing users service and many others for performing
message transfer. Public discussion of the architecture has not kept
pace with the real-world technical and operational refinements. This
document offers an enhanced Internet Mail architecture to reflect the
current service.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . 4
1.2 Discussion venue . . . . . . . . . . . . . . . . . . . . . 5
1.3 Changes . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 6
2.1 User Actors . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 MHS Actors . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Administrative Actors . . . . . . . . . . . . . . . . . . 11
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . 13
3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Message Identifiers . . . . . . . . . . . . . . . . . . . 15
3.4 Identity Referencing Convention . . . . . . . . . . . . . 15
4. Services . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1 Message . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2 Mail User Agent (MUA) . . . . . . . . . . . . . . . . . . 20
4.3 Mail Submission Agent (MSA) . . . . . . . . . . . . . . . 22
4.4 Mail Transfer Agent (MTA) . . . . . . . . . . . . . . . . 23
4.5 Mail Delivery Agent (MDA) . . . . . . . . . . . . . . . . 25
4.6 Message Store (MS) . . . . . . . . . . . . . . . . . . . . 26
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1 Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.2 ReSending . . . . . . . . . . . . . . . . . . . . . . . . 30
5.3 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 31
5.4 Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.5 Boundary Filter . . . . . . . . . . . . . . . . . . . . . 35
6. Security Considerations . . . . . . . . . . . . . . . . . . . 35
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.1 References - Normative . . . . . . . . . . . . . . . . . . 35
7.2 Reference - Descriptive . . . . . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38
Intellectual Property and Copyright Statements . . . . . . . . 39
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1. Introduction
Over its thirty-four year history, Internet Mail has undergone
significant changes in scale and complexity, as it has become a
global infrastructure service.
The first standardized architecture for email specified a simple
split between the user world, in the form of Mail User Agents (MUA),
and the transmission world, in the form of the Mail Handling Service
(MHS) composed of Mail Transfer Agents (MTA). The MHS is responsible
for accepting a message from one User and delivering it to one or
more others.
+--------+
+---------------->| User |
| +--------+
| .
+--------+ | +--------+ .
| User +--+--------->| User | .
+--------+ | +--------+ .
. | . .
. | +--------+ . .
. +-->| User | . .
. +--------+ . .
. . . .
. . . .
. . . .
+--------------------------------------+
| Mail Handling Service (MHS) |
+--------------------------------------+
Figure 1: Basic Internet Mail Service Model
Today, Internet Mail is marked by many independent operators, many
different components for providing users service and many other
components for performing message transfer. So it is not surprising
that the operational service has sub-divided each of these "layers"
into more specialized modules. Core aspects of the service, such as
address and message style, have remained remarkably constant.
However public discussion of the architecture has not kept pace with
the real-world refinements. This document offers an enhanced
Internet Mail architecture to reflect the current service. The
original distinction between user-level concerns and transfer-level
concerns is retained, and the elaboration to each "level" of the
architecture is discussed separately. The term "Internet Mail" is
used to refer to the entire collection of user and transfer
components.
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For Internet Mail, the term "end-to-end" usually refers to a single
posting and the set of deliveries directly resulting from its single
transiting of the MHS. However, note that some 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 [RFC2423], EDI over email [RFC1767], and
facsimile over email.[ID-ffpim]
The current draft seeks to:
o Document refinements to the email model
o Clarify functional roles for the architectural components
o Clarify identity-related issues, across the email service
o Provide a document that serves as a common venue for further
defining and citing modern Internet Mail architecture
NOTE:
Any attempt to provide a retroactive description, for a service
that evolved so extensively, is certain to claim definitions and
relationships that do not match the equally reasonable views of
some portion of the technical community. Ultimately, the
"correct" choices are determined solely by the willingness of that
community to use the descriptions.
1.1 Service Overview
End-to-end Internet Mail exchange is accomplished by using a
standardized infrastructure comprising:
o An email object
o Global addressing
o An asynchronous sequence of point-to-point transfer mechanisms
o No prior arrangement between Originator and Recipient
o No prior arrangement between point-to-point transfer services,
over the open Internet
o No requirement for Originator and Recipient to be online at the
same time.
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The end-to-end portion of the service is the email object, called a
message. Broadly the message, itself, is divided between handling
control information and user message content.
A precept to the design of Internet Mail is permitting 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.
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 Discussion venue
Discussion about this document should be directed to the IETF-SMTP
mailing list <http://www.imc.org/ietf-smtp>. It is the most active,
long-standing venue for discussing email architecture. Although it
is primarily for discussing only the SMTP protocol, it is recommended
that discussion of this draft take place on that mailing list because
it attends to end-to-end infrastructure and architecture issues more
than other email-related mailing lists.
1.3 Changes
This is intended to be the last major revision, prior to seeking
publication.
