< draft-crocker-email-arch-03.txt   draft-crocker-email-arch-04.txt >
©À
SMTP D. Crocker SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking Internet-Draft Brandenburg InternetWorking
Expires: August 15, 2005 February 14, 2005 Expires: September 29, 2005 March 28, 2005
Internet Mail Architecture Internet Mail Architecture
draft-crocker-email-arch-03 draft-crocker-email-arch-04
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions This document is an Internet-Draft and is subject to all provisions
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
Over its thirty-four year history, Internet mail has undergone Over its thirty-four year history, Internet Mail has undergone
significant changes in scale and complexity. The first standardized significant changes in scale and complexity, as it has become a
architecture for email specified a simple split between the user global infrastructure service. The first standardized architecture
world, in the form of Mail User Agents (MUA), and the transmission for email specified a simple split between the user world, in the
world, in the form of the Mail Handling Service (MHS) composed of form of Mail User Agents (MUA), and the transmission world, in the
Mail Transfer Agents (MTA). Core aspects of the service, such as form of the Mail Handling Service (MHS) composed of Mail Transfer
address and message style, have remained remarkably constant. Agents (MTA). Core aspects of the service, such as address and
However public discussion of the architecture has not kept pace with message style, have remained remarkably constant. Today, Internet
the real-world refinements. This document offers an enhanced Mail is marked by many independent operators, many different
Internet Mail architecture to reflect the current service. 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 Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . 4
1.2 Discussion venue . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Discussion venue . . . . . . . . . . . . . . . . . . . . . 5
1.3 Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Changes . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 5 2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 6
2.1 User Actors . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 User Actors . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 MHS Actors . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 MHS Actors . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Administrative Actors . . . . . . . . . . . . . . . . . . . . 11 2.3 Administrative Actors . . . . . . . . . . . . . . . . . . 11
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . . . 13 3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . 13
3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Message Identifiers . . . . . . . . . . . . . . . . . . . . . 14 3.3 Message Identifiers . . . . . . . . . . . . . . . . . . . 15
3.4 Identity Referencing Convention . . . . . . . . . . . . . . . 15 3.4 Identity Referencing Convention . . . . . . . . . . . . . 15
4. Services . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4. Services . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1 Message . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.1 Message . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2 Mail User Agent (MUA) . . . . . . . . . . . . . . . . . . . . 19 4.2 Mail User Agent (MUA) . . . . . . . . . . . . . . . . . . 20
4.3 Mail Submission Agent (MSA) . . . . . . . . . . . . . . . . . 21 4.3 Mail Submission Agent (MSA) . . . . . . . . . . . . . . . 22
4.4 Mail Transfer Agent (MTA) . . . . . . . . . . . . . . . . . . 22 4.4 Mail Transfer Agent (MTA) . . . . . . . . . . . . . . . . 23
4.5 Mail Delivery Agent (MDA) . . . . . . . . . . . . . . . . . . 24 4.5 Mail Delivery Agent (MDA) . . . . . . . . . . . . . . . . 25
4.6 Message Store (MS) . . . . . . . . . . . . . . . . . . . . . . 25 4.6 Message Store (MS) . . . . . . . . . . . . . . . . . . . . 26
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1 Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.1 Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.2 ReSending . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5.2 ReSending . . . . . . . . . . . . . . . . . . . . . . . . 30
5.3 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . . . 30 5.3 Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 31
5.4 Gateways . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 5.4 Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.5 Security Filter . . . . . . . . . . . . . . . . . . . . . . . 34 5.5 Boundary Filter . . . . . . . . . . . . . . . . . . . . . 35
6. Security Considerations . . . . . . . . . . . . . . . . . . . 34 6. Security Considerations . . . . . . . . . . . . . . . . . . . 35
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.1 References - Normative . . . . . . . . . . . . . . . . . . . . 34 7.1 References - Normative . . . . . . . . . . . . . . . . . . 35
7.2 Reference - Descriptive . . . . . . . . . . . . . . . . . . . 36 7.2 Reference - Descriptive . . . . . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 37 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 37 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38
Intellectual Property and Copyright Statements . . . . . . . . 38 Intellectual Property and Copyright Statements . . . . . . . . 39
1. Introduction 1. Introduction
Over its thirty-four year history, Internet mail has undergone Over its thirty-four year history, Internet Mail has undergone
significant changes in scale and complexity. The first standardized significant changes in scale and complexity, as it has become a
architecture for email specified a simple split between the user global infrastructure service.
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 The first standardized architecture for email specified a simple
delivering it to one or more others. 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 | . | User +--+--------->| User | .
+--------+ | +--------+ . +--------+ | +--------+ .
. | . . . | . .
. | +--------+ . . . | +--------+ . .
. +-->| User | . . . +-->| User | . .
. +--------+ . . . +--------+ . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
+--------------------------------------+ +--------------------------------------+
| Mail Handling Service (MHS) | | Mail Handling Service (MHS) |
+--------------------------------------+ +--------------------------------------+
Figure 1: Basic Email Service Model Figure 1: Basic Internet Mail Service Model
Over time the operational service has sub-divided each of these Today, Internet Mail is marked by many independent operators, many
"layers" into more specialized modules. Core aspects of the service, different components for providing users service and many other
such as address and message style, have remained remarkably constant. 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 However public discussion of the architecture has not kept pace with
the real-world refinements. This document offers an enhanced the real-world refinements. This document offers an enhanced
Internet Mail architecture to reflect the current service. The Internet Mail architecture to reflect the current service. The
original distinction between user-level concerns and transfer-level original distinction between user-level concerns and transfer-level
concerns is retained, and the elaboration to each "level" of the concerns is retained, and the elaboration to each "level" of the
architecture is discussed separately. architecture is discussed separately. The term "Internet Mail" is
used to refer to the entire collection of user and transfer
components.
For Internet mail, the term "end-to-end" usually refers to a single For Internet Mail, the term "end-to-end" usually refers to a single
posting and the set of deliveries directly resulting from its single posting and the set of deliveries directly resulting from its single
transiting of the MHS. However, note that some uses of email transiting of the MHS. However, note that some uses of email
consider the entire email service -- including Originator and consider the entire email service -- including Originator and
Recipient -- as a subordinate component. For these services, Recipient -- as a subordinate component. For these services, "end-
"end-to-end" refers to points outside of the email service. Examples to-end" refers to points outside of the email service. Examples are
are voicemail over email [RFC2423], EDI over email [RFC1767], and voicemail over email [RFC2423], EDI over email [RFC1767], and
facsimile over email.[ID-ffpim] facsimile over email.[ID-ffpim]
The current draft seeks to: The current draft seeks to:
o Document refinements to the email model o Document refinements to the email model
o Clarify functional roles for the architectural components o Clarify functional roles for the architectural components
o Clarify identity-related issues, across the email service o Clarify identity-related issues, across the email service
o Provide a document that serves as a common venue for further o Provide a document that serves as a common venue for further
defining and citing modern Internet mail architecture 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 1.1 Service Overview
End-to-end Internet mail exchange is accomplished by using a End-to-end Internet Mail exchange is accomplished by using a
standardized infrastructure comprising: standardized infrastructure comprising:
o An email object o An email object
o Global addressing o Global addressing
o A connected sequence of point-to-point transfer mechanisms o An asynchronous sequence of point-to-point transfer mechanisms
o No prior arrangement between Originator and Recipient o No prior arrangement between Originator and Recipient
o No prior arrangement between point-to-point transfer services, o No prior arrangement between point-to-point transfer services,
over the open Internet over the open Internet
The end-to-end portion of the service is the message. Broadly the o No requirement for Originator and Recipient to be online at the
message, itself, is divided between handling control information and same time.
user message content.
A precept to the design of Internet mail is permitting user-to-user 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 and MTA-to-MTA interoperability with no prior, direct administrative
arrangement. That is, all participants rely on having the core arrangement. That is, all participants rely on having the core
services be universally supported, either directly or through services be universally supported, either directly or through
Gateways that translate between Internet mail standards and other Gateways that translate between Internet Mail standards and other
email conventions. email conventions.
For localized environments (Edge networks) prior, administrative For localized environments (Edge networks) prior, administrative
arrangement can include access control, routing constraints and arrangement can include access control, routing constraints and
lookup service configuration. In recent years one change to local lookup service configuration. In recent years one change to local
environments is an increased requirement for authentication or, at environments is an increased requirement for authentication or, at
least, accountability. In these cases, the server performs explicit least, accountability. In these cases, the server performs explicit
validation of the client's identity. validation of the client's identity.
1.2 Discussion venue 1.2 Discussion venue
skipping to change at page 5, line 22 skipping to change at page 5, line 40
it attends to end-to-end infrastructure and architecture issues more it attends to end-to-end infrastructure and architecture issues more
than other email-related mailing lists. than other email-related mailing lists.
1.3 Changes 1.3 Changes
This is intended to be the last major revision, prior to seeking This is intended to be the last major revision, prior to seeking
publication. publication.
Significant changes to this version: Significant changes to this version:
Administrative Domain: Extensive discussion of this operational Administrative Unit: Changed from Administrative Domain to
construct, including distinguishing User, Edge and Transit ADs. Administrative Unit, to remove possible confusion with "domain
This elaborates the reference to "providers" in earlier drafts. name". Added Tussle reference
Mediator: Extensive revision both to the description of Mediator
and use of the construct throughout the document.
Gateway: The construct of a gateway is elaborated. Sieve: Noted ability to have other places to run sieve
instructions.
Set by: Tables that had an entry for "Actor:" have been changed to Word Smithing: Assorted small tweaks to definitions, diagrams and
"Set by:" in order to clarify the nature of the Actor reference comments.
