< draft-crocker-email-arch-07.txt   draft-crocker-email-arch-08.txt >
SMTP D. Crocker SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track May 6, 2007 Intended status: Standards Track May 20, 2007
Expires: November 7, 2007 Expires: November 21, 2007
Internet Mail Architecture Internet Mail Architecture
draft-crocker-email-arch-07 draft-crocker-email-arch-08
Status of this Memo Status of this Memo
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skipping to change at page 1, line 34 skipping to change at page 1, line 34
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
Over its thirty-five year history Internet Mail has undergone Over its thirty-five year history Internet Mail has undergone
significant changes in scale and complexity, as it has become a significant changes in scale and complexity, as it has become a
global infrastructure service. The first standardized architecture global infrastructure service. The first standardized architecture
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architecture that targets description of the existing service, in architecture that targets description of the existing service, in
order to facilitate clearer and more efficient technical, operations order to facilitate clearer and more efficient technical, operations
and policy discussions about email. and policy discussions about email.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Service Overview . . . . . . . . . . . . . . . . . . . . . 5 1.2. Service Overview . . . . . . . . . . . . . . . . . . . . . 5
1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6 1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6
2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 6 2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 7
2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 10 2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 10
2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 13 2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 13
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 16 3.2. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 17
3.3. Message Identifier . . . . . . . . . . . . . . . . . . . . 17 3.3. Message Identifier . . . . . . . . . . . . . . . . . . . . 17
4. Services and Standards . . . . . . . . . . . . . . . . . . . . 18 4. Services and Standards . . . . . . . . . . . . . . . . . . . . 19
4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 21 4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 22
4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 26 4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 27
4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 28 4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 29
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 31 5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.1. Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.1. Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.2. Re-Sending . . . . . . . . . . . . . . . . . . . . . . . . 34 5.2. Re-Sending . . . . . . . . . . . . . . . . . . . . . . . . 34
5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 36 5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 36
5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 37 5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 39 5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 38
6. Security Considerations . . . . . . . . . . . . . . . . . . . 39 6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 39
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 6.1. Security Considerations . . . . . . . . . . . . . . . . . 39
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39 6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 39
8.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 39 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
8.2. Descriptive . . . . . . . . . . . . . . . . . . . . . . . 42 7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 39
7.2. Descriptive . . . . . . . . . . . . . . . . . . . . . . . 41
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 42 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 42
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 43 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 42
Intellectual Property and Copyright Statements . . . . . . . . . . 44 Intellectual Property and Copyright Statements . . . . . . . . . . 43
1. Introduction 1. Introduction
Over its thirty-five year history Internet Mail has undergone Over its thirty-five year history Internet Mail has undergone
significant changes in scale and complexity, as it has become a significant changes in scale and complexity, as it has become a
global infrastructure service. The changes have been evolutionary, global infrastructure service. The changes have been evolutionary,
rather than revolutionary, reflecting a strong desire to preserve its rather than revolutionary, reflecting a strong desire to preserve its
installed base of users and utility. Today, Internet Mail is marked installed base of users and utility. Today, Internet Mail is marked
by many independent operators, many different components for by many independent operators, many different components for
providing service to users and many other components for performing providing service to users and many other components for performing
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to one or more others, creating a virtual MUA-to-MUA exchange to one or more others, creating a virtual MUA-to-MUA exchange
environment. environment.
As shown in Figure 1 this defines two logical "layers" of As shown in Figure 1 this defines two logical "layers" of
interoperability. One is directly between Users. The other is interoperability. One is directly between Users. The other is
between the neighboring components, along the transfer path. In between the neighboring components, along the transfer path. In
addition, there is interoperability between the layers, first when a addition, there is interoperability between the layers, first when a
message is posted from the User to the MHS and later when it is message is posted from the User to the MHS and later when it is
delivered from the MHS to the User. delivered from the MHS to the User.
As it has evolved, the operational service has sub-divided each of
these layers into more specialized modules. Core aspects of the
service, such as mailbox addressing and message format style, have
remained remarkably constant. So the original distinction between
user-level concerns and transfer-level concerns is retained, but with
an elaboration to each level of the architecture. The term "Internet
Mail" is used to refer to the entire collection of user and transfer
components and services.
For Internet Mail the term "end-to-end" usually refers to a single
posting and the set of deliveries directly resulting from its single
transiting of the MHS. A common exception is with group dialogue
that is mediated via a mailing list, so that two postings occur
before intended recipients receive an originator's message, as
discussed in Section 2.1.4. In fact some uses of email consider the
entire email service -- including Originator and Recipient -- as a
subordinate component. For these services "end-to-end" refers to
points outside of the email service. Examples are voicemail over
email [RFC3801], EDI over email [RFC1767] and facsimile over email.
[RFC4142]
+--------+ +--------+
+---------------->| User | +---------------->| User |
| +--------+ | +--------+
| ^ | ^
+--------+ | +--------+ . +--------+ | +--------+ .
| User +--+--------->| User | . | User +--+--------->| User | .
+--------+ | +--------+ . +--------+ | +--------+ .
. | ^ . . | ^ .
. | +--------+ . . . | +--------+ . .
. +-->| User | . . . +-->| User | . .
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| . . . | | . . . |
| +......................>+ . | | +......................>+ . |
| . . | | . . |
| +.............................>+ | | +.............................>+ |
| | | |
| Mail Handling Service (MHS) | | Mail Handling Service (MHS) |
+--------------------------------------+ +--------------------------------------+
Figure 1: Basic Internet Mail Service Model Figure 1: Basic Internet Mail Service Model
As it has evolved, the operational service has sub-divided each of
these layers into more specialized modules. Core aspects of the
service, such as mailbox addressing and message format style, have
remained remarkably constant. So the original distinction between
user-level concerns and transfer-level concerns is retained, but with
an elaboration to each level of the architecture. The term "Internet
Mail" is used to refer to the entire collection of user and transfer
components and services.
For Internet Mail the term "end-to-end" usually refers to a single
posting and the set of deliveries directly resulting from its single
transiting of the MHS. A common exception is with group dialogue
that is mediated via a mailing list, so that two postings occur
before intended recipients receive an originator's message, as
discussed in Section 2.1.3. In fact some uses of email consider the
entire email service -- including Originator and Recipient -- as a
subordinate component. For these services "end-to-end" refers to
points outside of the email service. Examples are voicemail over
email [RFC3801], EDI over email [RFC1767] and facsimile over email.
[RFC4142]
1.2. Service Overview 1.2. 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:
* An email object * An email object
* Global addressing * Global addressing
* An asynchronous sequence of point-to-point transfer mechanisms * An asynchronous sequence of point-to-point transfer mechanisms
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two-part dotted notation. The first part cites the document that two-part dotted notation. The first part cites the document that
contains the specification for the field and the second is the name contains the specification for the field and the second is the name
of the field. Hence <RFC2822.From> is the From field in an email of the field. 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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as specified in RFC 2119 [RFC2119]. document are to be interpreted as specified in RFC 2119 [RFC2119].
Discussion venue: Please direct discussion about this document to Discussion venue: Please direct discussion about this document
the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>. to the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>.
Changes:
Added text to explain utility of having an architecture document.
Added text explaining benefit of the ADMD construct.
Added commentary on List-ID. Changes: Removed "associated with" construct, relying only on
"set by"
Made identity table and discussions more consistent.
Moved Bounce out of MHS in figure. Restricted "envelope" only to refer to SMTP information,
calling RFC2822-level fields "Trace information".
Moved "generic" Identity field case-analysis text into common area Moved "Bounce" to be User Actor.
after Table 1, reserving per-role text for per-role peculiarities.
Extensive word-smithing and cleanup. Extended introduction to acronyms in Services and Standards
section.
2. Responsible Actor Roles 2. Responsible 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 3 basic types: actors serving different roles. These divide into 3 basic types:
* User * User
* Mail Handling Service (MHS) * Mail Handling Service (MHS)
* ADministrative Management Domain (ADMD) * ADministrative Management Domain (ADMD)
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 functionality concerns participant responsibilities, rather than on functionality
of modules. Hence the labels used are different than for classic of modules. Hence the labels used are different than for classic
email architecture diagrams. email architecture diagrams.