Significant changes to this version:
Administrative Unit: Changed from Administrative Domain to
Administrative Unit, to remove possible confusion with "domain
name". Added Tussle reference
Sieve: Noted ability to have other places to run sieve
instructions.
Word Smithing: Assorted small tweaks to definitions, diagrams and
comments.
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Notices, Bounces and Disp: Added Bounce module to Services diagram,
to make clear that MHS return messages can go to an independent
address. Dotted link to MSA shows responsibility for setting
Notices address. Changed "Notification" to "Bounce", to use more
popular term and to avoid confusion with MDN notices. Added Disp
module to Services, to distinguish DSN traffic from MDN.
2. Email Actor Roles
Internet Mail is a highly distributed service, with a variety of
actors serving different roles. These divide into:
o User
o Mail Handling Service (MHS)
o Administrative Unit
Although related to a technical architecture, the focus on Actors
concerns participant responsibilities, rather than on functional
modules. Hence the labels used are different than for classic email
architecture diagrams. Actors often will be associated with
different organizations. This operational independence provides the
motivation for distinguishing Administrative Units.
2.1 User Actors
Users are the sources and sinks of messages. They may be humans or
processes. 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 types of user-level
Actors:
o Originators
o Recipients
o Mediators
From the User-level perspective all mail transfer activities are
performed by a monolithic Mail Handling Service (MHS), even though
the actual service may be provided by many independent organizations.
Users are customers of this service.
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The following depicts the flow of messages among Actors.
+------------+
| Originator |<--------------+
+-+---+----+-+ |
| | | |
| | V |
| | +-----------+ |
| | | Recipient | |
| | +-----------+ |
| | |
| | +----------+ |
| | | | |
| V V | |
| +-----------+ +---+---+---+
| | Mediator +--->| Recipient |
| +-----------+ +-----------+
|
V
+-----------+ +-----------+ +-----------+
| Mediator +--->| Mediator +--->| Recipient |
+-----------+ +-----------+ +-----------+
Figure 2: Relationships Among User Actors
2.1.1 Originator
Also called "Author", this is the user-level participant responsible
for creating original content and requesting its transmission. The
MHS operates to send and deliver mail among Originators and
Recipients. As described below, the MHS has a "Source" role, that
correlates with the Author role.
2.1.2 Recipient
The Recipient is a consumer of delivered content. As described
below, the MHS has a "Dest" role, that correlates with the Recipient
role.
A Recipient may close the user-level communication loop by creating
and submitting a new message that replies to an Originator. An
example of an automated form of reply is the Message Disposition
Notification, which informs the Originator about the Recipient's
disposition of the message. See Section 4.1.
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2.1.3 Mediator
A Mediator receives, aggregates, reformulates and redistributes
messages as part of a potentially-protracted, higher-level exchange
among Users. Example uses of Mediators include group dialogue and
organizational message flow, as occurs with a purchase approval
process. Note that it is easy to confuse this user-level activity
with the underlying MHS transfer exchanges. However they serve very
different purposes and operate is very different ways. Mediators are
considered extensively in Section 5.
When mail is delivered to an envelope address, a Mediator is viewed
by the Mail Handling Service as a Recipient. When submitting
messages, the Mediator is an Originator. What is distinctive is that
a Mediator preserves the Originator information of the message it
reformulates, but may make meaningful changes to the content. Hence
the MHS sees a new message, but Users receive a message that is
interpreted as primarily being from -- or, at least, initiated by --
the author of the original message. The role of a Mediator permits
distinct, active creativity, rather than being limited to the more
constrained job of merely connecting together other participants.
Hence it is really the Mediator that is responsible for the new
message.
A Mediator's task may be complex and contingent, 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 request.
Because a Mediator originates messages, it might also receive
replies. So, a Mediator really is a full-fledged User.
Gateway: A Gateway is a particularly interesting form of Mediator.
It is a hybrid of User and Relay that interconnects heterogeneous
mail services. Its goal is to emulate a Relay, so Gateway is
described in more detail, in the next section.
2.2 MHS Actors
The Mail Handling Service (MHS) has the task of performing a single,
email-level end-to-end transfer, on behalf of the Originator and
reaching the Recipient address(es) specified in the envelope.
Mediated or protracted, iterative exchanges, such as those used for
collaboration over time, are part of the User-level service, and are
not part of this Transfer-level Handling Service.
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The following depicts the relationships among transfer participants
in Internet Mail. It shows the Source as distinct from the
Originator, and Destination as distinct from Recipient, although it
is common for each pair to be the same actor. The figure also shows
multiple Relays in the sequence. It is legal to have no separate
Relay, where the Source and Dest interact directly. For intra-
organization mail services, it is common to have only one Relay.