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 Notices, Bounces and Disp: Added Bounce module to Services diagram,
document. 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 2. Email Actor Roles
Internet Mail is a highly distributed service, with a variety of Internet Mail is a highly distributed service, with a variety of
actors serving different roles. These divide into: actors serving different roles. These divide into:
o User o User
o Mail Handling Service (MHS) o Mail Handling Service (MHS)
o Administrative Domain o Administrative Unit
Although related to a technical architecture, the focus on Actors Although related to a technical architecture, the focus on Actors
concerns participant responsibilities, rather than on functional concerns participant responsibilities, rather than on functional
modules. Hence the labels used are different than for classic email modules. Hence the labels used are different than for classic email
architecture diagrams. Actors often will be associated with architecture diagrams. Actors often will be associated with
different organizations. This operational independence provides the different organizations. This operational independence provides the
motivation for distinguishing Administrative Domains. motivation for distinguishing Administrative Units.
2.1 User Actors 2.1 User Actors
Users are the sources and sinks of messages. They may have an Users are the sources and sinks of messages. They may be humans or
exchange that iterates and they may expand or contract the set of processes. They may have an exchange that iterates and they may
Users participating in a set of exchanges. In Internet Mail there expand or contract the set of Users participating in a set of
are three types of user-level Actors: exchanges. In Internet Mail there are three types of user-level
Actors:
o Originators o Originators
o Recipients o Recipients
o Mediators o Mediators
From the User-level perspective all mail transfer activities are From the User-level perspective all mail transfer activities are
performed by a monolithic, shared MHS. Users are customers of this performed by a monolithic Mail Handling Service (MHS), even though
service. the actual service may be provided by many independent organizations.
Users are customers of this service.
The following depicts the relationships among them. The following depicts the flow of messages among Actors.
+------------+ +------------+
| Originator |<--------------+ | Originator |<--------------+
+-+---+----+-+ | +-+---+----+-+ |
| | | | | | | |
| | V | | | V |
| | +-----------+ | | | +-----------+ |
| | | Recipient | | | | | Recipient | |
| | +-----------+ | | | +-----------+ |
| | | | | |
skipping to change at page 7, line 28 skipping to change at page 7, line 28
| V V | | | V V | |
| +-----------+ +---+---+---+ | +-----------+ +---+---+---+
| | Mediator +--->| Recipient | | | Mediator +--->| Recipient |
| +-----------+ +-----------+ | +-----------+ +-----------+
| |
V V
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+ +-----------+
| Mediator +--->| Mediator +--->| Recipient | | Mediator +--->| Mediator +--->| Recipient |
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+ +-----------+
Figure 2: Relationships Among User Actors Figure 2: Relationships Among User Actors
2.1.1 Originator 2.1.1 Originator
Also called "Author", this is the user-level participant responsible Also called "Author", this is the user-level participant responsible
for creating original content and requesting its transmission. The for creating original content and requesting its transmission. The
MHS operates to send and deliver mail among Originators and MHS operates to send and deliver mail among Originators and
Recipients. Recipients. As described below, the MHS has a "Source" role, that
correlates with the Author role.
2.1.2 Recipient 2.1.2 Recipient
The Recipient is a consumer of delivered content. 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 A Recipient may close the user-level communication loop by creating
and submitting a new message that replies to an Originator. An and submitting a new message that replies to an Originator. An
example of an automated form of reply is the Message Disposition example of an automated form of reply is the Message Disposition
Notification, which informs the Originator about the Recipient's Notification, which informs the Originator about the Recipient's
disposition of the message. See Section 4.1. disposition of the message. See Section 4.1.
2.1.3 Mediator 2.1.3 Mediator
A Mediator receives, aggregates, reformulates and redistributes A Mediator receives, aggregates, reformulates and redistributes
messages as part of a potentially-protracted, higher-level exchange messages as part of a potentially-protracted, higher-level exchange
among Users. Example uses of Mediators include group dialogue and among Users. Example uses of Mediators include group dialogue and
organizational message flow, as occurs with a purchase approval organizational message flow, as occurs with a purchase approval
process. Note that it is easy to confuse this user-level activity process. Note that it is easy to confuse this user-level activity
with the underlying MHS exchanges. However they serve very different with the underlying MHS transfer exchanges. However they serve very
purposes and operate is very different ways. Mediators are different purposes and operate is very different ways. Mediators are
considered extensively in Section 5. considered extensively in Section 5.
When mail is delivered to an envelope address, a Mediator is viewed When mail is delivered to an envelope address, a Mediator is viewed
by the Mail Handling Service as a Recipient. When submitting by the Mail Handling Service as a Recipient. When submitting
messages, the Mediator is an Originator. What is distinctive is that messages, the Mediator is an Originator. What is distinctive is that
a Mediator preserves the Originator information of the message it a Mediator preserves the Originator information of the message it
reformulates, but may make meaningful changes to the content. Hence reformulates, but may make meaningful changes to the content. Hence
the MHS sees a new message, but Users receive a message that is the MHS sees a new message, but Users receive a message that is
interpreted as primarily being from -- or, at least, initiated by -- interpreted as primarily being from -- or, at least, initiated by --
the author of the original message. The role of a Mediator permits the author of the original message. The role of a Mediator permits
skipping to change at page 8, line 34 skipping to change at page 8, line 40
and adding content or regulating which users may participate and and adding content or regulating which users may participate and
when. The popular example of this role is a group mailing list. A 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 sequence of mediators may even perform a series of formal steps, such
as reviewing, modifying and approving a purchase request. as reviewing, modifying and approving a purchase request.
Because a Mediator originates messages, it might also receive Because a Mediator originates messages, it might also receive
replies. So, a Mediator really is a full-fledged User. replies. So, a Mediator really is a full-fledged User.
Gateway: A Gateway is a particularly interesting form of Mediator. Gateway: A Gateway is a particularly interesting form of Mediator.
It is a hybrid of User and Relay that interconnects heterogeneous It is a hybrid of User and Relay that interconnects heterogeneous
mail services. Its goal of emulating a Relay, so Gateway is mail services. Its goal is to emulate a Relay, so Gateway is
described in the next section. described in more detail, in the next section.
2.2 MHS Actors 2.2 MHS Actors
The Mail Handling Service (MHS) has the task of performing a single, The Mail Handling Service (MHS) has the task of performing a single,
email-level end-to-end transfer, on behalf of the Originator and email-level end-to-end transfer, on behalf of the Originator and
reaching the Recipient address(es) specified in the envelope. reaching the Recipient address(es) specified in the envelope.
Mediated or protracted, iterative exchanges, such as those used for Mediated or protracted, iterative exchanges, such as those used for
collaboration over time, are part of the User-level service, and are collaboration over time, are part of the User-level service, and are
not part of this Transfer-level service. not part of this Transfer-level Handling Service.
The following depicts the relationships among transfer participants The following depicts the relationships among transfer participants
in Internet Mail. It shows the Source as distinct from the in Internet Mail. It shows the Source as distinct from the
Originator, and Destination as distinct from Recipient, although it Originator, and Destination as distinct from Recipient, although it
is common for each pair to be the same actor. The figure also shows is common for each pair to be the same actor. The figure also shows
multiple Relays in the sequence. It is legal to have only one, and multiple Relays in the sequence. It is legal to have no separate
for intra-organization mail services, this is common. Relay, where the Source and Dest interact directly. For intra-
organization mail services, it is common to have only one Relay.
+------------+ +-----------+ +------------+ +-----------+
| Originator | | Recipient | | Originator | | Recipient |
+-----+------+ +-----------+ +-----+------+ +-----------+
| ^ | ^
| Mail Handling Service | | Mail Handling Service |
/+=================================================+\ /+=================================================+\
|| | | || || | | ||
|| | | || || | | ||
V | V |
+---------+ +--------+ +----+----+ +---------+ +--------+ +----+----+
| | | |<------------+ | | | | |<------------+ |
| Source +...>| Notice | | Dest | | Source +...>| Bounce | | Dest |
| | | |<---+ | | | | | |<---+ | |
+----+----+ +--------+ | +---------+ +----+----+ +--------+ | +---------+
| | ^ | | ^
V | | V | |
+---------+ +----+----+ +----+----+ +---------+ +----+----+ +----+----+
| Relay +-->.......-->| Relay +-->| Relay | | Relay +-->.......-->| Relay +-->| Relay |
+---------+ +----+----+ +---------+ +---------+ +----+----+ +---------+
| |
V V
+---------+ +---------+
| Gateway +-->... | Gateway +-->...
+---------+ +---------+
Figure 3: Relationships Among MHS Actors Figure 3: Relationships Among MHS Actors
2.2.1 Source 2.2.1 Source
The Source role is responsible for ensuring that a message is valid The Source role is responsible for ensuring that a message is valid
for posting and then submitting it to a Relay. Validity includes for posting and then submitting it to a Relay. Validity includes
conformance with Internet mail standards, as well as with local conformance with Internet Mail standards, as well as with local
operational policies. The source may simply review the message for operational policies. The source may simply review the message for
conformance, and reject it if there are errors, or it may create some conformance, and reject it if there are errors, or it may create some
or all of the necessary information. or all of the necessary information.
The Source operates with dual "allegiance". It serves the Originator The Source operates with dual "allegiance". It serves the Originator
and often it is the same entity. However its role in assuring and often it is the same entity. However its role in assuring
validity means that it must also represent the local operator of the validity means that it must also represent the local operator of the
MHS, that is, the local Administrative Domain. MHS, that is, the local Administrative Domain.
The Source also has the responsibility for any post-submission, The Source also has the responsibility for any post-submission,
Originator-related administrative tasks associated with message Originator-related administrative tasks associated with message
transmission and delivery. Notably this pertains to error and transmission and delivery. Notably this pertains to error and
delivery notices. Hence, Source is best held accountable for the delivery notices. Hence, Source is best held accountable for the
message content, even when they did not create any or most of it. message content, even when they did not create any or most of it.