2.1. User Actors 2.1. User Actors
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The Recipient is a consumer of delivered content. As described The Recipient is a consumer of delivered content. As described
below, the MHS has a "Dest[ination]" role that correlates with the below, the MHS has a "Dest[ination]" role that correlates with the
user-level Recipient role. user-level Recipient role.
A Recipient can close the user-level communication loop by creating A Recipient can 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
handling of the message. (See Section 4.1.) handling of the message. (See Section 4.1.)
2.1.3. Mediator 2.1.3. Bounce Handler
The Bounce Handler receives and services notifications that are
generated by the MHS, as a result of efforts to transfer or deliver
the message. Notices can be about failures or completions and are
sent to an address that is specified by the Source. This Bounce
handling address (also known as a Return address) might have no
visible characteristics in common with the address of the Originator
or Source.
NOTE: The choice of the label "Bounce" is unfortunate, due to
its negative implication and narrow focus. However it is the
most popular term for the address.
2.1.4. 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 Mediators include group dialogue, such as among Users. Example Mediators include group dialogue, such as
collaboration via mailing lists, and organizational message flow, as collaboration via mailing lists, and organizational message flow, as
occurs with a purchase approval process. Note that it is easy to occurs with a purchase approval process. Note that it is easy to
confuse this user-level activity with the underlying MHS transfer confuse this user-level activity with the underlying MHS transfer
exchanges. However they serve very different purposes and operate in exchanges. However they serve very different purposes and operate in
very different ways. Mediators are considered extensively in very different ways. Mediators are considered extensively in
Section 5. Section 5.
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A Mediator's task can be complex and contingent, such as by modifying A Mediator's task can be complex and contingent, such as by modifying
and adding content or regulating which users are allowed to and adding content or regulating which users are allowed to
participate and when. The popular example of this role is a group participate and when. The popular example of this role is a group
mailing list. A sequence of Mediators may even perform a series of mailing list. A sequence of Mediators may even perform a series of
formal steps, such as reviewing, modifying and approving a purchase formal steps, such as reviewing, modifying and approving a purchase
request. request.
Because a Mediator originates messages, it can also receive replies. Because a Mediator originates messages, it can also receive replies.
So a Mediator really is a full-fledged User. 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
It is a hybrid of User and Relay that interconnects heterogeneous Mediator. It is a hybrid of User and Relay that interconnects
mail services. Its goal is to emulate a Relay, and a detailed heterogeneous mail services. Its goal is to emulate a Relay,
discussion is in Section 2.2.4. and a detailed discussion is in Section 2.2.3.
2.2. Mail Handling Service (MHS) Actors 2.2. Mail Handling Service (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,
end-to-end transfer on behalf of the Originator and reaching the end-to-end transfer on behalf of the Originator and reaching the
Recipient address(es) specified in the original RFC2821.RcptTo Recipient address(es) specified in the original RFC2821.RcptTo
commands. Mediated or protracted, iterative exchanges, such as those commands. Mediated or protracted, iterative exchanges, such as those
used for collaboration over time, are part of the User-level service, used for collaboration over time, are part of the User-level service,
and are not part of this transfer-level Handling Service. and are not part of this transfer-level Handling Service.
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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 Management Domain (ADMD). MHS, that is, the local ADministrative Management Domain (ADMD).
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. Bounce Handler 2.2.2. Relay
The Bounce Handler processes service notifications that are generated
by the MHS, as a result of its efforts to transfer or deliver the
message. Notices can be about failures or completions and are sent
to an address that is specified by the Source. This Bounce handling
address (also known as a Return address) might have no visible
characteristics in common with the address of the Originator or
Source.
NOTE:
The choice of the label "Bounce" is unfortunate, due to its
negative implication and narrow focus. However it is the most
popular term for the address.
2.2.3. Relay
A mail Relay performs email transfer-service routing and store-and- A mail Relay performs email transfer-service routing and store-and-
forward by (re-)transmitting the message on towards its Recipient(s). forward by (re-)transmitting the message on towards its Recipient(s).
A Relay can add information to the envelope, such as with trace A Relay can add trace information. However it does not modify
information. However it does not modify existing envelope existing envelope information or the message content semantics. It
information or the message content semantics. It can modify message can modify message content syntax, such as a change from text to
content syntax, such as a change from text to binary transfer- binary transfer-encoding form, only as required to meet the
encoding form, only as required to meet the capabilities of the next capabilities of the next hop in the MHS.
hop in the MHS.
A set of Relays composes a Mail Handling Service (MHS) network. This A set of Relays composes a Mail Handling Service (MHS) network. This
is above any underlying packet-switching network that they might be is above any underlying packet-switching network that they might be
using and below any gateways or other user-level Mediators. using and below any gateways or other user-level Mediators.
In other words, interesting email scenarios can involve three In other words, interesting email scenarios can involve three
distinct architectural layers of store-and-forward service: distinct architectural layers of store-and-forward service:
* User Mediators * User Mediators
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* Packet Switches * Packet Switches
with the bottom-most usually being the Internet's IP service. The with the bottom-most usually being the Internet's IP service. The
most basic email scenarios involve Relays and Switches. 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 Bounce address. The Originator and send an error message to the Bounce address. The
potential for looping is avoided by having this message, itself, potential for looping is avoided by having this message, itself,
contain no Bounce address. contain no Bounce address.
2.2.4. Gateway 2.2.3. 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. Its purpose is simply to emulate a heterogeneous mail services. Its purpose is simply to emulate a
Relay and the closer it comes to this, the better. However it Relay and the closer it comes to this, the better. However it
operates at the User level, because it MUST be able to modify message operates at the User level, because it MUST be able to modify message
content. content.
Differences between mail services can be as small as minor syntax Differences between mail services can be as small as minor syntax
variations, but usually encompass significant, semantic distinctions. variations, but usually encompass significant, semantic distinctions.
One difference could have the concept of an email address be a One difference could have the concept of an email address be a
skipping to change at page 13, line 10 skipping to change at page 13, line 29
components in the Originator's or Recipient's mail services, other components in the Originator's or Recipient's mail services, other
than adding the Gateway. To each of these otherwise independent than adding the Gateway. To each of these otherwise independent
services, the Gateway will appear to be a "native" participant. services, the Gateway will appear to be a "native" participant.
However the ultimate test of a Gateway's adequacy is whether the However the ultimate test of a Gateway's adequacy is whether the
Originator and Recipient can sustain a dialogue. In particular can a Originator and Recipient can sustain a dialogue. In particular can a
Recipient's MUA automatically formulate a valid Reply that will reach Recipient's MUA automatically formulate a valid Reply that will reach
the initial Originator? the initial Originator?
2.3. Administrative Actors 2.3. Administrative Actors
Actors often will are associated with different organizations, each Actors often are associated with different organizations, each with
with its own administrative authority. This operational its own administrative authority. This operational independence,
independence, coupled with the need for interaction between groups, coupled with the need for interaction between groups, provides the
provides the motivation for distinguishing among ADministrative motivation for distinguishing among ADministrative Management Domains
Management Domains (ADMD). Each ADMD can have vastly different (ADMD). Each ADMD can have vastly different operating policies and
operating policies and trust-based decision-making. An obvious trust-based decision-making. An obvious example is the distinction
example is the distinction between mail that is exchanged within a between mail that is exchanged within a single organization, versus
single organization, versus mail that is exchanged between mail that is exchanged between independent organizations. The rules
independent organizations. The rules for handling these two types of for handling these two types of traffic tend to be quite different.
traffic tend to be quite different. That difference requires That difference requires defining the boundaries of each, and this
defining the boundaries of each, and this requires the ADMD requires the ADMD construct.
construct.