+------------+ +-----------+
| Originator | | Recipient |
+-----+------+ +-----------+
| ^
| Mail Handling Service |
/+=================================================+\
|| | | ||
|| | | ||
V |
+---------+ +--------+ +----+----+
| | | |<------------+ |
| Source +...>| Bounce | | Dest |
| | | |<---+ | |
+----+----+ +--------+ | +---------+
| | ^
V | |
+---------+ +----+----+ +----+----+
| Relay +-->.......-->| Relay +-->| Relay |
+---------+ +----+----+ +---------+
|
V
+---------+
| Gateway +-->...
+---------+
Figure 3: Relationships Among MHS Actors
2.2.1 Source
The Source role is responsible for ensuring that a message is valid
for posting and then submitting it to a Relay. Validity includes
conformance with Internet Mail standards, as well as with local
operational policies. The 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.
The 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 also represent the local operator of the
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MHS, that is, the local Administrative Domain.
The Source also has the responsibility for any post-submission,
Originator-related administrative tasks associated with message
transmission and delivery. Notably this pertains to error and
delivery notices. Hence, Source is best held accountable for the
message content, even when they did not create any or most of it.
2.2.2 Bounce Handler
The Bounce Handler processes service notifications that are generated
by the MHS, as a result of its efforts to transfer or deliver the
message. Notices may be about failures or completions and are sent
to an address that is specified by the Source. This Bounce handling
address (also known as a Return address) might have no visible
characteristics in common with the 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-level handling information and then
(re-)transmits the message on towards its Recipient(s). A Relay may
add information to the envelope, such as with trace information.
However it does not modify existing envelope information or the
message content semantics. It may modify message content syntax,
such as a change from text to binary transfer-encoding form, only as
required to meet the capabilities of the next hop in the MHS.
A set of Relays composes a Mail Handling Service network. This is
above any underlying packet-switching network that they might be
using and below any gateways or other user-level Mediators.
In other words, interesting email scenarios can involve three,
distinct architectural layers of store-and-forward service:
o User Mediators
o MHS Relays
o Packet Switches
with the bottom-most usually being the Internet's IP service. The
most basic email scenarios involve Relays and Switches.
Aborting a message transfer results in having the Relay become an
Originator and send an error message to the Bounce (Bounce) address.
(The potential for looping is avoided by having this message, itself,
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contain no Bounce address.)
2.2.4 Gateway
A Gateway is a hybrid form of User and Relay that interconnects
heterogeneous mail services. Its purpose is simply to emulate a
Relay and the closer it comes to this, the better. However it
operates at the User level, because it must be able to modify message
content.
Differences between mail services can be as small as minor syntax
variations, but usually encompass significant, 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 content and multi-media. Hence the
Relay function in a Gateway offers the minor challenge in design.
The more significant challenge is in ensuring the user-to-user
functionality that matches syntax and semantics of independent email
standards suites.
The basic test of a Gateway's adequacy is, of course, whether an
Originator on one side of a Gateway can send a message to a Recipient
on the other side, without requiring changes to any of the components
in the Originator's or Recipient's mail services, other than adding
the Gateway. To each of these otherwise independent services, the
Gateway will appear to be a "native" participant. However the
ultimate test of a Gateway's adequacy is whether the Originator and
Recipient can sustain a dialogue. In particular, can a Recipient's
MUA automatically formulate a valid Reply?
2.3 Administrative Actors
Operation of Internet Mail services is apportioned to different
providers (or operators). Each can be composed of an independent
Administrative Unit (AU). 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 Unit potentially having a complex arrangement of
functional components. Figure 4 depicts the relationships among AUs.
Perhaps the most salient aspect of an AU is the differential trust
that determines its policies for activities within the AU, versus
those involving interactions with other AUs. The architectural
impact of needing to have boundaries between AU's is discussed in
[Tussle]
Basic components of AU distinction include:
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Edge: Independent transfer services, in networks at the edge of the
Internet Mail service.
User: End-user services. This might be subsumed under the Edge
service, such as is common for web-based email access.
Transit: These are Mail Service Providers (MSP) offering value-
added capabilities for Edge AUs, such as aggregation and
filtering.
Note that Transit services are quite different from packet-level
transit operation. Whereas end-to-end packet transfers usually go
through intermediate routers, email exchange across the open Internet
is often directly between the Edge AUs, at the email level.
+------ +------+ +------+
| AU-1 | | AU-3 | | AU-4 |
| ---- | | ---- | | ---- |
| | +---------------------->| | | |
| User | | |-Edge-+---->|-User |
| | | | +--->| | | |
| V | | | +------+ +------+
| Edge-+----+ |
| | | +---------+ |
+------+ | | AU-2 | |
| | ------- | |
| | | |
+--->|-Transit-+---+
| |
+---------+
Figure 4: Administrative Units (AU) Example
Edge networks may use proprietary email standards internally.
