wdiff draft-crocker-email-arch-02.txt draft-crocker-email-arch-03.txt

SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking
Expires: July 27, August 15, 2005 January 26, February 14, 2005

Internet Mail Architecture
draft-crocker-email-arch-02
draft-crocker-email-arch-03

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Abstract

Over its thirty thirty-four 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) (MUA), and the transmission
world, in the form of the Mail Handling Service (MHS) composed of
Mail Transfer Agents (MTA). Over time each Core aspects of these has divided into
multiple, specialized modules. Public discussion and agreement about the nature service, such as
address and message style, have remained remarkably constant.
However public discussion of the changes to Internet mail architecture has not kept pace, and
abuses of pace with
the Internet mail service have brought these issues into
stark relief. real-world refinements. This draft document offers clarifications and enhancements, an enhanced
Internet Mail architecture to
provide a more consistent base for community discussion of email
service problems and proposed email service enhancements. reflect the current service.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Document Changes Discussion venue . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Discussion venue Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 5
2.1 User-Level User Actors . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Transfer-Level MHS Actors . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Administrative Actors . . . . . . . . . . . . . . . . . . . . 11
3. Email Identities . . . . . . . . . . . . . . . . . . . . . . . 11 . . . 13
3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . . . 12 13
3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . . 13 14
3.3 Message Identifers Identifiers . . . . . . . . . . . . . . . . . . . . . . 13 14
3.4 Identity Referencing Convention . . . . . . . . . . . . . . . 13 15
4. Protocols and Services . . . . . . . . . . . . . . . . . . . . 13 . . . . . . . 15
4.1 Service Components Message . . . . . . . . . . . . . . . . . . . . . . 15 . . . . . 17
4.2 Operational Configuration Mail User Agent (MUA) . . . . . . . . . . . . . . . . . . 21 . . 19
4.3 Layers of Identity References Mail Submission Agent (MSA) . . . . . . . . . . . . . . . . . 21
5.
4.4 Mail Transfer Agent (MTA) . . . . . . . . . . . . . . . . . . 22
4.5 Mail Delivery Agent (MDA) . . . . . . . . . . . . . . . . . . 24
4.6 Message Data Store (MS) . . . . . . . . . . . . . . . . . . . . . . 25
5. Mediators . . . . . 22 . . . . . . . . . . . . . . . . . . . . . 25
5.1 Envelope Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 27
5.2 Message Header Fields ReSending . . . . . . . . . . . . . . . . . . . . 22 . . . . . . 28
5.3 Body Mailing Lists . . . . . . . . . . . . . . . . . . . . . . . . 30
5.4 Gateways . . . . . 23
6. Two Levels of Store-And-Forward . . . . . . . . . . . . . . . 23
6.1 MTA Relaying . . . . . . . 33
5.5 Security Filter . . . . . . . . . . . . . . . . . . 23
6.2 MUA Forwarding . . . . . 34
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23 34
7. Security Considerations References . . . . . . . . . . . . . . . . . . . 30
8. . . . . . . . 34
7.1 References - Normative . . . . . . . . . . . . . . . . . . . . 34
7.2 Reference - Descriptive . . . . . . 31 . . . . . . . . . . . . . 36
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 33 37
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33 37
Intellectual Property and Copyright Statements . . . . . . . . 34 38

1. Introduction

There are two, basic categories of participants transmission
world, in Internet Mail.
Users are customers the form of the Mail Handling Service (MHS). They
represent the sources and sinks (MHS) composed of that service. The
Mail Handling
Service Transfer Agents (MTA).

The MHS is responsible for accepting a message from one user User and
delivering it 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

Over time the operational service has sub-divided each of reference 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 changes, and abuses of the real-world refinements. This document offers an enhanced
Internet mail service
have brought this into stark relief. So, it is necessary Mail architecture to produce
a revised architecture. However it is important that reflect the current service. 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 retained, and the elaboration to confuse this user-level activity with each "level" of the underlying mail
transmission service exchanges.
architecture is discussed separately.