Operation of Internet Mail services is apportioned to different Operation of Internet Mail services is apportioned to different
providers (or operators). Each can be an independent ADMD. This providers (or operators). Each can be an independent ADMD. This
independence of administrative decision-making defines boundaries independence of administrative decision-making defines boundaries
that distinguish different portions of the Internet Mail service. that distinguish different portions of the Internet Mail service.
Examples include an end-user operating their desktop client, a Examples include an end-user operating their desktop client, a
department operating a local Relay, an IT department operating an department operating a local Relay, an IT department operating an
enterprise Relay and an ISP operating a public shared email service. enterprise Relay and an ISP operating a public shared email service.
These can be configured into many combinations of administrative and These can be configured into many combinations of administrative and
operational relationships, with each ADMD potentially having a operational relationships, with each ADMD potentially having a
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| | +---------------------->| | | | | | +---------------------->| | | |
| User | | |-Edge--+--->|-User | | User | | |-Edge--+--->|-User |
| | | | +---------+ +--->| | | | | | | | +---------+ +--->| | | |
| V | | | ADMD2 | | +-------+ +-------+ | V | | | ADMD2 | | +-------+ +-------+
| Edge--+---+ | ----- | | | Edge--+---+ | ----- | |
| | | | | | | | | | | |
+-------+ +----|-Transit-+---+ +-------+ +----|-Transit-+---+
| | | |
+---------+ +---------+
Figure 4: ADministrative Management Domains (ADMD) Example Figure 4: ADMD Example
Edge networks can use proprietary email standards internally. Edge networks can 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 distinction calls for additional administrations. In particular this distinction calls for additional
care in assessing transitions of responsibility, as well as the care in assessing transitions of responsibility, as well as the
accountability and authorization relationships among participants in accountability and authorization relationships among participants in
email transfer. email transfer.
skipping to change at page 17, line 22 skipping to change at page 17, line 43
identifier, or performing transfer processing. identifier, or performing transfer processing.
3.3. Message Identifier 3.3. Message Identifier
There are two standardized tags, for identifying messages: Message-ID There are two standardized tags, for identifying messages: Message-ID
and ENVID. and ENVID.
3.3.1. Message-ID 3.3.1. Message-ID
The Message-ID is a user-level tag, primarily used for threading and The Message-ID is a user-level tag, primarily used for threading and
for eliminating duplicates. [RFC2822]. It is associated with the for eliminating duplicates. [RFC2822]. Any actor within the
RFC2822.From field, although any actor within the originating ADMD originating ADMD might assign the Message-ID, although it is
might assign it. The recipient's ADMD is the intended consumer of typically created by an actor within the Originating ADMD.. The
the Message-ID, although any actor along the transfer path might use recipient's ADMD is the intended consumer of the Message-ID, although
it. Internet Mail standards provide for a single Message-ID; however any actor along the transfer path might use it. Internet Mail
more than one is sometimes assigned. standards provide for a single Message-ID; however more than one is
sometimes assigned.
Like a mailbox address, a Message-ID has two distinct parts, divided Like a mailbox address, a Message-ID has two distinct parts, divided
by an at-sign ("@"). The right-hand side is globally interpreted and by an at-sign ("@"). The right-hand side is globally interpreted and
specifies the ADMD or host assigning the identifier. The left-hand specifies the ADMD or host assigning the identifier. The left-hand
side contains a string that is globally opaque and serves to uniquely side contains a string that is globally opaque and serves to uniquely
identify the message within the domain referenced on the right-hand identify the message within the domain referenced on the right-hand
side. The duration of uniqueness for the message identifier is side. The duration of uniqueness for the message identifier is
undefined. undefined.
When a message is revised in any way, the question of whether to When a message is revised in any way, the question of whether to
skipping to change at page 18, line 5 skipping to change at page 18, line 26
revisions to the message each receive new message identifiers." revisions to the message each receive new message identifiers."
However real-world experience dictates some flexibility. An However real-world experience dictates some flexibility. An
impossible test is whether the recipient will consider the new impossible test is whether the recipient will consider the new
message to be equivalent to the old. For most components of Internet message to be equivalent to the old. For most components of Internet
Mail, there is no way to predict a specific recipient's preferences Mail, there is no way to predict a specific recipient's preferences
on this matter. Both creating and failing to create a new Message-ID on this matter. Both creating and failing to create a new Message-ID
have their downsides. have their downsides.
The best that can be offered, here, are some guidelines and examples: The best that can be offered, here, are some guidelines and examples:
o If a message is changed only in terms of form, such as character- * If a message is changed only in terms of form, such as
encoding, it clearly is still the same message. character-encoding, it clearly is still the same message.
o If a message has minor additions to the content, such as a mailing * If a message has minor additions to the content, such as a
list tag at the beginning of the RFC2822.Subject header field, or mailing list tag at the beginning of the RFC2822.Subject header
some mailing list administrative information added to the end of field, or some mailing list administrative information added to
the primary body-part's text, then it probably is still the same the end of the primary body-part's text, then it probably is
message. still the same message.
o If a message has viruses deleted from it, it probably is still the * If a message has viruses deleted from it, it probably is still
same message. the same message.
o If a message has offensive words deleted from it, then some * If a message has offensive words deleted from it, then some
recipients will consider it the same message, but some will not. recipients will consider it the same message, but some will
not.
o If a message is translated into a different language, then some * If a message is translated into a different language, then some
recipients will consider it the same message, but some will not. recipients will consider it the same message, but some will
not.
The absence of objective, precise criteria for Message-ID re- The absence of objective, precise criteria for Message-ID re-
generation, along with the absence of strong protection associated generation, along with the absence of strong protection associated
with the string, means that the presence of an ID can permit an with the string, means that the presence of an ID can permit an
assessment that is marginally better than a heuristic, but the ID assessment that is marginally better than a heuristic, but the ID
certainly has no value on its own for strict formal reference or certainly has no value on its own for strict formal reference or
comparison. Hence it is not appropriate to use the Message-ID for comparison. Hence it is not appropriate to use the Message-ID for
any process that might be called "security". any process that might be called "security".
3.3.2. ENVID 3.3.2. ENVID
skipping to change at page 18, line 47 skipping to change at page 19, line 24
message posting, until the directly-resulting message deliveries. It message posting, until the directly-resulting message deliveries. It
does not survive re-postings. does not survive re-postings.
The format of an ENVID is free-form. Although its creator might The format of an ENVID is free-form. Although its creator might
choose to impose structure on the string, none is imposed by Internet choose to impose structure on the string, none is imposed by Internet
standards. By implication, the scope of the string is defined by the standards. By implication, the scope of the string is defined by the
domain name of the Bounce Address. domain name of the Bounce Address.
4. Services and Standards 4. Services and Standards
Internet Mail's architecture distinguishes among six different types Internet Mail's architecture distinguishes among six basic types of
of functional components, arranged to support a store-and-forward functional components, arranged to support a store-and-forward
service architecture: service architecture. As shown in Figure 5 these types can have
multiple instances, some of which represent specialized sub-roles.
This section considers the activities and relationships among these
components, and the Internet Mail standards used among them.
* Message 1. Message
* Mail User Agent (MUA) 2. Mail User Agent (MUA)
* Message Submission Agent (MSA) + Originating MUA (oMUA)
* Message Transfer Agent (MTA) + Receiving MUA (rMUA)
* Message Delivery Agent (MDA) 3. Message Submission Agent (MSA)
* Message Store (MS) + Originator-focussed MSA functions (oMSA)
+ MHS-focussed MSA functions (hMSA)
4. Message Transfer Agent (MTA)
5. Message Delivery Agent (MDA)
+ Recipient-focused MDA functions (rMDA)
+ MHS-focussed MDA functions (hMDA)
6. Message Store (MS)
+ Originator MS (oMS)
- oMS on a remote server (soMS)
- oMS co-located with the oMUA (uoMS)
+ Recipient MS (rms)
- rMS on a remote server (srM)
- rMS co-located with the rMUA (urMS)
This section describes each functional component for Internet Mail, This section describes each functional component for Internet Mail,
and the standards-based protocols that are associated with their and the standards-based protocols that are associated with their
operation. operation.