2.2.2 Notifications Handler 2.2.2 Bounce Handler
The Notifications Handler processes service notifications that are The Bounce Handler processes service notifications that are generated
generated by the MHS, as a result of its efforts to transfer or by the MHS, as a result of its efforts to transfer or deliver the
deliver the message. Notices may be about failures or completions message. Notices may be about failures or completions and are sent
and are sent to an address that is specified by the Source. This to an address that is specified by the Source. This Bounce handling
Notices handling address (also known as a Bounce or Return address) address (also known as a Return address) might have no visible
might have no visible characteristics in common with the address of characteristics in common with the address of the Originator or
the Originator or Source. Source.
2.2.3 Relay 2.2.3 Relay
A mail Relay performs email transfer-service routing and A mail Relay performs email transfer-service routing and store-and-
store-and-forward. It adds envelope-level handling information and forward. It adds envelope-level handling information and then
then (re-)transmits the message on towards its Recipient(s). A Relay (re-)transmits the message on towards its Recipient(s). A Relay may
may add information to the envelope, such as with trace information. add information to the envelope, such as with trace information.
However it does not modify existing envelope information or the However it does not modify existing envelope information or the
message content semantics. It may modify message content syntax, message content semantics. It may modify message content syntax,
such as a change from text to binary transfer-encoding form, only as 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. required to meet the capabilities of the next hop in the MHS.
A set of Relays composes a Mail Handling Service network. This is A set of Relays composes a Mail Handling Service network. This is
above any underlying packet-switching network that they might be above any underlying packet-switching network that they might be
using. Hence, interesting email scenarios can involve three levels using and below any gateways or other user-level Mediators.
of store-and-forward:
In other words, interesting email scenarios can involve three,
distinct architectural layers of store-and-forward service:
o User Mediators o User Mediators
o MHS Relays o MHS Relays
o Packet Switches 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 Aborting a message transfer results in having the Relay become an
Originator and send an error message to the Notifications (Bounce) Originator and send an error message to the Bounce (Bounce) address.
address. (The potential for looping is avoided by having this (The potential for looping is avoided by having this message, itself,
message, itself, contain no Notifications address.) contain no Bounce address.)
2.2.4 Gateway 2.2.4 Gateway
A Gateway is a hybrid form of User and Relay that interconnects A Gateway is a hybrid form of User and Relay that interconnects
heterogeneous mail services. It operates as a User process, but its heterogeneous mail services. Its purpose is simply to emulate a
purpose is simply to Relay messages. The more closely a Gateway is Relay and the closer it comes to this, the better. However it
able to operate as a Relay, the better. Differences between mail operates at the User level, because it must be able to modify message
services can be as small as minor syntax variations, but usually content.
encompass significant, semantic distinctions. For example, the
concept of an email address might be as different as a hierarchical, Differences between mail services can be as small as minor syntax
machine-specific address versus a flat, global name space. Or variations, but usually encompass significant, semantic distinctions.
between text-only content and multi-media. Hence the Relay function For example, the concept of an email address might be as different as
in a Gateway offers the minor challenge in design. The more a hierarchical, machine-specific address versus a flat, global name
significant challenge is in ensuring the user-to-user functionality space. Or between text-only content and multi-media. Hence the
that matches syntax and semantics of independent email standards Relay function in a Gateway offers the minor challenge in design.
suites. 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 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 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 on the other side, without requiring changes to any of the components
in the Originator's or Recipient's mail services, other than adding in the Originator's or Recipient's mail services, other than adding
the Gateway. To each of these otherwise independent services, the the Gateway. To each of these otherwise independent services, the
Gateway will appear to be a "native" participant. However the Gateway will appear to be a "native" participant. However the
ultimate test of a Gateway's adequacy is whether the Originator and ultimate test of a Gateway's adequacy is whether the Originator and
Recipient can sustain a dialogue. In particular, can a Recipient's Recipient can sustain a dialogue. In particular, can a Recipient's
MUA automatically formulate a valid Reply? MUA automatically formulate a valid Reply?
2.3 Administrative Actors 2.3 Administrative Actors
Operation of Internet mail services is apportioned to different Operation of Internet Mail services is apportioned to different
providers (or operators). Each can be composed of an independent providers (or operators). Each can be composed of an independent
Administrative Domain (AD). Examples include an end-user operating Administrative Unit (AU). Examples include an end-user operating
their desktop client, a department operating a local Relay, an IT their desktop client, a department operating a local Relay, an IT
department operating an enterprise Relay, and an ISP operating a department operating an enterprise Relay, and an ISP operating a
public, shared email service. These can be configured into many public, shared email service. These can be configured into many
combinations of administrative and operational relationships, with combinations of administrative and operational relationships, with
each Administrative Domain potentially having a complex arrangement each Administrative Unit potentially having a complex arrangement of
of functional components. Figure 4 depicts the relationships among functional components. Figure 4 depicts the relationships among AUs.
ADs. Perhaps the most salient aspect of an AD is the differential Perhaps the most salient aspect of an AU is the differential trust
trust that determines its policies for activities within the AD, that determines its policies for activities within the AU, versus
versus those involving interactions with other ADs. those involving interactions with other AUs. The architectural
impact of needing to have boundaries between AU's is discussed in
Basic components of AD distinction include: [Tussle]
Transit: These are Mail Service Providers (MSP) offering Basic components of AU distinction include:
value-added capabilities for Edge ADs, such as aggregation and
filtering.
Edge: Independent transfer services, in networks at the edge of the Edge: Independent transfer services, in networks at the edge of the
Internet mail service. Internet Mail service.
User: End-user services. This might be subsumed under the Edge User: End-user services. This might be subsumed under the Edge
service, such as is common for web-based email access. 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 Note that Transit services are quite different from packet-level
transit operation. Whereas end-to-end packet transfers usually go transit operation. Whereas end-to-end packet transfers usually go
through intermediate routers. Email exchange across the open through intermediate routers, email exchange across the open Internet
Internet is often directly between the Edge ADs, at the email level. is often directly between the Edge AUs, at the email level.
+------ +------+ +------+ +------ +------+ +------+
| AD-1 | | AD-3 | | AD-4 | | AU-1 | | AU-3 | | AU-4 |
| ---- | | ---- | | ---- | | ---- | | ---- | | ---- |
| | +---------------------->| | | | | | +---------------------->| | | |
| User | | |-Edge-+---->|-User | | User | | |-Edge-+---->|-User |
| | | | +--->| | | | | | | | +--->| | | |
| V | | | +------+ +------+ | V | | | +------+ +------+
| Edge-+----+ | | Edge-+----+ |
| | | +---------+ | | | | +---------+ |
+------+ | | AD-2 | | +------+ | | AU-2 | |
| | ------- | | | | ------- | |
| | | | | | | |
+--->|-Transit-+---+ +--->|-Transit-+---+
| | | |
+---------+ +---------+
Figure 4: Administrative Domains (AD) Figure 4: Administrative Units (AU) Example
Edge networks may use proprietary email standards internally. Edge networks may use proprietary email standards internally.
However the distinction between Transit network and Edge network However the distinction between Transit network and Edge network
transfer services is primarily significant because it highlights the transfer services is primarily significant because it highlights the
need for concern over interaction and protection between independent need for concern over interaction and protection between independent
administrations. In particular, this distinctions calls for administrations. In particular, this distinctions calls for
additional care in assessing transitions of responsibility, as well additional care in assessing transitions of responsibility, as well
as the accountability and authorization relationships among as the accountability and authorization relationships among
participants in email transfer. participants in email transfer.
The interactions between functional components within an The interactions between functional components within an
Administrative Domain are subject to the policies of that domain. Administrative Unit are subject to the policies of that domain.
Policies can cover such things as reliability, access control, Policies can cover such things as reliability, access control,
accountability and even content evaluation and modification. They accountability and even content evaluation and modification. They
may be implemented in different functional components, according to may be implemented in different functional components, according to
the needs of the Administrative Domain. For example, see the needs of the Administrative Unit. For example, see [ID-spamops].
[ID-spamops].
User, Edge and Transit services can be offered by providers that User, Edge and Transit services can be offered by providers that
operate component services or sets of services. Further, it is operate component services or sets of services. Further, it is
possible for one AD to host services for other ADs. Common AD possible for one AU to host services for other AUs. Common AU
examples are: examples are:
Enterprise Service Providers: Enterprise Service Providers:
Operating an organization's internal data and/or mail operations. Operating an organization's internal data and/or mail services.
Internet Service Providers: Internet Service Providers:
Operating underlying data communication services that, in turn, Operating underlying data communication services that, in turn,
are used by one or more Relays and Users. It is not their job to 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 perform email functions, but to provide an environment in which
those functions can be performed. those functions can be performed.
Mail Service Providers: Mail Service Providers:
Operate email services, such as for end-users, or mailing lists. Operating email services, such as for end-users, or mailing lists.
Operational pragmatics often dictate that providers be involved in Operational pragmatics often dictate that providers be involved in
detailed administration and enforcement issues, to help ensure the detailed administration and enforcement issues, to help ensure the
health of the overall Internet Mail Service. This can include health of the overall Internet Mail Service. This can include
operators of lower-level packet services. operators of lower-level packet services.
3. Identities 3. Identities
Internet mail uses three forms of identity. The most common is the Internet Mail uses three forms of identity. The most common is the
mailbox address <addr-spec> [RFC2822]. The other two are the domain mailbox address <addr-spec> [RFC2822] Also see <address> and
name <domain> [RFC1034] and message identifier <msg-id> [RFC2822]. <mailbox> in [RFC2821]. The other two are the domain name <domain>
Section 3.2 and message identifier <msg-id> [RFC2822].