For Internet mail, the term "end-to-end" usually refers to a single
posting and the set of deliveries directly resulting from a its single
transiting of the MHS. However, note that specialized 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 and [RFC2423], EDI over email. email [RFC1767], and
facsimile over email.[ID-ffpim]

The current draft seeks to:

o Document changes that have taken place in refining 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

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 A connected sequence of point-to-point transfer mechanisms

o No prior arrangement between originator Originator and recipient

5. Recipient

o 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. content.

A precept to the design of Internet mail is to permit 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
Gateways that translate between Internet mail standards and other
email conventions.

For localized environments (edge (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
'mediator'.

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:
<mailto:ietf-smtp@imc.org> mailing list. The the IETF-SMTP
mailing list
<http://www.imc.org/ietf-smtp/index.htm> <http://www.imc.org/ietf-smtp>. It is the most active,
long-standing venue for discussing email architecture. Although this
list it
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 because
it attends to end-to-end infrastructure and architecture issues more
than other email-related mailing lists.

2. Email Actor Roles

Discussion

1.3 Changes

This is intended to be the last major revision, prior to seeking
publication.

Significant changes to this version:

Administrative Domain: Extensive discussion of email architecture requires this operational
construct, including distinguishing different
actors within the service, User, Edge and being clear about Transit ADs.
This elaborates the job each
performs. The best way reference to "providers" in earlier drafts.

Mediator: Extensive revision both to maintain the distinction between user
activity description of Mediator
and handling activities use of the construct throughout the document.

Gateway: The construct of a gateway is elaborated.

Set by: Tables that had an entry for "Actor:" have been changed to depict their details
"Set by:" in
separate diagrams. Current order to clarify the nature of the Actor reference
being made. It is intended to indicate who is responsible for
setting the identity, rather than indicate what identity is
referred to. The specific references were carefully reviewed and
modified, to reflect this focus. The list of "set by" entries was
extensively reviewed, with substantial modifications made.

Editorial proofing: A complete word-smithing pass over the
document.

2. Email Actor Roles

Internet mail provides only Mail is a small set highly distributed service, with a variety of capabilities for supporting
actors serving different kinds of ongoing, user-level
exchanges. roles. These divide into:

o User

o Mail Handling Service (MHS)

o Administrative Domain
Although related to a technical architecture, the focus of a
discussions on Actors is on
concerns participant responsibilities, rather than on 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
different organizations. This operational independence provides the email service.
motivation for distinguishing Administrative Domains.

2.1 User-Level User 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
Users participating in a set of exchanges. In Internet Mail there
are three, basic three types of user-level Actors:
Originators, Recipients, and Mediators. Fromhe t

o Originators

o Recipients

o Mediators

From the User-level perspective all mail transfer activities are
performed by a monolithic, shared handling service. MHS. Users are customers of this
service.

The following depicts the relationships among them.

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

The functions of these

Figure 2: Relationships Among User Actors are:

2.1.1 Originator

Also called "Author", this is the user-level participant responsible
for creating original content and requesting its transmission. The
Mail Handling Service
MHS operates to send and deliver mail among Originators and
Recipients.

2.1.2 Recipient

The Recipient is a consumer of delivered content.

A recipient Recipient may close the user-level communication loop by creating
and submitting a new message that replies to an originator. Originator. An
automated, or semi-automated
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.

2.1.3 Mediator

A Mediator receives, aggregates, reformulates and distributes redistributes
messages as part of a potentially-protracted, higher-level exchange
among users. A 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 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, when
the Mediator's address is specified in the envelope. 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(s) message it
reformulates, but makes may make meaningful changes to the content. Hence
the
Mail Handling Service 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 passive
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, contingent complex and creative, 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. That is, So, a Mediator really is a full-fledged User.

Specialized Mediators include:

Forwarder:

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

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
Originator.

Mailing List: This Actor performs a task that can be viewed as an
elaboration particularly interesting form of the Redirector role. In addition to sending the
new message to Mediator.
It is a potentially large number hybrid of new Recipients,
content might be modified, such as deletion of attachments,
formatting conversion, User 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 Relay 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 interconnects heterogeneous
mail services. Its goal of content
classed as spam or emulating 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 Relay, so Gateway is
described in the addition
of content that records the actions. next section.

2.2 Transfer-Level MHS Actors

The Mail Handling Service (MHS) has the task of performing a single,
email-level end-to-end transfer transfer, on behalf of the originator Originator and
reaching the
recipient Recipient address(es) specified in the envelope. Protracted,
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 service.