Software implementations of these architectural components often Software implementations of these architectural components often
compress them, such as having the same software do MSA, MTA and MDA compress them, such as having the same software do MSA, MTA and MDA
functions. However the requirements for each of these components of functions. However the requirements for each of these components of
the service are becoming more extensive. So their separation is the service are becoming more extensive. So their separation is
increasingly common. increasingly common.
NOTE: NOTE: A discussion about any interesting system architecture is
often complicated by confusion between architecture versus
A discussion about any interesting system architecture is often implementation. An architecture defines the conceptual
complicated by confusion between architecture versus functions of a service, divided into discrete conceptual
implementation. An architecture defines the conceptual functions modules. An implementation of that architecture can combine or
of a service, divided into discrete conceptual modules. An separate architectural components, as needed for a particular
implementation of that architecture can combine or separate operational environment. It is important not to confuse the
architectural components, as needed for a particular operational engineering decisions that are made to implement a product,
environment. It is important not to confuse the engineering with the architectural abstractions used to define conceptual
decisions that are made to implement a product, with the functions.
architectural abstractions used to define conceptual functions.
The following figure shows function modules and the standardized The following figure shows function modules and the standardized
protocols used between them. Additional protocols and configurations protocols used between them. Additional protocols and configurations
are possible. Boxes defined by asterisks (*) represent functions are possible. Boxes defined by asterisks (*) represent functions
that often are distributed among two or more systems. that often are distributed among two or more systems.
+------+ +-------+ +------+ +-------+
............+ oMUA |..............................| Disp | ............+ oMUA |..............................| Disp |
. +--+-+-+ +-------+ . +--+-+-+ +-------+
. local,imap}| |{smtp,submission ^ . local,imap}| |{smtp,submission ^
skipping to change at page 20, line 23 skipping to change at page 21, line 23
. +------V-.---*------------+ * | | . +------V-.---*------------+ * | |
. MSA | +-------+ * +------+ | * | | . MSA | +-------+ * +------+ | * | |
. | | oMSA +--O-->| hMSA | | * | | . | | oMSA +--O-->| hMSA | | * | |
. | +-------+ * +--+---+ | * | | . | +-------+ * +--+---+ | * | |
. +------------*------+-----+ * | | . +------------*------+-----+ * | |
/+==========+\ * V {smtp * | | /+==========+\ * V {smtp * | |
|| MESSAGE || * +------+ * /+===+===+\ | || MESSAGE || * +------+ * /+===+===+\ |
||----------|| MHS * | MTA | * || dsn || | ||----------|| MHS * | MTA | * || dsn || |
|| Envelope || * +--+---+ * \+=======+/ | || Envelope || * +--+---+ * \+=======+/ |
|| SMTP || * V {smtp * ^ ^ | || SMTP || * V {smtp * ^ ^ |
|| RFC2822 || * +------+ * | | /+==+==+\ || Content || * +------+ * | | /+==+==+\
|| Content || * | MTA +----*-----+ | || mdn || || RFC2822 || * | MTA +----*-----+ | || mdn ||
|| RFC2822 || * +--+---+ * | \+=====+/ || MIME || * +--+---+ * | \+=====+/
|| MIME || * smtp}| {local * | | \+==========+/ * smtp}| {local * | |
\+==========+/ MDA * | {lmtp * | | MDA * | {lmtp * | |
. +------------+------V-----+ * | | . +------------+------V-----+ * | |
. | +------+ * +------+ | * | | . | +------+ * +------+ | * | |
. | | | * | | +--*---------+ | . | | | * | | +--*---------+ |
. | | rMDA |<--O---+ hMDA | | * | . | | rMDA |<--O---+ hMDA | | * |
. | | | * | | |<-*-------+ | . | | | * | | |<-*-------+ |
. | +-+----+ * +------+ | * | | . | +-+----+ * +------+ | * | |
. +---+--+-----*------------+ * | | . +---+--+-----*------------+ * | |
. | | ***************** | | . | | ***************** | |
. pop} +--+ +---+ | | . pop} +--+ +---+ | |
. imap} | | {local | | . imap} | | {local | |
skipping to change at page 21, line 13 skipping to change at page 22, line 13
Figure 5: Protocols and Services Figure 5: Protocols and Services
4.1. Message Data 4.1. Message Data
The purpose of the Mail Handling Service (MHS) is to exchange a The purpose of the Mail Handling Service (MHS) is to exchange a
message object among participants. , [RFC2822] [RFC0822] Hence all of message object among participants. , [RFC2822] [RFC0822] Hence all of
its underlying mechanisms are merely in the service of getting that its underlying mechanisms are merely in the service of getting that
message from its Originator to its Recipients. A message can be message from its Originator to its Recipients. A message can be
explicitly labeled as to its nature. [RFC3458] explicitly labeled as to its nature. [RFC3458]
A message comprises a transit handling envelope and the end-user A message comprises a transit handling envelope and the message
message content. The envelope contains handling information used by content. The envelope contains information used by the MHS. The
the MHS, or generated by it. The content is divided into a content is divided into a structured header and the body. The header
structured header and the body. The body may be unstructured simple comprises transit trace information and end-user structured fields.
lines of text, or it may be a MIME tree of multi-media subordinate The body may be unstructured simple lines of text, or it may be a
objects, called body-parts, or attachments. [RFC2045], [RFC2046], MIME tree of multi-media subordinate objects, called body-parts, or
[RFC2047], [RFC4288], [RFC4289], [RFC2049]. attachments. [RFC2045], [RFC2046], [RFC2047], [RFC4288], [RFC4289],
[RFC2049].
In addition, Internet Mail has a few conventions for special control In addition, Internet Mail has a few conventions for special control
data: data --
Delivery Status Notification (DSN): Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) is a message that can be A Delivery Status Notification (DSN) is a message that can be
generated by the MHS (MSA, MTA or MDA) and sent to the generated by the MHS (MSA, MTA or MDA) and sent to the
RFC2821.MailFrom address. The mailbox for this is shown as RFC2821.MailFrom address. The mailbox for this is shown as
Bounces in Figure 5. It provides information about message Bounces in Figure 5. DSNs provide 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) is a message that A Message Disposition Notification (MDN) is a message that
provides information about user-level, Recipient-side message provides information about user-level, Recipient-side message
processing, such as indicating that the message has been processing, such as indicating that the message has been
displayed [RFC3798] or the form of content that can be displayed [RFC3798] or the form of content that can be
supported. [RFC3297] It can be generated by an rMUA and is supported. [RFC3297] It can be generated by an rMUA and is
sent to the Disposition-Notification-To address(es). The sent to the Disposition-Notification-To address(es). The
mailbox for this is shown as Disp in Figure 5. It mailbox for this is shown as Disp in Figure 5.
Message Filtering (SIEVE): Message Filtering (SIEVE):
SIEVE is a scripting language that permits specifying SIEVE is a scripting language that permits specifying
conditions for differential handling of mail, typically at the conditions for differential handling of mail, typically at the
time of delivery. [RFC3028] It can be conveyed in a variety of time of delivery. [RFC3028] It can be conveyed in a variety of
ways, as a MIME part. Figure 5 shows a Sieve specification ways, as a MIME part. Figure 5 shows a Sieve specification
going from the rMUA to the MDA. However filtering can be done going from the rMUA to the MDA. However filtering can be done
at many different points along the transit path and any one or at many different points along the transit path and any one or
more of them might be subject to Sieve directives, especially more of them might be subject to Sieve directives, especially
within a single ADMD. Hence the Figure shows only one within a single ADMD. Hence the Figure shows only one
relationship, for (relative) simplicity. relationship, for (relative) simplicity.