However the distinction between Transit network and Edge network
transfer services is primarily significant because it highlights the
need for concern over interaction and protection between independent
administrations. In particular, this distinctions calls for
additional care in assessing transitions of responsibility, as well
as the accountability and authorization relationships among
participants in email transfer.
The interactions between functional components within an
Administrative Unit are subject to the policies of that domain.
Policies can cover such things as reliability, access control,
accountability and even content evaluation and modification. They
may be implemented in different functional components, according to
the needs of the Administrative Unit. For example, see [ID-spamops].
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User, Edge and Transit services can be offered by providers that
operate component services or sets of services. Further, it is
possible for one AU to host services for other AUs. Common AU
examples are:
Enterprise Service Providers:
Operating an organization's internal data and/or mail services.
Internet Service Providers:
Operating underlying data communication services that, in turn,
are used by one or more Relays and Users. It is not their job to
perform email functions, but to provide an environment in which
those functions can be performed.
Mail Service Providers:
Operating 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 ensure the
health of the overall Internet Mail Service. This can include
operators of lower-level packet services.
3. Identities
Internet Mail uses three forms of identity. The most common is the
mailbox address <addr-spec> [RFC2822] Also see <address> and
<mailbox> in [RFC2821]. The other two are the domain name <domain>
Section 3.2 and message identifier <msg-id> [RFC2822].
3.1 Mailbox Addresses
"A mailbox sends and receives mail. It is a conceptual entity
which does not necessarily pertain to file storage." [RFC2822]
A mailbox is specified as an Internet Mail address <addr-spec>. It
has two distinct parts, divided by an at-sign ("@"). The right-hand
side contains a globally interpreted name for an Administrative Unit.
Domain Names are discussed in Section 3.2.
The portion to the left of the at-sign contains a string that is
globally opaque and is called the <local-part>. It is to be
interpreted only by the entity specified 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 public distribution is equivalent to sending a
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"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 addr-spec.
A small class of addresses has 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 "+16137637582@vpim.example.com",
and iFax [RFC2304], such as
"FAX=+12027653000/T33S=1387@ifax.example.com".
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
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. Conceptually, the name might encompass an entire
organization, or a collection of machines integrated into a
homogeneous service, or only a single machine. A domain name can be
administered to refer to individual users, but this is not common
practice. The name is structured as a hierarchical sequence of sub-
names, separated by dots ("."). Domain names are defined and
operated through the Domain Name Service (DNS) [RFC1034], [RFC1035],
[RFC2181].
When not part of a mailbox address, a domain name is used in Internet
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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 Identifiers
Like mailbox addresses, message identifiers have two distinct parts,
divided by an at-sign ("@"). The right-hand side is globally
interpreted and specifies the Administrative Unit 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, within the AU responsible for the indicated
domain. 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. Services
The Internet's MHS architecture distinguishes six types of functional
components, arranged to support a store-and-forward service
architecture:
o Message
o Mail User Agent (MUA)
o Message Submission Agent (MSA)
o Message Transfer Agent (MTA)
o Message Delivery Agent (MDA)
o Message Store (MS)
This section describes the specific functional components for
Internet Mail, and the standard protocols associated with performing
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them.
Software implementations of these architectural components often
compress them, such as having the same software do MSA, MTA and MDA
functions. However the requirements for each of these components of
the service are becoming more extensive. So, their separation is
increasingly common.
NOTE:
A discussion about any interesting system architecture is often
complicated by confusion between architecture versus
implementation. An architecture defines the conceptual functions
of a service, divided into discrete conceptual modules. An
implementation of that architecture may combine or separate
architectural components, as needed for a particular operational
environment. It is important not to confuse the engineering
decisions that are made to implement a product, with the
architectural abstractions used to define conceptual functions.
This figure shows function modules and the protocols used between
them. Additional protocols and configurations are possible.