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 them each pair 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 |
+---------+ +----+----+ +---------+
|
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 mail relay. Relay. Validity includes
conformance with Internet mail standards, as well as with local
operational policies. Source 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. "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 Mail Handling
Service.
MHS, that is, the local Administrative Domain.

The Source also has the responsibility for any post-submission,
originator-related
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 Notices Notifications Handler

Transfer efforts might result in

The Notifications Handler processes service notifications that are
generated by the generation MHS, as a result of service reporting
information its efforts to transfer or
deliver the message. Notices may be about failures or completions. These Transfer or
Delivery notification messages completions
and are sent to an address that is specified by the Source. A This
Notices handling address (also known as a Bounce or Return address)
might have no visible characteristics in common the with the address of
the Originator or Source.

2.2.3 Relay

A mail relay Relay performs email transfer-service routing and
store-and-forward. It adds envelope-related envelope-level handling information and
then (re-)transmits the message on towards its recipient(s). 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 contents.

A basic transfer operation is between a client and content semantics. It may modify message content syntax,
such as a server Relay. 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. Hence, interesting email scenarios can involve three levels
of store-and-forward:

o User Mediators

o MHS Relays

o Packet Switches

Aborting a 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. Notifications address.)

2.2.4 Gateway

A Gateway is a special hybrid form of User and Relay that interconnects
heterogeneous mail services. It operates as a User process, but its
purpose is simply to Relay messages. The more closely a Gateway is
able to operate as a Relay, the better. Differences between the mail
services can be as small as minor syntax variations, but usually
encompass much more basic, 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, Hence the Relay function of
in a gateway is Gateway offers the minor
component. 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 Gateway's adequacy is, of course, whether an
originator
Originator on one side of a Gateway can send a message to a recipient, Recipient
on the other side, without requiring any changes to any of the components
in the originator's mail service Originator's or the
recipient's Recipient's mail service, services, other than adding
the gateway. Gateway. To each of these otherwise independent services, the gateway
Gateway will appear to be a "native" participant. However the
ultimate test of a gateway's Gateway's adequacy is whether the originator Originator and recipient
Recipient can sustain a dialogue. In particular, can a recipient 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 is operators). Each can be composed of an independent
Administrative Domain. Domain (AD). Examples include an end-user operating
their desktop client, a department operating a local relay, Relay, an IT
department operating an enterprise relay, 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. Figure 4 depicts the relationships among
ADs. Perhaps the most salient aspect of an AD is the differential
trust that determines its policies for activities within the AD,
versus those involving interactions with other ADs.

Basic components of AD distinction include:

Transit: These are Mail Service Providers (MSP) offering
value-added capabilities for Edge ADs, such as aggregation and
filtering.

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.

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 ADs, at the email level.

+------ +------+ +------+
| AD-1 | | AD-3 | | AD-4 |
| ---- | | ---- | | ---- |
| | +---------------------->| | | |
| User | | |-Edge-+---->|-User |
| | | | +--->| | | |
| V | | | +------+ +------+
| Edge-+----+ |
| | | +---------+ |
+------+ | | AD-2 | |
| | ------- | |
| | | |
+--->|-Transit-+---+
| |
+---------+

Figure 4: Administrative Domains (AD)

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 Domain 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 Domain.

2.3.1 Provider

Providers For example, see
[ID-spamops].

User, Edge and Transit services can be offered by providers that
operate component services or sets of services. It Further, it is
possible for Providers one AD to host services for other Providers. ADs. Common AD
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 providers be involved in
detailed administration and enforcement issues, to help insure ensure the
health of the overall Internet Mail Service. This can include
operators of lower-level packet services.

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

3.1 Mailbox Addresses

An addr-spec

"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
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 Domain Name Service (DNS) [RFC1034], [RFC1035]. [RFC1035], [RFC2181].

The left-hand side 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
"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. addr-spec.

A small class of addresses have 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, Recipient
Gateways, and they are invisible to the public email transfer
infrastructure. When an originator Originator is explicitly sending via a
gateway
Gateway out of the Internet, there are coding conventions for the
local-part, so that the originator Originator can formulate instructions for the
gateway.
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. 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

Like mailbox addresses, message identifiers have two distinct parts,
divided by an at-sign ("@"). The right-hand side contains a is globally
interpreted name for and specifies 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. path, within the AD 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. 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
decisions that are made to implement a product, with the
architectural abstractions used to define conceptual functions.