4.1.1. Envelope 4.1.1. Envelope
Information that is directly used by, or produced by, the MHS is Internet Mail has a fragmented framework for transit-related
called the "envelope". It controls and records handling activities "handling" information. Information that is directly used by the MHS
by the transfer service. Internet Mail has a fragmented framework is called the "envelope". It directs handling activities by the
for handling this "handling" information. The envelope exists partly transfer service as is carried in transfer service commands. That
in the transfer protocol SMTP [RFC2821] and partly in the message is, The envelope exists in the transfer protocol SMTP [RFC2821].
object [RFC2822]. The SMTP specification uses the term to refer only
to the transfer-protocol information.
NOTE:
Due to the frequent use of the term "envelope" to refer only to
SMTP constructs, there has been some call for using a different
term, to label the larger set of information defined here. So
far, no alternative term has developed any community support.
Direct envelope addressing information, as well as optional transfer Trace information records handling activity and is recorded in the
directives, are carried within the SMTP control channel. Other message Header.
envelope information, such as trace records, is carried within the
message object header fields. Upon delivery, some SMTP-level
envelope information is typically encoded within additional message
object header fields, such as Return-Path. [RFC2821],[RFC2822]
4.1.2. Header Fields 4.1.2. Header Fields
Header fields are attribute name/value pairs covering an extensible Header fields are attribute name/value pairs covering an extensible
range of email service, user content and user transaction meta- range of email service, user content and user transaction meta-
information. The core set of header fields is defined in [RFC2822], information. The core set of header fields is defined in [RFC2822],
[RFC0822]. It is common to extend this set, for different [RFC0822]. It is common to extend this set, for different
applications. Procedures for registering header fields are defined applications. Procedures for registering header fields are defined
in [RFC4021]. An extensive set of existing header field in [RFC4021]. An extensive set of existing header field
registrations is provided in [RFC3864]. registrations is provided in [RFC3864].
skipping to change at page 23, line 9 skipping to change at page 24, line 5
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 hierarchically structured into a composition of multi-media body- be hierarchically structured into a composition of multi-media body-
part attachments, using MIME. [RFC2045], [RFC2046], [RFC2047], part attachments, using MIME. [RFC2045], [RFC2046], [RFC2047],
[RFC4288], [RFC2049] MIME structures each body-part into a recursive [RFC4288], [RFC2049] MIME structures each body-part into a recursive
set of MIME header field meta-data and MIME Content sections. set of MIME header field meta-data and 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 identifiers combine into: For a message in transit, the core uses of identifiers combine into:
+-----------------------+-------------+---------------------+ +-----------------------+----------------+---------------------+
| Layer | Field | Set By | | Layer | Field | Set By |
+-----------------------+-------------+---------------------+ +-----------------------+----------------+---------------------+
| Message Body | MIME Header | Originator | | Message Body | MIME Header | Originator |
| Message header fields | From | Originator | | Message header fields | From | Originator |
| | Sender | Source | | | Sender | Source |
| | Reply-To | Originator | | | Reply-To | Originator |
| | To, CC, BCC | Originator | | | To, CC, BCC | Originator |
| | Message-ID | Source | | | Message-ID | Source |
| | Received | Source, Relay, Dest | | | Received | Source, Relay, Dest |
| | Return-Path | MDA, from MailFrom | | | Return-Path | MDA, from MailFrom |
| | Resent-* | Mediator | | | Resent-* | Mediator |
| SMTP | HELO | Latest Relay Client | | SMTP | HELO | Latest Relay Client |
| | MailFrom | Source | | | ENVID | Source |
| | RcptTo | Originator | | | MailFrom | Source |
| IP | IP Address | Latest Relay Client | | | RcptTo | Originator |
+-----------------------+-------------+---------------------+ | IP | Source Address | Latest Relay Client |
+-----------------------+----------------+---------------------+
Layered Identities Layered Identities
The most common address-related fields are: The most common address-related fields are:
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.
RFC2822.Reply-To RFC2822.Reply-To Set by: Originator
Set by: Originator
If a message Recipient sends a reply message that would otherwise If a message Recipient sends a reply message that would otherwise
use the RFC2822.From field address(es) that are contained in the use the RFC2822.From field address(es) that are contained in the
original message, then they are instead to use the address(es) in original message, then they are instead to use the address(es) in
the RFC2822.Reply-To field. In other words this field is a direct the RFC2822.Reply-To field. In other words this field is a direct
override of the From field, for responses from Recipients. override of the From field, for responses from Recipients.
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 can be into the transfer service. For efficiency this field can 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. field MUST be used.
Specification of the error return addresses -- the "Bounce" Specification of the error return addresses -- the "Bounce"
address, contained in RFC2821.MailFrom -- is made by the address, contained in RFC2821.MailFrom -- is made by the
RFC2822.Sender. Typically the Bounce address is the same as the RFC2822.Sender. Typically the Bounce address is the same as the
Sender address. However some usage scenarios require it to be Sender address. However some usage scenarios require it to be
different. different.
RFC2822.To, RFC2822.CC RFC2822.To/.CC Set by: Originator
Set by: Originator
These specify MUA Recipient addresses. However some or all of the These specify MUA Recipient addresses. However some or all of the
addresses in these fields might not be present in the addresses in these fields might not be present in the
RFC2821.RcptTo commands, due to handling process that might RFC2821.RcptTo commands, due to handling process that might
transfer from the former to the latter. transfer from the former to the latter.
The distinction between To and CC is subjective. Generally a To The 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. field indicates a message copy to such a Recipient.
Typically, the field lists no addresses or only lists the address Typically, the field lists no addresses or only lists the address
of the Recipient receiving this copy. An MUA will typically make of the 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 of the Originator. preferences of the Originator.
RFC2821.HELO/.EHLO RFC2821.HELO/.EHLO Set by: Source
Set by: Source
The MSA can specify its hosting domain identity for the SMTP HELO The MSA can specify its hosting domain identity for the SMTP HELO
or EHLO command operation. or EHLO command operation.
RFC2821.MailFrom RFC3461.ENVID Set by: Source
Set by: Source The MSA can specify an opaque string, to be included in a DSN, as
a means of assisting the Bounce address recipient in identifying
the message that produced a DSN.
RFC2821.MailFrom 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 transmission- determine what address needs to be informed about 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 mailbox, while the RFC2821.RcptTo might specify a mailing list mailbox, while the RFC2821.RcptTo
address specifies a member of that list. address specifies a member of that list.
RFC2821.Received RFC2821.Received Set by: Source, Relay, Mediator, Dest
Set by: Source, Relay, Mediator, Dest
This indicates trace information, including originating host, This indicates trace information, including originating host,
relays, Mediators, and MSA host domain names and/or IP Addresses. relays, Mediators, and MSA host domain names and/or IP Addresses.
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.
RFC2919.List-Id RFC2919.List-Id Set by: Mediator Originator
Set by: Mediator Originator
This provides a globally unique mailing list naming framework that This provides a globally unique mailing list naming framework that
is independent of particular hosts. [RFC2919] is independent of particular hosts. [RFC2919]
The identifier is in the form of a domain name; however the string The identifier is in the form of a domain name; however the string
usually is constructed by combining the two parts of an email usually is constructed by combining the two parts of an email
address and the result rarely is a true domain name, listed in the address and the result rarely is a true domain name, listed in the
domain name service -- although it can be. domain name service -- although it can be.
RFC2369.List-* RFC2369.List-* Set by: Mediator Originator
Set by: Mediator Originator
[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 for mailing lists. In effect they supply list-specific parameters for
common mailing list user operations. The identifiers for these common mailing list user operations. The identifiers for these
operations are for the list, itself, and the user-as-subscriber. operations are for the list, itself, and the user-as-subscriber.
[RFC2369] [RFC2369]
RFC0791.SourceAddr Set by: The Client SMTP sending host immediately
preceding the current receiving SMTP server.
[RFC0791] defines the basic unit of data transfer for the
Internet, the IP Datagram. It contains a "Source Address" field
that specifies the IP Address for the host (interface) from which
the datagram was sent. This information is set and provided by
the IP layer, and is therefore independent of mail-level
mechanisms. As such, it is often taken to be authoritative,
although it is possible to provide false addresses.