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+------+ +---------+
...............+ oMUA |...| Disp |<----------------+
. +--+---+ +---------+ |
. | {smtp, |
. V {submission |
. +------+ +---------+ |
. | MSA |...| Bounces |< -----+ |
. +--+---+ +---------+ | |
. | | |
. V {smtp | |
. +------+ /+===+===+\ |
. | MTA | || dsn || |
/+==========+\ +--+---+ \+=======+/ |
|| MESSAGE || . ^ ^ |
||----------|| . {smtp | | |
|| Envelope || . | | |
|| SMTP || V | | |
|| RFC2822 || +------+ | | /+==+==+\
|| Content || | MTA +-------------------+ | || mdn ||
|| RFC2822 || +--+---+ | \+=====+/
|| MIME || | {local, smtp, | |
\+==========+/ V {lmtp | |
. +------+ | |
. | +-----------------------+ |
. | MDA | |
. | |<--------------------+ |
. +-+--+-+ | |
. local} | | | |
. V | | |
. +------+ | /+===+===+\ |
. | sMS | | || sieve || |
. +-+--+-+ | \+=======+/ |
. | | | {pop, imap ^ |
. | V V | |
. | +------+ | |
. | | uMS | | |
. | +--+---+ | |
. | | {pop, imap, | |
. V V {local | |
. +------+ | |
. | +---- -------------------+ |
...........>| rMUA | |
| +----------------------------------+
+------+
Figure 5: Protocols and Services
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4.1 Message
The purpose of the Mail Handling Service is to exchange a message
object among participants. Hence, all of the underlying mechanisms
are merely in the service of getting that message from its Originator
to its Recipients. A message may be explicitly labeled as to its
nature. [RFC3458]
A message comprises a transit handling envelope and the end-user
message content. The envelope contains handling information used by
the Message Handling Service, or generated by it. The content is
divided into a structured header and the body. The body may be
unstructured, simple text, or it may be a tree of multi-media
subordinate objects, called body-parts.
Internet Mail has distinguished some special versions of messages,
for exchanging control information:
Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) may be generated by the Mail
Handling Service (MSA, MTA or MDA) and sent to the
RFC2821.MailFrom address. It provides information about message
transit, such as transmission errors or successful delivery.
[RFC3461]
Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) may be generated by an
rMUA and is sent to the Disposition-Notification-To address(es).
It provides information about user-level, Recipient-side message
processing, such as indicating that the message has been read
[RFC2298] or the form of content that can be supported. [RFC3297]
Message Filtering (SIEVE):
SIEVE provides a means of specifying conditions for differential
handling of mail, at the time of delivery [RFC3028]. Figure 5
shows a Sieve specification going from the rMUA to the MDA.
However filtering can be done at many different points along the
transit path and any one or more of them might be subject to Sieve
directives, especially within a single AU. Hence, the Figure
shows only one relationship, for simplicity.
4.1.1 Envelope
Information that is directly used by, or produced by, the MHS is
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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]. The SMTP specification uses the term to refer only
to the transfer-protocol information.
NOTE:
Due to the frequent use of the term "envelope" to refer only to
SMTP constructs, there has been some call for using a different
term, to label the larger set of information defined here. So
far, no alternative term has developed any community support.
Direct envelope addressing information, as well as optional transfer
directives, are carried within the SMTP control channel. Other
envelope information, such as trace records, is carried within the
content header fields. Upon delivery, some SMTP-level envelope
information is typically encoded within additional content header
fields, such as Return-Path.
4.1.2 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. Procedures
for registering headers are provided in [RFC4021]. A complete set of
registered header fields is being developed through [ID-hdr-reg].
One danger with placing additional information in header fields is
that Gateways often alter or delete them.
4.1.3 Body
The body of a message might simply be lines of ASCII text or it might
be structured into a composition of multi-media, body-part
attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],
and [RFC2049]. MIME structures each body-part into a recursive set
of MIME header field meta-data and MIME Content sections.
4.1.4 Identity References in a Message
For a message in transit, the core uses of identity references
combine into:
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+-----------------------+-------------+---------------------+
| Layer | Field | Set By |
+-----------------------+-------------+---------------------+
| Message Body | MIME Header | Originator |
| Message header fields | From | Originator |
| | Sender | Source |
| | Reply-To | Originator |
| | To, CC, BCC | Originator |
| | Message-ID | Source |
| | Received | Source, Relay, Dest |
| | Return-Path | MDA, from MailFrom |
| | Resent-* | Mediator |
| SMTP | HELO | Latest Relay Client |
| | MailFrom | Source |
| | RcptTo | Originator |
| IP | IP Address | Latest Relay Client |
+-----------------------+-------------+---------------------+
4.2 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 implementation, such as a
"thin" user interface module on a limited end-user device, with the
bulk of the MUA functionality operated remotely on a more capable
server. An example of such an architecture might use IMAP [RFC3501]
for most of the interactions between an MUA client and an MUA server.
A Mediator is special class of MUA. It performs message re-posting,
as discussed in Section 2.1.
Identity fields relevant to the MUA include:
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RFC2822.From
Set by: Originator
Names and addresses for author(s) of the message content are
listed in the From field
RFC2822.Reply-To
Set by: Originator
If a message Recipient sends a reply message that would otherwise
use the 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.
RFC2822.Sender
Set by: Source
This specifies the address responsible for submitting the message
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 "Bounce" address, contained in RFC2821.MailFrom -- is made
by the RFC2822.Sender. Typically the Bounce address is the same
as the Sender address. However some usage scenarios require it to
be different.