3.1 Mailbox Addresses 3.1 Mailbox Addresses
"A mailbox sends and receives mail. It is a conceptual entity "A mailbox sends and receives mail. It is a conceptual entity
which does not necessarily pertain to file storage." [RFC2822] which does not necessarily pertain to file storage." [RFC2822]
A mailbox is specified as an Internet mail address <addr-spec>. It A mailbox is specified as an Internet Mail address <addr-spec>. It
has two distinct parts, divided by an at-sign ("@"). The right-hand has two distinct parts, divided by an at-sign ("@"). The right-hand
side contains a globally interpreted name for an administrative side contains a globally interpreted name for an Administrative Unit.
domain. This domain name might refer to an entire organization, or Domain Names are discussed in Section 3.2.
to a collection of machines integrated into a homogeneous service, or
to a single machine. Domain names are defined and operated through
the Domain Name Service (DNS) [RFC1034], [RFC1035], [RFC2181].
The portion to the left of the at-sign contains a string that is 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 globally opaque and is called the <local-part>. It is to be
interpreted only by the entity specified in the address's right-hand interpreted only by the entity specified in the address's right-hand
side. All other entities must treat the local-part as a side. All other entities must treat the local-part as a
uninterpreted, literal string and must preserve all of its original uninterpreted, literal string and must preserve all of its original
details. As such, its public distribution is equivalent to sending a details. As such, its public distribution is equivalent to sending a
"cookie" that is only interpreted upon being returned to its "cookie" that is only interpreted upon being returned to its
originator. originator.
3.1.1 Global Standards for Local-Part 3.1.1 Global Standards for Local-Part
It is common for sites to have local structuring conventions for the It is common for sites to have local structuring conventions for the
left-hand side (local-part) of an addr-spec. This permits left-hand side (local-part) of an addr-spec. This permits sub-
sub-addressing, such as for distinguishing different discussion addressing, such as for distinguishing different discussion groups by
groups by the same participant. However it must be stressed that the same participant. However it must be stressed that these
these conventions are strictly private to the user's organization and conventions are strictly private to the user's organization and must
must not be interpreted by any domain except the one listed in the not be interpreted by any domain except the one listed in the right-
right-hand side of the addr-spec. hand side of the addr-spec.
A small class of addresses has an elaboration on basic email A small class of addresses has an elaboration on basic email
addressing, with a standardized, global schema for the local-part. addressing, with a standardized, global schema for the local-part.
These are conventions between originating end-systems and Recipient These are conventions between originating end-systems and Recipient
Gateways, and they are invisible to the public email transfer Gateways, and they are invisible to the public email transfer
infrastructure. When an Originator is explicitly sending via a infrastructure. When an Originator is explicitly sending via a
Gateway out of the Internet, there are coding conventions for the Gateway out of the Internet, there are coding conventions for the
local-part, so that the Originator can formulate instructions for the local-part, so that the Originator can formulate instructions for the
Gateway. Standardized examples of this are the telephone numbering Gateway. Standardized examples of this are the telephone numbering
formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com", formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com",
and iFax [RFC2304], such as "FAX=+12027653000/ and iFax [RFC2304], such as
T33S=1387@ifax.example.com". "FAX=+12027653000/T33S=1387@ifax.example.com".
3.1.2 Scope of Email Address Use 3.1.2 Scope of Email Address Use
Email addresses are being used far beyond their original email Email addresses are being used far beyond their original email
transfer and delivery role. In practical terms, email strings have transfer and delivery role. In practical terms, email strings have
become a common form of user identity on the Internet. What is 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 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 identity string in a particular context and to be clear about the
entity responsible for setting that string. entity responsible for setting that string.
3.2 Domain Names 3.2 Domain Names
A domain name is a global reference to an Internet resource, such as 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 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 IP Addresses. Conceptually, the name might encompass an entire
individual users, but this is not common practice. The name is organization, or a collection of machines integrated into a
structure as a hierarchical sequence of sub-names, separated by dots 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 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 Mail to refer to a node that took action upon the message, such as
providing the administrative scope for a message identifier, or providing the administrative scope for a message identifier, or
performing transfer processing. performing transfer processing.
3.3 Message Identifiers 3.3 Message Identifiers
Like mailbox addresses, message identifiers have two distinct parts, Like mailbox addresses, message identifiers have two distinct parts,
divided by an at-sign ("@"). The right-hand side is globally divided by an at-sign ("@"). The right-hand side is globally
interpreted and specifies the administrative domain assigning the interpreted and specifies the Administrative Unit assigning the
identifier. The left-hand side of the at-sign contains a string that identifier. The left-hand side of the at-sign contains a string that
is globally opaque and serves to uniquely identify the message within is globally opaque and serves to uniquely identify the message within
the domain referenced on the right-hand side. The duration of the domain referenced on the right-hand side. The duration of
uniqueness for the message identifier is undefined. uniqueness for the message identifier is undefined.
The identifier may be assigned by the user or by any component of the The identifier may be assigned by the user or by any component of the
system along the path, within the AD responsible for the indicated system along the path, within the AU responsible for the indicated
domain. Although Internet mail standards provide for a single domain. Although Internet Mail standards provide for a single
identifier, more than one is sometimes assigned. identifier, more than one is sometimes assigned.
3.4 Identity Referencing Convention 3.4 Identity Referencing Convention
In this document, fields references to identities are labeled in a In this document, fields references to identities are labeled in a
two-part, dotted notation. The first part cites the document two-part, dotted notation. The first part cites the document
defining the identity and the second defines the name of the defining the identity and the second defines the name of the
identity. Hence, <RFC2822.From> is the From field in an email identity. Hence, <RFC2822.From> is the From field in an email
content header, and <RFC2821.MailFrom> is the address in the SMTP content header, and <RFC2821.MailFrom> is the address in the SMTP
"Mail From" command. "Mail From" command.
skipping to change at page 15, line 47 skipping to change at page 16, line 6
o Message Transfer Agent (MTA) o Message Transfer Agent (MTA)
o Message Delivery Agent (MDA) o Message Delivery Agent (MDA)
o Message Store (MS) o Message Store (MS)
This section describes the specific functional components for This section describes the specific functional components for
Internet Mail, and the standard protocols associated with performing Internet Mail, and the standard protocols associated with performing
them. 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 This figure shows function modules and the protocols used between
them. them. Additional protocols and configurations are possible.
+------+ +------+ +---------+
...............+ oMUA |<------------------------------+ ...............+ oMUA |...| Disp |<----------------+
. +--+---+ | . +--+---+ +---------+ |
. | {smtp, submission | . | {smtp, |
. V | . V {submission |
. +------+ | . +------+ +---------+ |
. | MSA |<--------------------+ | . | MSA |...| Bounces |< -----+ |
. +--+---+ | | . +--+---+ +---------+ | |
. | {smtp | | . | | |
. V | | . V {smtp | |
. +------+ /+===+===+\ | . +------+ /+===+===+\ |
. | MTA | || dsn || | . | MTA | || dsn || |
/+==========+\ +--+---+ \+=======+/ | /+==========+\ +--+---+ \+=======+/ |
|| MESSAGE || . {smtp ^ ^ | || MESSAGE || . ^ ^ |
||----------|| . | | | ||----------|| . {smtp | | |
|| Envelope || . | | | || Envelope || . | | |
|| SMTP || V | | | || SMTP || V | | |
|| RFC2822 || +------+ | | /+==+==+\ || RFC2822 || +------+ | | /+==+==+\
|| Content || | MTA +-------------------+ | || mdn || || Content || | MTA +-------------------+ | || mdn ||
|| RFC2822 || +--+---+ | \+=====+/ || RFC2822 || +--+---+ | \+=====+/
|| MIME || | {local, smtp, lmtp | | || MIME || | {local, smtp, | |
\+==========+/ V | | \+==========+/ V {lmtp | |
. +------+ | | . +------+ | |
. | +-----------------------+ | . | +-----------------------+ |
. | MDA | | . | MDA | |
. | |<--------------------+ | . | |<--------------------+ |
. +-+--+-+ | | . +-+--+-+ | |
. local} | | | | . local} | | | |
. V | | | . V | | |
. +------+ | /+===+===+\ | . +------+ | /+===+===+\ |
. | MS-1 | | || sieve || | . | sMS | | || sieve || |
. +-+--+-+ | \+=======+/ | . +-+--+-+ | \+=======+/ |
. | | | {pop, imap ^ | . | | | {pop, imap ^ |
. | V V | | . | V V | |
. | +------+ | | . | +------+ | |
. | | MS-2 | | | . | | uMS | | |
. | +--+---+ | | . | +--+---+ | |
. | | {pop, imap, local | | . | | {pop, imap, | |
. V V | | . V V {local | |
. +------+ | | . +------+ | |
...........>| rMUA +------------------------+---------+ . | +---- -------------------+ |
...........>| rMUA | |
| +----------------------------------+
+------+ +------+
Figure 5: Protocols and Services 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 4.1 Message
The purpose of the Mail Handling Service is to exchange a message The purpose of the Mail Handling Service is to exchange a message
object among participants. Hence, all of the underlying mechanisms object among participants. Hence, all of the underlying mechanisms
are merely in the service of getting that message from its Originator are merely in the service of getting that message from its Originator
to its Recipients. A message may be explicitly labeled as to its to its Recipients. A message may be explicitly labeled as to its
nature. [RFC3458] nature. [RFC3458]
A message comprises a transit handling envelope and the end-user A message comprises a transit handling envelope and the end-user
message content. The envelope contains handling information used by message content. The envelope contains handling information used by
the Message Handling Service, or generated by it. The content is the Message Handling Service, or generated by it. The content is
divided into a structured header and the body. The body may be divided into a structured header and the body. The body may be
unstructured, simple text, or it may be a tree of multi-media unstructured, simple text, or it may be a tree of multi-media
subordinate objects. subordinate objects, called body-parts.