4.2. User-Level Services 4.2. User-Level Services
Interactions at the user level entail protocol exchanges, distinct Interactions at the user level entail protocol exchanges, distinct
from those that occur at lower layers of the Internet Mail from those that occur at lower layers of the Internet Mail
architecture, which is above the Internet Transport layer. Because architecture, which is above the Internet Transport layer. Because
the motivation for email, and much of its use, is for interaction the motivation for email, and much of its use, is for interaction
among humans, the nature and details of these protocol exchanges among humans, the nature and details of these protocol exchanges
often are determined by the needs of human and group communication. often are determined by the needs of human and group communication.
In terms of efforts to specify behaviors, one effect of this is to In terms of efforts to specify behaviors, one effect of this is to
require subjective guidelines, rather than strict rules, for some require subjective guidelines, rather than strict rules, for some
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folder for messages waiting to be sent (Queued or Unsent) and a folder for messages waiting to be sent (Queued or Unsent) and a
folder for messages that have been successfully posted for folder for messages that have been successfully posted for
transmission (Sent). transmission (Sent).
The Recipient-side MUA (rMUA) works on behalf of the end-user The Recipient-side MUA (rMUA) works on behalf of the end-user
Recipient to process received mail. This includes generating user- Recipient to process received mail. This includes generating user-
level return control messages, displaying and disposing of the level return control messages, displaying and disposing of the
received message, and closing or expanding the user communication received message, and closing or expanding the user communication
loop, by initiating replies and forwarding new messages. loop, by initiating replies and forwarding new messages.
NOTE: Although not shown in Figure 5, an MUA can, itself, have a NOTE: Although not shown in Figure 5, an MUA can, itself, have a
distributed implementation, such as a "thin" user interface module distributed implementation, such as a "thin" user interface
on a limited end-user device, with the bulk of the MUA module on a limited end-user device, with the bulk of the MUA
functionality operated remotely on a more capable server. An functionality operated remotely on a more capable server. An
example of such an architecture might use IMAP [RFC3501] for most example of such an architecture might use IMAP [RFC3501] for
of the interactions between an MUA client and an MUA server. A most of the interactions between an MUA client and an MUA
standardized approach for such scenarios is defined by [RFC4550]. server. A standardized approach for such scenarios is defined
by [RFC4550].
A Mediator is special class of MUA. It performs message re-posting, A Mediator is special class of MUA. It performs message re-posting,
as discussed in Section 2.1. as discussed in Section 2.1.
Identity fields relevant to a typical end-user MUA include: Identity fields relevant to a typical end-user MUA include:
RFC2822.From RFC2822.From
RFC2822.Reply-To RFC2822.Reply-To
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an Origination-side Ms (oMS) and a Recipient-side MS (rMS). There is an Origination-side Ms (oMS) and a Recipient-side MS (rMS). There is
a rich set of choices for configuring a store, because any MS may a rich set of choices for configuring a store, because any MS may
comprise a distributed set of component stores. In Figure 5, the rMS comprise a distributed set of component stores. In Figure 5, the rMS
demonstrates this by showing an rMS that is located on a remote demonstrates this by showing an rMS that is located on a remote
server (srMS) and an rMS that is on the same machine as the MUA server (srMS) and an rMS that is on the same machine as the MUA
(urMS). The relationship between two message stores, themselves, can (urMS). The relationship between two message stores, themselves, can
vary. vary.
As discussed in [RFC1733] the operational relationship among MSs can As discussed in [RFC1733] the operational relationship among MSs can
be -- be --
Online:
Only a remote MS is used, with messages being accessible only Online: Only a remote MS is used, with messages being accessible
when the MUA is attached to the MS, and the MUA repeatedly only when the MUA is attached to the MS, and the MUA repeatedly
fetches all or part of a message, from one session to the next. fetches all or part of a message, from one session to the next.
Offline: Offline: The MS is local to the user, and messages are
completely moved from any remote store, rather than (also)
The MS is local to the user, and messages are completely moved being retained there.
from any remote store, rather than (also) being retained there.
Disconnected:
An rMS and a uMS are kept synchronized, for all or part of Disconnected: An rMS and a uMS are kept synchronized, for all or
their contents, while there is a connection between them. part of their contents, while there is a connection between
While they are disconnected, mail can continue to arrive at the them. While they are disconnected, mail can continue to arrive
rMS and the user may continue to make changes to the uMS. Upon at the rMS and the user may continue to make changes to the
reconnection, the two stores are re-synchronized. uMS. Upon reconnection, the two stores are re-synchronized.
4.3. MHS-Level Services 4.3. MHS-Level Services
4.3.1. Mail Submission Agent (MSA) 4.3.1. 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 ADMD and the oMUA and enforces the policies of the hosting ADMD and the
requirements of Internet standards. An MSA represents an unusual requirements of Internet standards. An MSA represents an unusual
functional dichotomy. A portion of its task is to represent MUA functional dichotomy. A portion of its task is to represent MUA
(uMSA) interests during message posting, to facilitate posting (uMSA) interests during message posting, to facilitate posting
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formal RFC2822 representation. formal RFC2822 representation.
Historically, standards-based MUA/MSA interactions have used SMTP Historically, standards-based MUA/MSA interactions have used SMTP
[RFC2821]. A recent alternative is SUBMISSION [RFC4409]. Although [RFC2821]. A recent alternative is SUBMISSION [RFC4409]. Although
SUBMISSION derives from SMTP, it uses a separate TCP port and imposes SUBMISSION derives from SMTP, it uses a separate TCP port and imposes
distinct requirements, such as access authorization. distinct requirements, such as access authorization.
Identities relevant to the MSA include: Identities relevant to the MSA include:
RFC2821.HELO/.EHLO RFC2821.HELO/.EHLO
RFC3461.ENVID
RFC2821.MailFrom RFC2821.MailFrom
RFC2821.RcptTo RFC2821.RcptTo
RFC2821.Received RFC2821.Received
4.3.2. Mail Transfer Agent (MTA) 4.3.2. Mail Transfer Agent (MTA)
A Mail Transfer Agent (MTA) relays mail for one application-level A Mail Transfer Agent (MTA) relays mail for one application-level
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The primary "routing" mechanism for Internet Mail is the DNS MX The primary "routing" mechanism for Internet Mail is the DNS MX
record [RFC1035], which specifies a host through which the queried record [RFC1035], which specifies a host through which the queried
domain can be reached. This presumes a public -- or at least a domain can be reached. This presumes a public -- or at least a
common -- backbone that permits any attached host to connect to any common -- backbone that permits any attached host to connect to any
other. other.
Identities relevant to the MTA include: Identities relevant to the MTA include:
RFC2821.HELO/.EHLO RFC2821.HELO/.EHLO
RFC3461.ENVID
RFC2821.MailFrom RFC2821.MailFrom
RFC2821.RcptTo RFC2821.RcptTo
RFC2822.Received RFC2822.Received Set by: Relay Server
Set by: Relay Server
4.3.3. Mail Delivery Agent (MDA) 4.3.3. 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 ADMD, such as at an organizational border in the Recipient's ADMD, such as at an organizational border
(Boundary) Relay. However it is required for the MDA, if only (Boundary) Relay. However it is required for the MDA, if only
because the MDA must know where to deliver the message. because the MDA must know where to deliver the message.
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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:
RFC2821.Return-Path RFC2821.Return-Path Set by: Originator Source or Mediator Source
Set by: Originator Source or Mediator 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: MDA server
Set by: MDA server
An MDA can record a Received header field to indicate trace An MDA can record a Received header field to indicate trace
information, including source host and receiving host domain information, including source host and receiving host domain
names and/or IP Addresses. names and/or IP Addresses.