RFC2822.To, RFC2822.CC
Set by: Originator
These specify MUA Recipient addresses. The addresses in the
fields might not be present in the RFC2821.RcptTo command. The
distinction between To and CC is subjective. Generally, a To
addressee is considered primary and is expected to take action on
the message. A CC addressee typically receives a copy only for
their information.
RFC2822.BCC
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Set by: Originator
A message might be copied to an addressee whose participation is
not to be disclosed to the RFC2822.To or RFC2822.CC Recipients
and, usually, not to the other BCC Recipients. The BCC header
field indicates a message copy to such a Recipient. Typically,
the field lists no addresses or only lists the address of the
Recipient receiving this copy. An MUA will typically make
separate postings for TO and CC Recipients, versus BCC Recipients.
The former will see no indication that any BCCs were sent, whereas
the latter have a BCC field present. It might be empty, contain a
comment, or contain one or more BCC addresses, depending upon the
preferences or the Originator.
4.3 Mail Submission Agent (MSA)
A Mail Submission Agent (MSA) accepts the message submission from the
oMUA and enforces the policies of the hosting AU 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
employees.
The MUA/MSA interface can be implemented within a single host and use
private conventions for its 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 authorization.
Identities relevant to the MSA include:
RFC2821.HELO or RFC2821.EHLO
Set by: Source
The MSA may specify its hosting domain identity for the SMTP HELO
or EHLO command operation.
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RFC2821.MailFrom
Set by: Source
This is an end-to-end string that specifies an email address for
receiving return control information, such as "bounces". The name
of this field is misleading, because it is not required to specify
either the author or the 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.)
RFC2821.RcptTo
Set by: Originator
This specifies the MUA mailbox address of a recipient. The string
might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.RcptTo
address specifies a member of that list.
RFC2821.Received
Set by: 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.
4.4 Mail Transfer Agent (MTA)
A Mail Transfer Agent (MTA) relays mail for one, application-level
"hop". It is like a packet-switch or IP router in that its job is to
make routing assessments and to move the message closer to the
Recipient(s). Relaying is performed by a sequence of MTAs, until the
message reaches its destination MDA(s). Hence an MTA implements both
client and server MTA functionality. It does not make changes to
addresses in the envelope or reformulate the content, except as
transfer-encoding requirements dictate. Also it may add trace
information. Of course email objects are typically much larger than
the payload of a packet or datagram, and the end-to-end latencies are
typically much higher.
Internet Mail primarily uses SMTP [RFC2821], [RFC0821] to effect
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point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]. As with
most network layer mechanisms Internet Mail's SMTP supports a basic
level of reliability, by virtue of providing for retransmission after
a temporary transfer failure. Contrary to typical packet switches
(and Instant Messaging services) Internet Mail MTAs typically store
messages in a manner that allows recovery across service
interruptions, such as host system shutdown. However the degree of
such robustness and persistence by an MTA can be highly variable.
The primary "routing" mechanism for Internet Mail is the DNS MX
record [RFC1035], which specifies a host, through which the queried
domain can be reached. This presumes a public -- or at least a
common -- backbone that permits any attached host to connect to any
other.
An important characteristic of MTA-MTA communications, over the open
Internet, is that they do not require prior arrangement between the
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 relevant to the MTA include:
RFC2821.HELO
Set by: Relay
The MTA may specify its hosting domain identity for the SMTP HELO
or EHLO command. This is the only standardized way of identifying
the agent responsible for operation of the Relay, during the
transfer operation.
RFC2821.MailFrom
Set by: Source
This is an MHS end-to-end string that specifies an email address
for receiving return control Bounce, such as delivery
confirmations and error notices. The protocol 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
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
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(and, possibly, successes.)
RFC2821.RcptTo
Set by: Originator
This specifies the MUA mailbox address of a Recipient. The string
might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.RcptTo
address specifies a member of that list.
RFC2822.Received
Set by: 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.
4.5 Mail Delivery Agent (MDA)
A Mail Delivery Agent (MDA) delivers email to the Recipient's
mailbox. It can provide distinctive, address-based functionality,
made possible by its detailed knowledge of the properties of the
destination address. This knowledge might also be present elsewhere
in the Recipient's Administrative Unit, such as at an organizational
border Relay. However it is required for the MDA, if only because
the MDA must know where to deliver the message.
Using Internet protocols, delivery can be effected by a variety of
standard protocols. When coupled with an internal, local mechanism,
SMTP [RFC2821] and LMTP [RFC2033] permit "push" delivery to the
Recipient system, at the initiative of the upstream email service.
POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the
initiative of the Recipient system. POP and IMAP can also be used
for repeated access to messages on a remote MS.
Identities relevant to the MDA include:
RFC2821.Return-Path
Set by: Source
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The MDA records the RFC2821.MailFrom address into the
RFC2822.Return-Path field.