Modern Internet email

The Internet's MHS architecture distinguishes four 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
them.

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

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

Figure 5: Protocols and Services

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.

4.1 Message

The purpose of the Mail Handling Service Components 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.

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. It
provides information about 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]

4.1.1 Envelope

Information that is directly used by, 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]. 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, 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. 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]. It should be noted that 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:

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 Recipient-side rMUA works on behalf of the end-user recipient 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 Mediator is special class of MUA performs message re-posting, as
discussed in
the <Mediator> section. Section 2.1.

Identity fields set by relevant to the MUA include:

RFC2822.From

Actor:

Set by: Originator

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

RFC2822.Reply-To

Actor:

Set by: Originator

If a message recipient 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. Recipients.

RFC2822.Sender

Actor:

Set by: Source

This specifies the address responsible for submission 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
"notifications" "Notifications" (or "bounces") address, contained in
RFC2821.MailFrom -- is made by the RFC2822.Sender. Typically the
notifications
Notifications address is the same as the Sender address. However
some usage scenarios require it to be different.

RFC2822.To, RFC2822.CC

Actor: Recipient

Set by: Originator

These specify MUA recipient 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

Actor: Recipient

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. Recipients
and, usually, not to the other BCC Recipients. The BCC header
field indicates a message copy to such a recipient. Recipient. Typically,
the field lists no addresses or only lists the address of the single recipient
Recipient receiving the this 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, Recipients, versus BCC recipients. 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.

4.1.2

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 network AD 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 their 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 set by relevant to the MSA include:

RFC2821.HELO or RFC2821.EHLO

Actor:
Set by: Source

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

RFC2821.MailFrom

Actor:

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.Rcpt-To

Actor: Recipient successes.)

RFC2821.RcptTo

Set by: Originator

This specifies the MUA inbox 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.Rcpt-To RFC2821.RcptTo
address specifies a member of that list.

RFC2821.Received

Actor:

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.

4.1.3

4.4 Mail Transfer Agent (MTA)

A Mail Transfer Agent (MTA)

An <MTA> relays mail. It is like a message to another other MTA packet-switch
or to an <MDA>, IP router in a
point-to-point exchange. 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.
Hence an MTA implements both client and server MTA functionality.

The basic functionality of an MTA is similar It
does not make changes to that of a packet
switch addresses in the envelope or IP router. That is, it does email store-and-forward email,
with a routing decision determining where reformulate the next-hop destination
shall be.
content, except as transfer-encoding requirements dictate. Also it
may add trace information.

The primary "routing" mechanism for Internet mail is the DNS MX
record [RFC1035]. As with most "link layer" network layer mechanisms Internet
mail's SMTP supports a basic level of reliability, by virtue of
providing for retransmission after al a temporary transfer failure.
However the degree of persistence by an MTA can be highly variable.

However

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. Contrary to typical packet switches (and Instant Messaging
services) Internet mail MTAs typically store messages in a manner
that allows recovery across services service 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] [RFC2645].

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 set by relevant to the MTA include:

RFC2821.HELO

Actor:

Set by: Relay

The MTA may specify its hosting domain identity for the SMTP HELO
or EHLO command command. This is the only standardized way of identifying
the agent responsible for operation of the Relay, during the
transfer operation.

RFC2821.Return-Path

Actor:

RFC2821.MailFrom

Set by: Source

This is an end-to-end string that specifies an email address for
receiving return control Notifications, such as "bounces". The MDA records
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 (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 into an RFC2822 header field named Return-Path. fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.RcptTo
address specifies a member of that list.

RFC2822.Received

Actor:

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.

4.1.4

4.5 Mail Delivery Agent (MDA)

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

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 Recipient's Administrative Domain, such as at an
organizational border gateway. Relay. However it is required for the MDA, if
only because the MDA must know where to store deliver the message.
This knowledge is used to achieve differential handling of messages.