Internet mail has distinguished some special versions of messages, Internet Mail has distinguished some special versions of messages,
for exchanging control information: for exchanging control information:
Delivery Status Notification (DSN): Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) may be generated by the Mail A Delivery Status Notification (DSN) may be generated by the Mail
Handling Service (MSA, MTA or MDA) and sent to the Handling Service (MSA, MTA or MDA) and sent to the
RFC2821.MailFrom address. It provides information about message RFC2821.MailFrom address. It provides information about message
transit, such as transmission errors or successful delivery. transit, such as transmission errors or successful delivery.
[RFC3461] [RFC3461]
Message Disposition Notification (MDN): Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) may be generated by an A Message Disposition Notification (MDN) may be generated by an
rMUA and is sent to the Disposition-Notification-To address. It rMUA and is sent to the Disposition-Notification-To address(es).
provides information about Recipient-side message processing, such It provides information about user-level, Recipient-side message
as indicating that the message has been read [RFC2298] or the form processing, such as indicating that the message has been read
of content that can be supported. [RFC3297] [RFC2298] or the form of content that can be supported. [RFC3297]
Message Filtering (SIEVE): Message Filtering (SIEVE):
SIEVE provides a means of specifying conditions for differential SIEVE provides a means of specifying conditions for differential
handling of mail, at the time of delivery. [RFC3028] 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 4.1.1 Envelope
Information that is directly used by, or produced by, the email Information that is directly used by, or produced by, the MHS is
transfer service is called the "envelope". It controls and records called the "envelope". It controls and records handling activities
handling activities by the transfer service. Internet mail has a by the transfer service. Internet Mail has a fragmented framework
fragmented framework for handling this "handling" information. The for handling this "handling" information. The envelope exists partly
envelope exists partly in the transfer protocol SMTP [RFC2821] and in the transfer protocol SMTP [RFC2821] and partly in the message
partly in the message object [RFC2822]. The SMTP specification uses object [RFC2822]. The SMTP specification uses the term to refer only
the term to refer only to the transfer-protocol information. to the transfer-protocol information.
NOTE: NOTE:
Due to the frequent use of the term "envelope" to refer only to Due to the frequent use of the term "envelope" to refer only to
SMTP constructs, there has been some call for using a different SMTP constructs, there has been some call for using a different
term, to label the larger set of information defined here. So term, to label the larger set of information defined here. So
far, no alternative term has developed any community support. far, no alternative term has developed any community support.
Direct envelope addressing information, as well as optional transfer Direct envelope addressing information, as well as optional transfer
directives, are carried within the SMTP control channel. Other directives, are carried within the SMTP control channel. Other
envelope information, such as trace records, is carried within the envelope information, such as trace records, is carried within the
content header fields. Upon delivery, SMTP-level envelope content header fields. Upon delivery, some SMTP-level envelope
information is typically encoded within additional content header information is typically encoded within additional content header
fields, such as Return-Path. fields, such as Return-Path.
4.1.2 Message Header Fields 4.1.2 Message Header Fields
Header fields are attribute/value pairs covering an extensible range Header fields are attribute/value pairs covering an extensible range
of email service, user content and user transaction meta-information. of email service, user content and user transaction meta-information.
The core set of header fields is defined in [RFC2822], [RFC0822]. It The core set of header fields is defined in [RFC2822], [RFC0822]. It
is common to extend this set, for different applications. A complete is common to extend this set, for different applications. Procedures
set of registered header fields is being developed through for registering headers are provided in [RFC4021]. A complete set of
[ID-hdr-reg]. registered header fields is being developed through [ID-hdr-reg].
One danger with placing additional information in header fields is One danger with placing additional information in header fields is
that Gateways often alter or delete them. that Gateways often alter or delete them.
4.1.3 Body 4.1.3 Body
The body of a message might simply be lines of ASCII text or it might 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 be structured into a composition of multi-media, body-part
attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048], attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],
and [RFC2049]. It should be noted that MIME structures each and [RFC2049]. MIME structures each body-part into a recursive set
body-part into a recursive set of MIME header field meta-data and of MIME header field meta-data and MIME Content sections.
MIME Content sections.
4.1.4 Identity References in a Message 4.1.4 Identity References in a Message
For a message in transit, the core uses of identity references For a message in transit, the core uses of identity references
combine into: combine into:
+-----------------------+-------------+---------------------+ +-----------------------+-------------+---------------------+
| Layer | Field | Set By | | Layer | Field | Set By |
+-----------------------+-------------+---------------------+ +-----------------------+-------------+---------------------+
| Message Body | MIME Header | Originator | | Message Body | MIME Header | Originator |
skipping to change at page 19, line 47 skipping to change at page 20, line 40
infrastructure, via a Mail Submission Agent (MSA). It may also infrastructure, via a Mail Submission Agent (MSA). It may also
perform any creation- and posting-time archival. An MUA outbox is perform any creation- and posting-time archival. An MUA outbox is
part of the origination-side MUA. part of the origination-side MUA.
The Recipient-side rMUA works on behalf of the end-user Recipient to The Recipient-side rMUA works on behalf of the end-user Recipient to
process received mail. This includes generating user-level return process received mail. This includes generating user-level return
control messages, display and disposition of the received message, control messages, display and disposition of the received message,
and closing or expanding the user communication loop, by initiating and closing or expanding the user communication loop, by initiating
replies and forwarding new messages. replies and forwarding new messages.
An MUA may, itself, have a distributed architecture, such as An MUA may, itself, have a distributed implementation, such as a
implementing a "thin" user interface module on a limited end-user "thin" user interface module on a limited end-user device, with the
device, with the bulk of the MUA functionality operated remotely on a bulk of the MUA functionality operated remotely on a more capable
more capable server. An example of such an architecture might use server. An example of such an architecture might use IMAP [RFC3501]
IMAP [RFC3501] for most of the interactions between an MUA client and for most of the interactions between an MUA client and an MUA server.
an MUA server.
A Mediator is special class of MUA performs message re-posting, as A Mediator is special class of MUA. It performs message re-posting,
discussed in Section 2.1. as discussed in Section 2.1.
Identity fields relevant to the MUA include: Identity fields relevant to the MUA include:
RFC2822.From RFC2822.From
Set by: Originator Set by: Originator
Names and addresses for author(s) of the message content are Names and addresses for author(s) of the message content are
listed in the From field listed in the From field
skipping to change at page 20, line 39 skipping to change at page 21, line 32
RFC2822.Sender RFC2822.Sender
Set by: Source Set by: Source
This specifies the address responsible for submitting the message This specifies the address responsible for submitting the message
into the transfer service. For efficiency, this field should be into the transfer service. For efficiency, this field should be
omitted if it contains the same address as RFC2822.From. However omitted if it contains the same address as RFC2822.From. However
this does not mean there is no Sender specified. Rather, it means 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 that that header field is virtual and that the address in the From
field must be used. Specification of the error return addresses field must be used. Specification of the error return addresses
-- the "Notifications" (or "bounces") address, contained in -- the "Bounce" address, contained in RFC2821.MailFrom -- is made
RFC2821.MailFrom -- is made by the RFC2822.Sender. Typically the by the RFC2822.Sender. Typically the Bounce address is the same
Notifications address is the same as the Sender address. However as the Sender address. However some usage scenarios require it to
some usage scenarios require it to be different. be different.
RFC2822.To, RFC2822.CC RFC2822.To, RFC2822.CC
Set by: Originator Set by: Originator
These specify MUA Recipient addresses. The addresses in the These specify MUA Recipient addresses. The addresses in the
fields might not be present in the RFC2821.RcptTo command. The fields might not be present in the RFC2821.RcptTo command. The
distinction between To and CC is subjective. Generally, a To distinction between To and CC is subjective. Generally, a To
addressee is considered primary and is expected to take action on addressee is considered primary and is expected to take action on
the message. A CC addressee typically receives a copy only for the message. A CC addressee typically receives a copy only for
skipping to change at page 21, line 8 skipping to change at page 22, line 4
Set by: Originator Set by: Originator
These specify MUA Recipient addresses. The addresses in the These specify MUA Recipient addresses. The addresses in the
fields might not be present in the RFC2821.RcptTo command. The fields might not be present in the RFC2821.RcptTo command. The
distinction between To and CC is subjective. Generally, a To distinction between To and CC is subjective. Generally, a To
addressee is considered primary and is expected to take action on addressee is considered primary and is expected to take action on
the message. A CC addressee typically receives a copy only for the message. A CC addressee typically receives a copy only for
their information. their information.
RFC2822.BCC RFC2822.BCC
Set by: Originator Set by: Originator
A message might be copied to an addressee whose participation is A message might be copied to an addressee whose participation is
not to be disclosed to the RFC2822.To or RFC2822.CC Recipients not to be disclosed to the RFC2822.To or RFC2822.CC Recipients
and, usually, not to the other BCC Recipients. The BCC header and, usually, not to the other BCC Recipients. The BCC header
field indicates a message copy to such a Recipient. Typically, field indicates a message copy to such a Recipient. Typically,
the field lists no addresses or only lists the address of the the field lists no addresses or only lists the address of the
Recipient receiving this copy. An MUA will typically make Recipient receiving this copy. An MUA will typically make
separate postings for TO and CC Recipients, versus BCC Recipients. separate postings for TO and CC Recipients, versus BCC Recipients.
The former will see no indication that any BCCs were sent, whereas 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 the latter have a BCC field present. It might be empty, contain a
comment, or contain one or more BCC addresses, depending upon the comment, or contain one or more BCC addresses, depending upon the
preferences or the Originator. preferences or the Originator.