RFC2822.Received
Set by: Destination
An MDA must record a Received header field, to indicate trace
information, including source host and receiving host domain names
and/or IP Addresses.
4.6 Message Store (MS)
An MUA can use 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 a server-based MS
(sMS) and user-based MS (uMS) in Figure 5. sMS is shown as being
remote from the MUA and uMS 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
reconnection.
5. Mediators
Basic email transfer is accomplished with an asynchronous store-and-
forward communication infrastructure, in a sequence of independent
transmissions through some number of MTAs. A very different task is
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a User-level sequence of postings and deliveries, through Mediators.
For such re-postings, a Mediator does share some functionality with
basic MTA relaying, but it enjoys a degree of freedom with both
addressing and content that is not available to MTAs.
RFC2821.HELO or RFC2821.EHLO
Set by: Source or Relay
The MSA may specify its hosting domain identity for the SMTP HELO
or EHLO command operation.
RFC2821.MailFrom
Set by: Source
This is an end-to-end string that specifies an email address for
receiving return control 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.)
RFC2821.RcptTo
Set by: Mediator
This specifies the MUA mailbox address of a Recipient. The string
might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.RcptTo
address specifies a member of that list.
RFC2821.Received
Set by: Mediator
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.
The salient aspect of a Mediator, that distinguishes it from any
other MUA creating an entirely new message, is that a Mediator
preserves the integrity and tone of the original message, including
the essential aspects of the original origination information. The
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Mediator might also add commentary.
Examples of MUA message creation that are NOT performed by Mediators
include:
New message forwarding existing message:
This action rather curiously provides a basic template for a class
of Mediators. However for it's typical occurrence it is not
itself an example of a Mediator. The new message is viewed as
being from the Agent doing the forwarding, rather than being from
the original Originator.
A new message encapsulates the original message and is seen as
strictly "from" the Mediator. The Mediator might add commentary
and certainly has the opportunity to modify the original message
content. The forwarded message is therefore independent of the
original message exchange and creates a new message dialogue.
However the final Recipient sees the contained message as from the
original Originator.
Reply:
When a Recipient formulates a response to a message, the new
message is not typically viewed as being a "forwarding" of the
original. It's focus is the new content; any inclusion of
material from the original message is contextual and secondary.
Annotator:
The integrity of the original message is usually preserved, but
one or more comments about the message are added in a manner that
distinguishes commentary from original text. The tone of the new
message is that it is primarily commentary from a new Originator,
similar to a Reply.
The remainder of this section describes common examples of Mediators.
5.1 Aliasing
A simple re-addressing facility that is available in most MDA
implementations is called Aliasing. It is performed just before
delivering a message to the specified Recipient's mailbox. 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. It resubmits the message, replacing the envelope
address, on behalf of the mailbox address that was listed in the
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envelope.
What is most distinctive about this forwarding mechanism is how
closely it compares to normal MTA store-and-forward Relaying. In
reality its only interesting difference is that it changes the
RFC2821.RcptTo value. Having the change be this small makes it easy
to view aliasing as a part of the lower-level mail relaying activity.
However the small change has a large semantic impact: The designated
recipient has chosen a new recipient. Hence, that original recipient
must become responsible for any handling issues.
An MDA that is re-posting a message to an alias typically changes
only envelope information:
RFC2822.TO, RFC2822.CC, RFC2822.BCC
Set by: Originator
These retain their original addresses.
RFC2821.RcptTo
Set by: Mediator
This field contains an alias address.
RFC2821.MailFrom
Set by: Mediator or original Source
The agent responsible for submission to an alias address will
often retain the original address to receive handling Bounces.
The benefit of retaining the original MailFrom value is to ensure
that the origination-side agent knows that there has been a
delivery problem. On the other hand, the responsibility for the
problem usually lies with the Recipient, since the Alias mechanism
is strictly under the Recipient's control.
RFC2821.Received
Set by: Mediator
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 therefore
include everything from original posting through final delivery to
the alias.
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5.2 ReSending
Also called ReDirecting, ReSending differs from Forwarding by virtue
of having the Mediator "splice" a message's addressing information,
to connect the Originator of the original message and the Recipient
of the new message. This permits them to have direct exchange, using
their normal MUA Reply functions. Hence the new Recipient sees the
message as being From the original Originator, even if the Mediator
adds commentary.
Identities specified in a resent message include
RFC2822.From
Set by: original 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.
RFC2822.Reply-To
Set by: original Originator
If this field is present in the original message, it is retained
in the Resent message.
RFC2822.Sender
Set by: original Source
This field is expected to contain the original Sender value.
RFC2822.TO, RFC2822.CC, RFC2822.BCC
Set by: original Originator
These specify the original message Recipients.