5. Mediators 5. Mediators
Basic email transfer from an Originator to the specified Recipients Basic email transfer from an Originator to the specified Recipients
is accomplished by using an asynchronous, store-and-forward is accomplished by using an asynchronous, store-and-forward
communication infrastructure, in a sequence of independent communication infrastructure, in a sequence of independent
transmissions through some number of MTAs. A very different task is transmissions through some number of MTAs. A very different task is
a User-level sequence of postings and deliveries, through Mediators. a User-level sequence of postings and deliveries, through Mediators.
A Mediator forwards a message, through a re-posting process. The A Mediator forwards a message, through a re-posting process. The
Mediator does share some functionality with basic MTA relaying, but Mediator does share some functionality with basic MTA relaying, but
it enjoys a degree of freedom with both addressing and content that it enjoys a degree of freedom with both addressing and content that
is not available to MTAs. is not available to MTAs.
RFC2821.HELO/.EHLO RFC2821.HELO/.EHLO Set by: Mediator Source
Set by: Mediator Source
RFC2821.MailFrom
Set by: Originator Source or Mediator Source
RFC2821.RcptTo RFC3461.ENVID Set by: Originator Source or Mediator Source
Set by: Mediator Originator RFC2821.MailFrom Set by: Originator Source or Mediator Source
RFC2821.Received RFC2821.RcptTo Set by: Mediator Originator
Set by: Mediator Dest RFC2821.Received Set by: Mediator Dest
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 its origination information. The Mediator the essential aspects of its origination information. The Mediator
might also add commentary. 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 that forwards an existing message: New message that forwards an existing message:
This action rather curiously provides a basic template for a class This action rather curiously provides a basic template for a
of Mediators. However for its typical occurrence it is not itself class of Mediators. However for its typical occurrence it is
an example of a Mediator. The new message is viewed as being from not itself an example of a Mediator. The new message is viewed
the Agent doing the forwarding, rather than being from the as being from the Agent doing the forwarding, rather than being
original Originator. from 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
and certainly has the opportunity to modify the original message commentary and certainly has the opportunity to modify the
content. The forwarded message is therefore independent of the original message content. The forwarded message is therefore
original message exchange and creates a new message dialogue. independent of the original message exchange and creates a new
However the final Recipient sees the contained message as from the message dialogue. However the final Recipient sees the
original Originator. contained message as from the original Originator.
Reply: Reply:
When a Recipient formulates a response back to the original When a Recipient formulates a response back to the original
message's author, the new message is not typically viewed as being message's author, the new message is not typically viewed as
a "forwarding" of the original. Its focus is the new content, being a "forwarding" of the original. Its focus is the new
although it might contain all or part of the material in the content, although it might contain all or part of the material
original message. Therefore the earlier material is merely in the original message. Therefore the earlier material is
contextual and secondary. merely contextual and secondary.
Annotation: Annotation:
The integrity of the original message is usually preserved, but The integrity of the original message is usually preserved, but
one or more comments about the message are added in a manner that one or more comments about the message are added in a manner
distinguishes commentary from original text. The tone of the new that distinguishes commentary from original text. The tone of
message is that it is primarily commentary from a new Originator, the new message is that it is primarily commentary from a new
similar to a Reply. Originator, similar to a Reply.
The remainder of this section describes common examples of Mediators. The remainder of this section describes common examples of Mediators.
5.1. Aliasing 5.1. Aliasing
Aliasing is a simple re-addressing facility that is available in most Aliasing is a simple re-addressing facility that is available in most
MDA implementations. It is performed just before placing a message MDA implementations. It is performed just before placing a message
into the specified Recipient's mailbox. Instead the message is into the specified Recipient's mailbox. Instead the message is
submitted back to the transfer service, for delivery to one or more submitted back to the transfer service, for delivery to one or more
alternate addresses. Although typically implemented as part of an alternate addresses. Although typically implemented as part of an
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aliasing as a part of the lower-level mail relaying activity. aliasing as a part of the lower-level mail relaying activity.
However the small change has a large semantic impact: The designated However the small change has a large semantic impact: The designated
recipient has chosen a new recipient. Hence that original recipient recipient has chosen a new recipient. Hence that original recipient
SHOULD become responsible for any handling issues. This change would SHOULD become responsible for any handling issues. This change would
be reflected by replacing the message's RFC2821.MailFrom address to be reflected by replacing the message's RFC2821.MailFrom address to
be one within the scope of the ADMD doing the aliasing. be one within the scope of the ADMD doing the aliasing.
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/.CC/.BCC Set by: Originator
Set by: Originator
These retain their original addresses. These retain their original addresses.
RFC2821.RcptTo RFC2821.RcptTo Set by: Mediator Originator
Set by: Mediator Originator
This field contains an alias address. This field contains an alias address.
RFC2821.MailFrom RFC2821.MailFrom Set by: Originator Source or Mediator Source
Set by: Originator Source or Mediator 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 Bounces. often retain the original address to receive handling Bounces.
The benefit of retaining the original MailFrom value is to The benefit of retaining the original MailFrom value is to
ensure that the origination-side agent knows that there has ensure that the origination-side agent knows that there has
been a delivery problem. On the other hand, the responsibility been a delivery problem. On the other hand, the responsibility
for the problem usually lies with the Recipient, since the for the problem usually lies with the Recipient, since the
Alias mechanism is strictly under the Recipient's control. Alias mechanism is strictly under the Recipient's control.
RFC2821.Received RFC2821.Received Set by: Mediator Dest
Set by: Mediator Dest
The agent can record Received information, to indicate the The agent can 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 can therefore address. The trace of Received header fields can therefore
include everything from original posting through final delivery include everything from original posting through final delivery
to a final delivery. to a final delivery.
5.2. Re-Sending 5.2. Re-Sending
Also called Re-Directing, Re-Sending differs from Forwarding by Also called Re-Directing, Re-Sending differs from Forwarding by
virtue of having the Mediator "splice" a message's addressing virtue of having the Mediator "splice" a message's addressing
information, to connect the Originator of the original message and information, to connect the Originator of the original message and
the Recipient of the new message. This permits them to have direct the Recipient of the new message. This permits them to have direct
exchange, using their normal MUA Reply functions. Hence the new exchange, using their normal MUA Reply functions. Hence the new
Recipient sees the message as being From the original Originator, Recipient sees the message as being From the original Originator,
even if the Mediator adds commentary. even if the Mediator adds commentary.
Identities specified in a resent message include Identities specified in a resent message include
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 retained. The free-form (display-name) message content are retained. The free-form (display-name)
portion of the address might be modified to provide informal portion of the address might be modified to provide informal
reference to the agent responsible for the redirection. reference to the agent responsible for the redirection.
RFC2822.Reply-To RFC2822.Reply-To Set by: original Originator
Set by: original Originator
If this field is present in the original message, it is If this field is present in the original message, it is
retained in the Resent message. retained in the Resent message.
RFC2822.Sender RFC2822.Sender Set by: Originator Source or Mediator Source.
Set by: Originator Source or Mediator Source
RFC2822.TO, RFC2822.CC, RFC2822.BCC RFC2822.To/.CC/.BCC Set by: original Originator
Set by: original Originator
These specify the original message Recipients. These specify the original message Recipients.
RFC2822.Resent-From RFC2822.Resent-From Set by: Mediator Originator
Set by: Mediator Originator
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
Set by: Mediator Source RFC2822.Resent-Sender Set by: Mediator Source
The address of the agent responsible for re-submitting the The address of the agent responsible for re-submitting the
message. As with RFC2822.Sender, this field is often omitted message. As with RFC2822.Sender, this field is often omitted
when it would merely contain the same address as when it would merely contain the same address as
RFC2822.Resent-From. RFC2822.Resent-From.
RFC2822.Resent-To, RFC2822.Resent-cc, RFC2822.Resent-bcc: RFC2822.Resent-To/-CC/-BCC: Set by: Mediator Originator
Set by: Mediator Originator
The addresses of the new Recipients who will now be able to The addresses of the new Recipients who will now be able to
reply to the original author. reply to the original author.