Using Internet protocols, delivery is can be effected with POP [RFC1939] or
IMAP [RFC3501]. by a variety of
standard protocols. When coupled with an internal, local mechanism,
SMTP
permits [RFC2821] and LMTP [RFC2033] permit "push" delivery to the recipient
Recipient system, at the initative initiative of the upstream email service.
POP is [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the
initiative of the recipient Recipient system. Notably, SMTP and POP effect a
transfer of message control from the email service to the recipient
host. In contrast, and IMAP provides on-going, interactive can also be used
for repeated access to messages on 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. remote MS.

Identities set by relevant to the MDA include:

RFC2821.HELO or RFC2821.EHLO

Actor: Relay

RFC2821.Return-Path

Set by: Source

The MDA may specify its hosting domain identity for records the SMTP HELO
or EHLO command operation. RFC2821.MailFrom address into the
RFC2822.Return-Path field.

RFC2822.Received

Actors: Source, Relay, Dest

Set by: Destination

An MTA MDA 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

4.6 Message Store (MS)

An MUA's uses 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 MS-1 and MS-2 in the
Figure.
Figure 5. 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
reconnection.

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:

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

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

5.3 Body

6. Two Levels of Store-And-Forward Mediators

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 infrastructure, in 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 User-level sequence of postings and deliveries,
through Mediators. For such re-postings, a new submission process,
after final delivery for an earlier transfer sequence. Such
MUA-based re-posting shares 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 primary "routing" mechanism MSA may specify its hosting domain identity for Internet mail is the DNS MX
record [RFC1035]. It SMTP HELO
or EHLO command operation.

RFC2821.MailFrom

Set by: Source

This is an advertisement, by a recipient domain, of
hosts end-to-end string that are able to relay mail to it, within the portion specifies an email address for
receiving return control Notifications, such as "bounces". The
name 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 field is to make routing assessments and to move the message
payload data closer to the recipient. It misleading, because it is not their job required to
reformulate
specify either the payload 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 change addresses be in the envelope or
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
content.

6.2 MUA Forwarding

As discussed in <Mediator> section, forwarding is performed by MUAs
that take mailbox address of a received Recipient. The string
might not be visible in the message and submit it back to content header. For example,
the transfer
service, for delivery to one or more different addresses. A
forwarded message may appear identical to a relayed message, destination address header fields, such as
for Alias forwarders, or it 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 have minimal similarity, as with record a
Reply.

6.2.1 MUA Basic Forwarding Received header field, to indicate initial
submission trace information, including originating host and MSA
host domain names and/or IP Addresses.

The simplest type salient aspect of forwarding involves a Mediator, that distinguishes it from any
other MUA creating an entirely new message, with new content, that includes the original message between
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 Mediator
preserves 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 integrity and new
Recipient-2. In this case, the content tone of the new message is the old original message, including preservation of
the essential aspects of the original message's origination information.

Identities specified in a resent message include

RFC2822.From

Actor: Originator

Names and email addresses for the original author(s) The
Mediator might also add commentary.

Examples of the MUA message content creation that 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

Actor: Originator

If not performed by Mediators
include:

New Message Forwarding Existing Message:

Curiously, this field action provides a basic template for a class of
Mediators. However by itself, it's typical occurrence is present in the original message, it should be
retained not, in the Re-sent message.

RFC2822.Sender

Actor: Source

This field is expected to contain the original Sender value.

RFC2822.TO, RFC2822.CC, RFC2822.BCC

Actor: Recipient
These specify the original message recipients.

RFC2822.Resent-From

Actor: Mediating Originator

The address
fact, an example of the original recipient who a Mediator. The new message is redirecting viewed as
being from the
message. Otherwise, Agent doing the same rules apply for forwarding, rather than being from
the Resent-From
field as for an original RFC2822.From field

RFC2822.Resent-Sender

Actor: Mediating Source

The address of the agent responsible for re-submitting the
message. For efficiency, this field should be omitted if it
contains Originator.

A new message encapsulates 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 original message 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 seen as
strictly "from" the Resent-Sender address. However
some usage scenarios require it to be different.

RFC2822.Resent-To, RFC2822.Resent-cc, RFC2822.Resent-bcc: Actor:
Recipient Mediator. The addresses of Mediator might add commentary
and certainly has the new recipients who will now be able to reply opportunity to modify the original author.