4.3 Mail Submission Agent (MSA) 4.3 Mail Submission Agent (MSA)
A Mail Submission Agent (MSA) accepts the message submission from the A Mail Submission Agent (MSA) accepts the message submission from the
oMUA and enforces the policies of the hosting AD and the requirements oMUA and enforces the policies of the hosting AU and the requirements
of Internet standards. Enforcement might be passive, involving of Internet standards. Enforcement might be passive, involving
review and approval or rejection, or it might be active, involving review and approval or rejection, or it might be active, involving
direct modification of the message. An MSA implements a server direct modification of the message. An MSA implements a server
function to MUAs and a client function to MTAs (or MDAs). function to MUAs and a client function to MTAs (or MDAs).
Examples of MSA-styled functions, in the world of paper mail, might Examples of MSA-styled functions, in the world of paper mail, might
range across the very different capabilities of administrative range across the very different capabilities of administrative
assistants, postal drop boxes, and post office front-counter assistants, postal drop boxes, and post office front-counter
employees. employees.
The MUA/MSA interface can be implemented within a single host and use The MUA/MSA interface can be implemented within a single host and use
private conventions for its interactions. Historically, private conventions for its interactions. Historically, standards-
standards-based MUA/MSA interactions have used SMTP [RFC2821]. based MUA/MSA interactions have used SMTP [RFC2821]. However a
However a recent alternative is SUBMISSION [RFC2476]. Although recent alternative is SUBMISSION [RFC2476]. Although SUBMISSION
SUBMISSION derives from SMTP, it operates on a separate TCP port, and derives from SMTP, it operates on a separate TCP port, and will
will typically impose distinct requirements, such as access typically impose distinct requirements, such as access authorization.
authorization.
Identities relevant to the MSA include: Identities relevant to the MSA include:
RFC2821.HELO or RFC2821.EHLO RFC2821.HELO or RFC2821.EHLO
Set by: Source Set by: Source
The MSA may specify its hosting domain identity for the SMTP HELO The MSA may specify its hosting domain identity for the SMTP HELO
or EHLO command operation. or EHLO command operation.
RFC2821.MailFrom RFC2821.MailFrom
Set by: Source Set by: Source
This is an end-to-end string that specifies an email address for This is an end-to-end string that specifies an email address for
receiving return control information, such as "bounces". The name receiving return control information, such as "bounces". The name
of this field is misleading, because it is not required to specify of this field is misleading, because it is not required to specify
either the author or the agent responsible for submitting the either the author or the agent responsible for submitting the
message. Rather, the agent responsible for submission specifies message. Rather, the agent responsible for submission specifies
the RFC2821.MailFrom address. Ultimately the simple basis for the RFC2821.MailFrom address. Ultimately the simple basis for
deciding what address needs to be in the RFC2821.MailFrom is to deciding what address needs to be in the RFC2821.MailFrom is to
determine what address needs to be informed about determine what address needs to be informed about transmission-
transmission-level problems (and, possibly, successes.) level problems (and, possibly, successes.)
RFC2821.RcptTo RFC2821.RcptTo
Set by: Originator Set by: Originator
This specifies the MUA mailbox address of a recipient. The string This specifies the MUA mailbox address of a recipient. The string
might not be visible in the message content header. For example, might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To, the message destination address header fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.RcptTo might specify a mailing list address, while the RFC2821.RcptTo
address specifies a member of that list. address specifies a member of that list.
skipping to change at page 22, line 43 skipping to change at page 23, line 39
RFC2821.Received RFC2821.Received
Set by: Source Set by: Source
An MSA may record a Received header field, to indicate initial An MSA may record a Received header field, to indicate initial
submission trace information, including originating host and MSA submission trace information, including originating host and MSA
host domain names and/or IP Addresses. host domain names and/or IP Addresses.
4.4 Mail Transfer Agent (MTA) 4.4 Mail Transfer Agent (MTA)
A Mail Transfer Agent (MTA) relays mail. It is like a packet-switch A Mail Transfer Agent (MTA) relays mail for one, application-level
or IP router in that its job is to make routing assessments and to "hop". It is like a packet-switch or IP router in that its job is to
move the message closer to the Recipient(s). Relaying is performed make routing assessments and to move the message closer to the
by a sequence of MTAs, until the message reaches its destination MDA. Recipient(s). Relaying is performed by a sequence of MTAs, until the
Hence an MTA implements both client and server MTA functionality. It message reaches its destination MDA(s). Hence an MTA implements both
does not make changes to addresses in the envelope or reformulate the client and server MTA functionality. It does not make changes to
content, except as transfer-encoding requirements dictate. Also it addresses in the envelope or reformulate the content, except as
may add trace information. transfer-encoding requirements dictate. Also it may add trace
information. Of course email objects are typically much larger than
The primary "routing" mechanism for Internet mail is the DNS MX the payload of a packet or datagram, and the end-to-end latencies are
record [RFC1035]. As with most network layer mechanisms Internet typically much higher.
mail's SMTP supports a basic level of reliability, by virtue of
providing for retransmission after a temporary transfer failure.
However the degree of persistence by an MTA can be highly variable.
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 service interruptions, such as host
system shutdown.
Internet mail primarily uses SMTP [RFC2821], [RFC0821] to effect Internet Mail primarily uses SMTP [RFC2821], [RFC0821] to effect
point-to-point transfers between peer MTAs. Other transfer point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]. 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 An important characteristic of MTA-MTA communications, over the open
Internet, is that they do not require prior arrangement between the Internet, is that they do not require prior arrangement between the
independent administrations operating the different MTAs. Given the independent administrations operating the different MTAs. Given the
importance of spontaneity and serendipity in the world of human importance of spontaneity and serendipity in the world of human
communications, this lack of prearrangement, between the communications, this lack of prearrangement, between the
participants, is a core benefit of Internet mail and remains a core participants, is a core benefit of Internet Mail and remains a core
requirement for it. requirement for it.
Identities relevant to the MTA include: Identities relevant to the MTA include:
RFC2821.HELO RFC2821.HELO
Set by: Relay Set by: Relay
The MTA may specify its hosting domain identity for the SMTP HELO The MTA may specify its hosting domain identity for the SMTP HELO
or EHLO command. This is the only standardized way of identifying or EHLO command. This is the only standardized way of identifying
the agent responsible for operation of the Relay, during the the agent responsible for operation of the Relay, during the
transfer operation. transfer operation.
RFC2821.MailFrom RFC2821.MailFrom
Set by: Source Set by: Source
This is an end-to-end string that specifies an email address for This is an MHS end-to-end string that specifies an email address
receiving return control Notifications, such as "bounces". The for receiving return control Bounce, such as delivery
name of this field is misleading, because it is not required to confirmations and error notices. The protocol name of this field
specify either the author or the agent responsible for submitting is misleading, because it is not required to specify either the
the message. Rather, the agent responsible for submission author or the agent responsible for submitting the message.
specifies the MailFrom address. Ultimately the simple basis for Rather, the agent responsible for submission specifies the
deciding what address needs to be in the RFC2821.MailFrom is to MailFrom address. Ultimately the simple basis for deciding what
determine what address needs to be informed about address needs to be in the RFC2821.MailFrom is to determine what
transmission-level problems (and, possibly, successes.) address needs to be informed about transmission-level problems
(and, possibly, successes.)
RFC2821.RcptTo RFC2821.RcptTo
Set by: Originator Set by: Originator
This specifies the MUA mailbox address of a Recipient. The string This specifies the MUA mailbox address of a Recipient. The string
might not be visible in the message content header. For example, might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To, the message destination address header fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.RcptTo might specify a mailing list address, while the RFC2821.RcptTo
address specifies a member of that list. address specifies a member of that list.
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An MTA must record a Received header field, to indicate trace An MTA must record a Received header field, to indicate trace
information, including source host and receiving host domain names information, including source host and receiving host domain names
and/or IP Addresses. and/or IP Addresses.
4.5 Mail Delivery Agent (MDA) 4.5 Mail Delivery Agent (MDA)
A Mail Delivery Agent (MDA) delivers email to the Recipient's A Mail Delivery Agent (MDA) delivers email to the Recipient's
mailbox. It can provide distinctive, address-based functionality, mailbox. It can provide distinctive, address-based functionality,
made possible by its detailed knowledge of the properties of the made possible by its detailed knowledge of the properties of the
destination address. This knowledge might also be present elsewhere destination address. This knowledge might also be present elsewhere
in the Recipient's Administrative Domain, such as at an in the Recipient's Administrative Unit, such as at an organizational
organizational border Relay. However it is required for the MDA, if border Relay. However it is required for the MDA, if only because
only because the MDA must know where to deliver the message. the MDA must know where to deliver the message.
Using Internet protocols, delivery can be effected by a variety of Using Internet protocols, delivery can be effected by a variety of
standard protocols. When coupled with an internal, local mechanism, standard protocols. When coupled with an internal, local mechanism,
SMTP [RFC2821] and LMTP [RFC2033] permit "push" delivery to the SMTP [RFC2821] and LMTP [RFC2033] permit "push" delivery to the
Recipient system, at the initiative of the upstream email service. Recipient system, at the initiative of the upstream email service.
POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the
initiative of the Recipient system. POP and IMAP can also be used initiative of the Recipient system. POP and IMAP can also be used
for repeated access to messages on a remote MS. for repeated access to messages on a remote MS.
Identities relevant to the MDA include: Identities relevant to the MDA include:
skipping to change at page 24, line 47 skipping to change at page 26, line 4
Recipient system, at the initiative of the upstream email service. Recipient system, at the initiative of the upstream email service.
POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the POP [RFC1939] and IMAP [RFC3501] are used for "pull" delivery at the
initiative of the Recipient system. POP and IMAP can also be used initiative of the Recipient system. POP and IMAP can also be used
for repeated access to messages on a remote MS. for repeated access to messages on a remote MS.