RFC2822.Resent-From
Set by: Mediator
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
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RFC2822.Resent-Sender
Set by: Mediator
The address of the agent responsible for re-submitting the
message. For efficiency this field is often 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 Notification 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:
Set by: Mediator
The addresses of the new Recipients who will now be able to reply
to the original author.
RFC2821.MailFrom
Set by: Mediator
The agent responsible for re-submission (RFC2822.Resent-Sender) is
also responsible for specifying the new MailFrom address.
RFC2821.RcptTo
Set by: Mediator
This will contain the address of a new Recipient
RFC2822.Received
Set by: Mediator
When resending a message, the submission agent may record a
Received header field, to indicate the transition from original
posting to resubmission.
5.3 Mailing Lists
Mailing lists have explicit email addresses and they forward messages
to a list of subscribed members. The Mailing List Actor performs a
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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, the Mediator can modify content, such as deleting
attachments, formatting conversion, and adding list-specific
comments. In addition, archiving list messages is common. Still,
the message retains characteristics of being "from" the original
Originator.
Identities relevant to a mailing list processor, when submitting a
message, include:
RFC2919.List-id
Set by: Mediator
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
adoption.
RFC2369.List-*
Set by: Mediator
[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.
RFC2822.From
Set by: original Originator
Names and email addresses for the original author(s) of the
message content are specified.
RFC2822.Reply-To
Set by: original Originator or Mediator
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.
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RFC2822.Sender
Set by: original Source or Mediator
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.
RFC2822.TO, RFC2822.CC
Set by: original Originator
These will usually contain the original list of Recipient
addresses.
RFC2821.MailFrom
Set by: original Source or Mediator
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 Bounces.
RFC2821.RcptTo
Set by: Mediator
This contains the address of a mailing list member.
RFC2821.Received
Set by: Mediator
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.
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5.4 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 distinction between an MTA
and a Gateway is that the latter transforms addresses and/or message
content, in order to map between the standards of two, different
messaging services. In virtually all cases, this mapping process
results in some degree of semantic loss. The challenge of Gateway
design is to minimize this loss.
A Gateway may set any identity field available to a regular MUA.
Identities typically relevant to Gateways include:
RFC2822.From
Set by: original 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
environment.
RFC2822.Reply-To
Set by: original 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.
RFC2822.Sender
Set by: original Source or Mediator
This may retain the original value or may be set to a new address
RFC2822.TO, RFC2822.CC, RFC2822.BCC
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Set by: original Recipient
These usually retain their original addresses.
RFC2821.MailFrom
Set by: original Source or Mediator
The agent responsible for gatewaying the message may choose to
specify a new address to receive handling notices.
RFC2822.Received
Set by: Mediator
The Gateway may record a Received header field, to indicate the
transition from original posting to the new messaging environment.
5.5 Boundary Filter
Organizations often enforce security boundaries by subjecting
messages to analysis, for conformance with the organization's safety
policies. An example is detection of content classed as spam or a
virus. A Filter might alter the content, to render it safe, such as
by removing content deemed unacceptable. Typically these actions
will result in the addition of content that records the actions.
6. 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.
In addition, email transfer protocols can operate over authenticated
and/or encrypted links, and message content can be authenticated or
encrypted.
7. References
7.1 References - Normative
[ID-hdr-reg]
"Registration of mail and MIME header fields",
draft-klyne-hdrreg-mail-04.txt (work in progress),
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Apr 2004.
[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
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.
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[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.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC3028] Showalter, T., "Sieve: A Mail Filtering Language",
RFC 3028, January 2001.
[RFC3297] Klyne, G., Iwazaki, R., and D. Crocker, "Content
Negotiation for Messaging Services based on Email",
RFC 3297, July 2002.
[RFC3458] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
[RFC3501] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, March 2005.
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7.2 Reference - Descriptive
[ID-ffpim]
Crocker, D. and G. Klyne, "Full-mode Fax Profile for
Internet Mail: FFPIM", March 2004.
[ID-spamops]
Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
E. Allman, "Email Submission Between Independent
Networks", draft-spamops-00 (work in progress),
March 2004.
[RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects",
RFC 1767, March 1995.
[Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
"Tussle in Cyberspace: Defining TomorrowÇÖs Internet",
ACM SIGCOMM, 2002.
Author's Address
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
Appendix A. Acknowledgements
This work derives from a section in draft-hutzler-spamops [ID-
spamops]. Discussion of the Source actor role was greatly clarified
during discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
insight on the framework and details of the original drafts.
Later reviews and suggestions were provided by Nathaniel Borenstein,
Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ned Freed,
Eric Hall, Brad Knowles, Bruce Lilly, Mark E. Mallett, David
MacQuigg, Chris Newman, Daryl Odnert, Rahmat M. Samik-Ibrahim, Hector
Santos, Jochen Topf, Willemien Hoogendoorn.
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Crocker Expires September 29, 2005 [Page 39]