RFC2821.MailFrom

Actor: Mediating Source message
content. The agent responsible for re-submission (RFC2822.Resent-Sender) forwarded message is
also responsible for specifying the new RFC2821.MailFrom address.

RFC2821.Rcpt-to

Actor: Recipient

This will contain the address therefore independent of the
original message exchange and creates a new recipient

RFC2822.Received
Actor: Mediating Source

When re-sending a message, message dialogue.
However the submission agent may record a
Received header field, to indicate final Recipient sees the transition contained message as from the
original
posting to resubmission.

6.2.3 MUA Reply Originator.

Reply:

When a recipient 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 It's focus is the basic routing and transfer work of message
relaying, but they also make new content; any message or address modifications
that are needed to send inclusion of
material from the original message into the next messaging
environment. When a gateway connects two differing messaging
services, its role is easy to identify contextual 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. secondary.

Annotator:

The critical distinguish between an MTA
and a gateway is that the latter modifies addresses and/or message
content.

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

RFC2822.From

Actor: Originator

Names and email addresses for integrity of the original author(s) of message is usually preserved, but
one or more comments about 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 added in the target
environment.

RFC2822.Reply-To

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.

RFC2822.Sender

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 manner that
distinguishes commentary from original addresses.

RFC2821.MailFrom

Actor: Source text. The agent responsible for gatewaying tone of the new
message may choose to
specify is that it is primarily commentary from a new address Originator,
similar to receive handling notices.

RFC2822.Receive

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 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
placing
delivering a message into to the specified recipient's inbox. 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 Recipient
user function. In effect it It resubmits the message to a new message, replacing the envelope
address, on behalf of the mailbox address that was listed recipient. in the
envelope.

What is most distinctive about this forwarding mechanism is how
closely it compares to normal MTA store-and-forward. store-and-forward Relaying. In
reality its only interesting difference is that it changes the RFC2821.RCPT-TO
RFC2821.RcptTo 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:

RFC2822.TO, RFC2822.CC, RFC2822.BCC

Actor: Recipient

Set by: Originator

These retain their original addresses.

RFC2821.Rcpt-To

Actor: Recipient

RFC2821.RcptTo

Set by: Mediator

This field contains an alias address.

RFC2821.MailFrom

Actor: Mediating

Set by: Mediator or original Source

The agent responsible for submission to an alias address will
usually
often retain the original address to receive handling
notifications.
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, Recipient,
since the Alias mechanism is strictly under the recipient's Recipient's
control.

RFC2821.Received

Actor: Mediating Recipient

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.

6.2.6

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

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
Notifications 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. Mailing list processing is a
user-level activity, outside of the core email transfer service. The
mailing list address is, therefore, associated with Mailing List Actor performs a distinct
user-level entity
task that can perform arbitrary actions upon be viewed as an elaboration of the
original message, before submitting it ReDirector role. In
addition to sending the mailing list
membership. Hence, mailing lists are similar new message to gateways. 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 set by relevant to a mailing list processor, when submitting a
message, include:

RFC2919.List-id

Actor: Mediating Originator

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-*

Actor: Mediating Recipient

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

Actor:

Set by: original Originator

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

RFC2822.Reply-To

Actor:

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.

RFC2822.Sender

Actor:

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, Relay, so that they preserve
as much of the original handling information as possible,
including the original RFC2822.Sender field.

RFC2822.TO, RFC2822.CC

Actor: Mediating Recipient

Set by: original Originator

These will usually contain the original list of recipient Recipient
addresses.

RFC2821.MailFrom

Actor: Mediating

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 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.

RFC2821.Rcpt-To

Actor: Recipient Notifications.

RFC2821.RcptTo

Set by: Mediator

This contains the address of a mailing list member.

RFC2821.Received

Actor: Mediating Recipient

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.

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

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 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.

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.

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), Apr
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.

[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.

[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
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.

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.

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

The Email Architecture section

This work derives from a section in 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, Frank Ellermann, Tony Finch,
Ned Freed, Eric Hall, Bruce Lilly,
Eric Hall, Mark E. Mallett, Chris Newman,
Daryl Odnert, Rahmat M. Samik-Ibrahim, Hector Santos, Jochen Topf. Topf,
Willemien.

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