Identities relevant to the MDA include: Identities relevant to the MDA include:
RFC2821.Return-Path RFC2821.Return-Path
Set by: Source Set by: Source
The MDA records the RFC2821.MailFrom address into the The MDA records the RFC2821.MailFrom address into the
RFC2822.Return-Path field. RFC2822.Return-Path field.
RFC2822.Received RFC2822.Received
Set by: Destination Set by: Destination
An MDA must record a Received header field, to indicate trace An MDA must record a Received header field, to indicate trace
information, including source host and receiving host domain names information, including source host and receiving host domain names
and/or IP Addresses. and/or IP Addresses.
4.6 Message Store (MS) 4.6 Message Store (MS)
An MUA can use a long-term Message Store (MS). A rich set of choices 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 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 associated with a single user, demonstrated as a server-based MS
Figure 5. MS-1 is shown as being remote from the MUA and MS-2 as (sMS) and user-based MS (uMS) in Figure 5. sMS is shown as being
being local. Further the relationship between two message store may remote from the MUA and uMS as being local. Further the relationship
vary. Between the MDA and the MUA, these choices are supported by a between two message store may vary. Between the MDA and the MUA,
wide variety of protocol options. these choices are supported by a wide variety of protocol options.
The operational relationship among two MSs can be: The operational relationship among two MSs can be:
Online: Online:
Only a remote MS is used, with messages being accessible only when 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 the MUA is attached to the MS, and the MUA repeatedly fetches all
or part of a message, from one session to the next. or part of a message, from one session to the next.
Offline: Offline:
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Disconnected: Disconnected:
A remote MS and a local MS synchronize all or parts of their A remote MS and a local MS synchronize all or parts of their
contents, while connected. The user may make changes while contents, while connected. The user may make changes while
disconnected, and the two stores are re-synchronized upon disconnected, and the two stores are re-synchronized upon
reconnection. reconnection.
5. Mediators 5. Mediators
Basic email transfer is accomplished with an asynchronous Basic email transfer is accomplished with an asynchronous store-and-
store-and-forward communication infrastructure, in a sequence of forward communication infrastructure, in a sequence of independent
independent transmissions through some number of MTAs. A very transmissions through some number of MTAs. A very different task is
different task is a User-level sequence of postings and deliveries, a User-level sequence of postings and deliveries, through Mediators.
through Mediators. For such re-postings, a Mediator does share some For such re-postings, a Mediator does share some functionality with
functionality with basic MTA relaying, but it enjoys a degree of basic MTA relaying, but it enjoys a degree of freedom with both
freedom with both addressing and content that is not available to addressing and content that is not available to MTAs.
MTAs.
RFC2821.HELO or RFC2821.EHLO RFC2821.HELO or RFC2821.EHLO
Set by: Source or Relay Set by: Source or Relay
The MSA may specify its hosting domain identity for the SMTP HELO The MSA may specify its hosting domain identity for the SMTP HELO
or EHLO command operation. or EHLO command operation.
RFC2821.MailFrom RFC2821.MailFrom
Set by: Source Set by: Source
This is an end-to-end string that specifies an email address for This is an end-to-end string that specifies an email address for
receiving return control Notifications, such as "bounces". The receiving return control Bounces. The name of this field is
name of this field is misleading, because it is not required to misleading, because it is not required to specify either the
specify either the author or the agent responsible for submitting author or the agent responsible for submitting the message.
the message. Rather, the agent responsible for submission Rather, the agent responsible for submission specifies the
specifies the RFC2821.MailFrom address. Ultimately the simple RFC2821.MailFrom address. Ultimately the simple basis for
basis for deciding what address needs to be in the deciding what address needs to be in the RFC2821.MailFrom is to
RFC2821.MailFrom is to determine what address needs to be informed determine what address needs to be informed about transmission-
about transmission-level problems (and, possibly, successes.) level problems (and, possibly, successes.)
RFC2821.RcptTo RFC2821.RcptTo
Set by: Mediator Set by: Mediator
This specifies the MUA mailbox address of a Recipient. The string This specifies the MUA mailbox address of a Recipient. The string
might not be visible in the message content header. For example, might not be visible in the message content header. For example,
the message destination address header fields, such as RFC2822.To, the message destination address header fields, such as RFC2822.To,
might specify a mailing list address, while the RFC2821.RcptTo might specify a mailing list address, while the RFC2821.RcptTo
address specifies a member of that list. address specifies a member of that list.
skipping to change at page 27, line 5 skipping to change at page 28, line 6
An MSA may record a Received header field, to indicate initial An MSA may record a Received header field, to indicate initial
submission trace information, including originating host and MSA submission trace information, including originating host and MSA
host domain names and/or IP Addresses. host domain names and/or IP Addresses.
The salient aspect of a Mediator, that distinguishes it from any The salient aspect of a Mediator, that distinguishes it from any
other MUA creating an entirely new message, is that a Mediator other MUA creating an entirely new message, is that a Mediator
preserves the integrity and tone of the original message, including preserves the integrity and tone of the original message, including
the essential aspects of the original origination information. The the essential aspects of the original origination information. The
Mediator might also add commentary. Mediator might also add commentary.
Examples of MUA message creation that are not performed by Mediators Examples of MUA message creation that are NOT performed by Mediators
include: include:
New Message Forwarding Existing Message: New message forwarding existing message:
Curiously, this action provides a basic template for a class of This action rather curiously provides a basic template for a class
Mediators. However by itself, it's typical occurrence is not, in of Mediators. However for it's typical occurrence it is not
fact, an example of a Mediator. The new message is viewed as itself an example of a Mediator. The new message is viewed as
being from the Agent doing the forwarding, rather than being from being from the Agent doing the forwarding, rather than being from
the original Originator. the original Originator.
A new message encapsulates the original message and is seen as A new message encapsulates the original message and is seen as
strictly "from" the Mediator. The Mediator might add commentary strictly "from" the Mediator. The Mediator might add commentary
and certainly has the opportunity to modify the original message and certainly has the opportunity to modify the original message
content. The forwarded message is therefore independent of the content. The forwarded message is therefore independent of the
original message exchange and creates a new message dialogue. original message exchange and creates a new message dialogue.
However the final Recipient sees the contained message as from the However the final Recipient sees the contained message as from the
original Originator. original Originator.
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the message is submitted back to the transfer service, for delivery the message is submitted back to the transfer service, for delivery
to one or more alternate addresses. Although implemented as part of to one or more alternate addresses. Although implemented as part of
the message delivery service, this facility is strictly a Recipient the message delivery service, this facility is strictly a Recipient
user function. It resubmits the message, replacing the envelope user function. It resubmits the message, replacing the envelope
address, on behalf of the mailbox address that was listed in the address, on behalf of the mailbox address that was listed in the
envelope. envelope.
What is most distinctive about this forwarding mechanism is how What is most distinctive about this forwarding mechanism is how
closely it compares to normal MTA store-and-forward Relaying. In closely it compares to normal MTA store-and-forward Relaying. In
reality its only interesting difference is that it changes the reality its only interesting difference is that it changes the
RFC2821.RcptTo value. 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 An MDA that is re-posting a message to an alias typically changes
only envelope information: only envelope information:
RFC2822.TO, RFC2822.CC, RFC2822.BCC RFC2822.TO, RFC2822.CC, RFC2822.BCC
Set by: Originator Set by: Originator
These retain their original addresses. These retain their original addresses.
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Set by: Mediator Set by: Mediator
This field contains an alias address. This field contains an alias address.
RFC2821.MailFrom RFC2821.MailFrom
Set by: Mediator or original Source Set by: Mediator or original Source
The agent responsible for submission to an alias address will The agent responsible for submission to an alias address will
often retain the original address to receive handling often retain the original address to receive handling Bounces.
Notifications. The benefit of retaining the original MailFrom The benefit of retaining the original MailFrom value is to ensure
value is to ensure that the origination-side agent knows that that the origination-side agent knows that there has been a
there has been a delivery problem. On the other hand, the delivery problem. On the other hand, the responsibility for the
responsibility for the problem usually lies with the Recipient, problem usually lies with the Recipient, since the Alias mechanism
since the Alias mechanism is strictly under the Recipient's is strictly under the Recipient's control.
control.
RFC2821.Received RFC2821.Received
Set by: Mediator Set by: Mediator
The agent should record Received information, to indicate the The agent should record Received information, to indicate the
delivery to the original address and submission to the alias delivery to the original address and submission to the alias
address. The trace of Received header fields should therefore address. The trace of Received header fields should therefore
include everything from original posting through final delivery to include everything from original posting through final delivery to
the alias. the alias.
skipping to change at page 30, line 4 skipping to change at page 31, line 6
RFC2822.Resent-From RFC2822.Resent-From
Set by: Mediator Set by: Mediator
The address of the original Recipient who is redirecting the The address of the original Recipient who is redirecting the
message. Otherwise, the same rules apply for the Resent-From message. Otherwise, the same rules apply for the Resent-From
field as for an original RFC2822.From field field as for an original RFC2822.From field
RFC2822.Resent-Sender RFC2822.Resent-Sender
Set by: Mediator Set by: Mediator
The address of the agent responsible for re-submitting the The address of the agent responsible for re-submitting the
message. For efficiency this field is often omitted if it message. For efficiency this field is often omitted if it
contains the same address as RFC2822.Resent-From. However this contains the same address as RFC2822.Resent-From. However this
does not mean there is no Resend-Sender specified. Rather, it does not mean there is no Resend-Sender specified. Rather, it
means that that header field is virtual and that the address in means that that header field is virtual and that the address in
the Resent-From field must be used. Specification of the error the Resent-From field must be used. Specification of the error
return addresses (the Notification address, contained in return addresses (the Notification address, contained in
RFC2821.MailFrom) is made by the Resent-Sender. Typically the RFC2821.MailFrom) is made by the Resent-Sender. Typically the
Notifications address is the same as the Resent-Sender address. Bounce address is the same as the Resent-Sender address. However
However some usage scenarios require it to be different. some usage scenarios require it to be different.
RFC2822.Resent-To, RFC2822.Resent-cc, RFC2822.Resent-bcc: RFC2822.Resent-To, RFC2822.Resent-cc, RFC2822.Resent-bcc:
Set by: Mediator Set by: Mediator
The addresses of the new Recipients who will now be able to reply The addresses of the new Recipients who will now be able to reply
to the original author. to the original author.
RFC2821.MailFrom RFC2821.MailFrom
skipping to change at page 31, line 29 skipping to change at page 32, line 31
standards-track specification, it has not gained significant standards-track specification, it has not gained significant
adoption. adoption.
RFC2369.List-* RFC2369.List-*
Set by: Mediator Set by: Mediator
[RFC2369] defines a collection of message header fields for use by [RFC2369] defines a collection of message header fields for use by
mailing lists. In effect, they supply list-specific parameters mailing lists. In effect, they supply list-specific parameters
for common mailing list user operations. The identifiers for for common mailing list user operations. The identifiers for
these operations are for the list, itself, and the these operations are for the list, itself, and the user-as-
user-as-subscriber. subscriber.
RFC2822.From RFC2822.From
Set by: original Originator Set by: original Originator
Names and email addresses for the original author(s) of the Names and email addresses for the original author(s) of the
message content are specified. message content are specified.
RFC2822.Reply-To RFC2822.Reply-To
skipping to change at page 32, line 30 skipping to change at page 33, line 30
addresses. addresses.
RFC2821.MailFrom RFC2821.MailFrom
Set by: original Source or Mediator Set by: original Source or Mediator
This may contain the original address to be notified of This may contain the original address to be notified of
transmission issues, or the mailing list agent may set it to transmission issues, or the mailing list agent may set it to
contain a new Notification address. Typically, the value is set contain a new Notification address. Typically, the value is set
to a new address, so that mailing list members and posters are not to a new address, so that mailing list members and posters are not
burdened with transmission-related Notifications. burdened with transmission-related Bounces.
RFC2821.RcptTo RFC2821.RcptTo
Set by: Mediator Set by: Mediator
This contains the address of a mailing list member. This contains the address of a mailing list member.
RFC2821.Received RFC2821.Received
Set by: Mediator Set by: Mediator
skipping to change at page 33, line 49 skipping to change at page 35, line 4
present. The ability to perform a successful reply by a Gatewayed present. The ability to perform a successful reply by a Gatewayed
Recipient is a typical test of Gateway functionality. Recipient is a typical test of Gateway functionality.
RFC2822.Sender RFC2822.Sender
Set by: original Source or Mediator Set by: original Source or Mediator
This may retain the original value or may be set to a new address This may retain the original value or may be set to a new address
RFC2822.TO, RFC2822.CC, RFC2822.BCC RFC2822.TO, RFC2822.CC, RFC2822.BCC
Set by: original Recipient Set by: original Recipient
These usually retain their original addresses. These usually retain their original addresses.
RFC2821.MailFrom RFC2821.MailFrom
Set by: original Source or Mediator Set by: original Source or Mediator
The agent responsible for gatewaying the message may choose to The agent responsible for gatewaying the message may choose to
specify a new address to receive handling notices. specify a new address to receive handling notices.
RFC2822.Received RFC2822.Received
Set by: Mediator Set by: Mediator
The Gateway may record a Received header field, to indicate the The Gateway may record a Received header field, to indicate the
transition from original posting to the new messaging environment. transition from original posting to the new messaging environment.
5.5 Security Filter 5.5 Boundary Filter
Organizations often enforce security boundaries by having message Organizations often enforce security boundaries by subjecting
subjected to analysis for conformance with the organization's safety messages to analysis, for conformance with the organization's safety
policies. Examples are detection of content classed as spam or a policies. An example is detection of content classed as spam or a
virus. A Security Filter might alter the content, to render it safe, virus. A Filter might alter the content, to render it safe, such as
such as by removing content deemed unacceptable. Typically these by removing content deemed unacceptable. Typically these actions
actions will result in the addition of content that records the will result in the addition of content that records the actions.
actions.
6. Security Considerations 6. Security Considerations
This document does not specify any new Internet mail functionality. This document does not specify any new Internet Mail functionality.
Consequently it should introduce no new security considerations. Consequently it should introduce no new security considerations.
However its discussion of the roles and responsibilities for However its discussion of the roles and responsibilities for
different mail service modules, and the information they create, different mail service modules, and the information they create,
highlights the considerable security considerations that must be highlights the considerable security considerations that must be
present when implementing any component of the Internet mail service. 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. References
7.1 References - Normative 7.1 References - Normative
[ID-hdr-reg] [ID-hdr-reg]
"Registration of mail and MIME header fields", "Registration of mail and MIME header fields",
draft-klyne-hdrreg-mail-04.txt (work in progress), Apr draft-klyne-hdrreg-mail-04.txt (work in progress),
2004. Apr 2004.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC [RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
821, August 1982. RFC 821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet [RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982. text messages", STD 11, RFC 822, August 1982.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
skipping to change at page 35, line 32 skipping to change at page 36, line 36
Bodies", RFC 2045, November 1996. Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996. November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text", Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996. RFC 2047, November 1996.
[RFC2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose [RFC2048] Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996. Procedures", BCP 13, RFC 2048, November 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996. Examples", RFC 2049, November 1996.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997. Specification", RFC 2181, July 1997.
skipping to change at page 36, line 13 skipping to change at page 37, line 17
Message Header Fields", RFC 2369, July 1998. Message Header Fields", RFC 2369, July 1998.
[RFC2421] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet [RFC2421] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
Mail - version 2", RFC 2421, September 1998. Mail - version 2", RFC 2421, September 1998.
[RFC2423] Vaudreuil, G. and G. Parsons, "VPIM Voice Message MIME [RFC2423] Vaudreuil, G. and G. Parsons, "VPIM Voice Message MIME
Sub-type Registration", RFC 2423, September 1998. Sub-type Registration", RFC 2423, September 1998.
[RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998. [RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998.
[RFC2476] Gellens, R. and J. Klensin, "Message Submission", RFC [RFC2476] Gellens, R. and J. Klensin, "Message Submission",
2476, December 1998. RFC 2476, December 1998.
[RFC2645] "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP [RFC2645] "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
Addresses", RFC 2465, August 1999. Addresses", RFC 2465, August 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001. April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April [RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
2001. April 2001.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field [RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists", and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001. RFC 2919, March 2001.
[RFC3028] Showalter, T., "Sieve: A Mail Filtering Language", RFC [RFC3028] Showalter, T., "Sieve: A Mail Filtering Language",
3028, January 2001. RFC 3028, January 2001.
[RFC3297] Klyne, G., Iwazaki, R. and D. Crocker, "Content [RFC3297] Klyne, G., Iwazaki, R., and D. Crocker, "Content
Negotiation for Messaging Services based on Email", RFC Negotiation for Messaging Services based on Email",
3297, July 2002. RFC 3297, July 2002.
[RFC3458] Burger, E., Candell, E., Eliot, C. and G. Klyne, "Message [RFC3458] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003. Context for Internet Mail", RFC 3458, January 2003.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC Extension for Delivery Status Notifications (DSNs)",
3461, January 2003. RFC 3461, January 2003.
[RFC3501] Crispin, M., "Internet Message Access Protocol - Version [RFC3501] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003. 4rev1", RFC 3501, March 2003.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, March 2005.
7.2 Reference - Descriptive 7.2 Reference - Descriptive
[ID-ffpim] [ID-ffpim]
Crocker, D. and G. Klyne, "Full-mode Fax Profile for Crocker, D. and G. Klyne, "Full-mode Fax Profile for
Internet Mail: FFPIM", March 2004. Internet Mail: FFPIM", March 2004.
[ID-spamops] [ID-spamops]
Hutzler, C., Crocker, D., Resnick, P., Sanderson, R. and Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
E. Allman, "Email Submission Between Independent E. Allman, "Email Submission Between Independent
Networks", draft-spamops-00 (work in progress), March Networks", draft-spamops-00 (work in progress),
2004. March 2004.
[RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects", RFC [RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects",
1767, March 1995. RFC 1767, March 1995.
[Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
"Tussle in Cyberspace: Defining Tomorrow‚ÇÖs Internet",
ACM SIGCOMM, 2002.
Author's Address Author's Address
Dave Crocker Dave Crocker
Brandenburg InternetWorking Brandenburg InternetWorking
675 Spruce Drive 675 Spruce Drive
Sunnyvale, CA 94086 Sunnyvale, CA 94086
USA USA
Phone: +1.408.246.8253 Phone: +1.408.246.8253
EMail: dcrocker@bbiw.net Email: dcrocker@bbiw.net
Appendix A. Acknowledgements Appendix A. Acknowledgements
This work derives from a section in draft-hutzler-spamops This work derives from a section in draft-hutzler-spamops [ID-
[ID-spamops]. Discussion of the Source actor role was greatly spamops]. Discussion of the Source actor role was greatly clarified
clarified during discussions in the IETF's Marid working group. during discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
insight on the framework and details of early drafts. insight on the framework and details of the original drafts.
Additional review and suggestions were provided by Nathaniel Later reviews and suggestions were provided by Nathaniel Borenstein,
Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ned Freed,
Ned Freed, Eric Hall, Bruce Lilly, Mark E. Mallett, Chris Newman, Eric Hall, Brad Knowles, Bruce Lilly, Mark E. Mallett, David
Daryl Odnert, Rahmat M. Samik-Ibrahim, Hector Santos, Jochen Topf, MacQuigg, Chris Newman, Daryl Odnert, Rahmat M. Samik-Ibrahim, Hector
Willemien. Santos, Jochen Topf, Willemien Hoogendoorn.
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
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