RFC2821.MailFrom RFC2821.MailFrom Set by: Mediator Source
Set by: Mediator Source
The agent responsible for re-submission (RFC2822.Resent-Sender) The agent responsible for re-submission (RFC2822.Resent-Sender)
is also responsible for specifying the new MailFrom address. is also responsible for specifying the new MailFrom address.
RFC2821.RcptTo RFC2821.RcptTo Set by: Mediator Originator
Set by: Mediator Originator
This will contain the address of a new Recipient
RFC2822.Received This will contain the address of a new Recipient.
Set by: Mediator Dest RFC2822.Received Set by: Mediator Dest
When resending a message the submission agent can record a When resending a message the submission agent can record a
Received header field, to indicate the transition from original Received header field, to indicate the transition from original
posting to resubmission. posting to resubmission.
5.3. Mailing Lists 5.3. Mailing Lists
Mailing lists have explicit email addresses and they re-post messages Mailing lists have explicit email addresses and they re-post messages
to a list of subscribed members. The Mailing List Actor performs a to a list of subscribed members. The Mailing List Actor performs a
task that can be viewed as an elaboration of the Re-Director role. task that can be viewed as an elaboration of the Re-Director role.
In addition to sending the new message to a potentially large number In addition to sending the new message to a potentially large number
of new Recipients, the Mediator can modify content, such as deleting of new Recipients, the Mediator can modify content, such as deleting
attachments, formatting conversion, and adding list-specific attachments, formatting conversion, and adding list-specific
comments. In addition, archiving list messages is common. Still the comments. In addition, archiving list messages is common. Still the
message retains characteristics of being "from" the original message retains characteristics of being "from" the original
Originator. Originator.
Identities relevant to a mailing list processor, when submitting a Identities relevant to a mailing list processor, when submitting a
message, include: message, include:
RFC2919.List-Id RFC2919.List-Id Set by: Mediator Originator
Set by: Mediator Originator
RFC2369.List-*
Set by: Mediator Originator
RFC2822.From RFC2369.List-* Set by: Mediator Originator
Set by: original Originator RFC2822.From 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 -- or, rather, retained. message content are specified -- or, rather, retained.
RFC2822.Reply-To RFC2822.Reply-To Set by: original Originator or Mediator
Originator
Set by: original Originator or Mediator Originator
RFC2822.Sender
Set by: Originator Source or Mediator Source RFC2822.Sender Set by: Originator Source or Mediator Source
This will usually specify the address of the agent responsible This will usually specify the address of the agent responsible
for mailing list operations. However some mailing lists for mailing list operations. However some mailing lists
operate in a manner very similar to a simple MTA Relay, so that operate in a manner very similar to a simple MTA Relay, so that
they preserve as much of the original handling information as they preserve as much of the original handling information as
possible, including the original RFC2822.Sender field. possible, including the original RFC2822.Sender field.
RFC2822.TO, RFC2822.CC RFC2822.To/.CC Set by: original Originator
Set by: original Originator
These usually contains the original list of Recipient
addresses.
RFC2821.MailFrom These usually contain the original list of Recipient addresses.
Set by: Originator Source or Mediator Source RFC2821.MailFrom Set by: Originator Source or Mediator Source
This can contain the original address to be notified of This can contain the original address to be notified of
transmission issues, or the mailing list agent can set it to transmission issues, or the mailing list agent can 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 to a new address, so that mailing list members and posters are
not burdened with transmission-related Bounces. not burdened with transmission-related Bounces.
RFC2821.RcptTo RFC2821.RcptTo Set by: Mediator Originator
Set by: Mediator Originator
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 Dest
Set by: Mediator Dest
A Mailing List Agent can record a Received header field, to A Mailing List Agent can record a Received header field, to
indicate the transition from original posting to mailing list indicate the transition from original posting to mailing list
forwarding. The Agent can choose to have the message retain forwarding. The Agent can choose to have the message retain
the original set of Received header fields or can choose to the original set of Received header fields or can choose to
remove them. In the latter case it can ensure that the remove them. In the latter case it can ensure that the
original Received header fields are otherwise available, to original Received header fields are otherwise available, to
ensure later accountability and diagnostic access to them. ensure later accountability and diagnostic access to them.
5.4. Gateways 5.4. Gateways
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The critical distinction between an MTA and a Gateway is that the The critical distinction between an MTA and a Gateway is that the
latter transforms addresses and/or message content, in order to map latter transforms addresses and/or message content, in order to map
between the standards of two, different messaging services. In between the standards of two, different messaging services. In
virtually all cases, this mapping process results in some degree of virtually all cases, this mapping process results in some degree of
semantic loss. The challenge of Gateway design is to minimize this semantic loss. The challenge of Gateway design is to minimize this
loss. loss.
A Gateway can set any identity field available to a regular MUA. A Gateway can set any identity field available to a regular MUA.
Identities typically relevant to Gateways include: Identities typically relevant to Gateways include:
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 retained. As for all original addressing message content are retained. As for all original addressing
information in the message, the Gateway can translate addresses information in the message, the Gateway can translate addresses
in whatever way will allow them continue to be useful in the in whatever way will allow them continue to be useful in the
target environment. target environment.
RFC2822.Reply-To RFC2822.Reply-To Set by: original Originator
Set by: original Originator
The Gateway SHOULD retain this information, if it is originally The Gateway SHOULD retain this information, if it is originally
present. The ability to perform a successful reply by a present. The ability to perform a successful reply by a
Gatewayed Recipient is a typical test of Gateway functionality. Gatewayed Recipient is a typical test of Gateway functionality.
RFC2822.Sender RFC2822.Sender Set by: Originator Source or Mediator Source
Set by: Originator Source or Mediator Source
This can retain the original value or can be set to a new This can retain the original value or can be set to a new
address address.
RFC2822.TO, RFC2822.CC, RFC2822.BCC
Set by: original Recipient RFC2822.To/.CC/.BCC Set by: original Recipient
These usually retain their original addresses. These usually retain their original addresses.
RFC2821.MailFrom RFC2821.MailFrom Set by: Originator Source or Mediator Source
Set by: Originator Source or Mediator Source
The agent responsible for gatewaying the message can choose to The agent responsible for gatewaying the message can 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 Dest
Set by: Mediator Dest
The Gateway can record a Received header field, to indicate the The Gateway can record a Received header field, to indicate the
transition from original posting to the new messaging transition from original posting to the new messaging
environment. environment.
5.5. Boundary Filter 5.5. Boundary Filter
Organizations often enforce security boundaries by subjecting Organizations often enforce security boundaries by subjecting
messages to analysis, for conformance with the organization's safety messages to analysis, for conformance with the organization's safety
policies. An example is detection of content classed as spam or a policies. An example is detection of content classed as spam or a
virus. A Filter might alter the content, to render it safe, such as virus. A Filter might alter the content, to render it safe, such as
by removing content deemed unacceptable. Typically these actions by removing content deemed unacceptable. Typically these actions
will result in the addition of content that records the actions. will result in the addition of content that records the actions.
6. Security Considerations 6. Considerations
6.1. Security Considerations
This document does not specify any new Internet Mail functionality. This document does not specify any new Internet Mail functionality.
Consequently it is not intended to introduce any security Consequently it is not intended to introduce any security
considerations. 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 degree to which security issues are highlights the considerable degree to which security issues are
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 In addition, email transfer protocols can operate over authenticated
and/or encrypted links, and message content or authorship can be and/or encrypted links, and message content or authorship can be
authenticated or encrypted. authenticated or encrypted.
7. IANA Considerations 6.2. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
8. References 7. References
8.1. Normative 7.1. Normative
[RFC0791] Postel, J., "Internet Protocol", 1981 September.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, [RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 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.
skipping to change at page 42, line 9 skipping to change at page 41, line 38
Internet Mail Extensions (MIME) Part Four: Registration Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 4289, December 2005. Procedures", BCP 13, RFC 4289, December 2005.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006. RFC 4409, April 2006.
[RFC4550] Maes, S., , S., and Isode Ltd., "Internet Email to Support [RFC4550] Maes, S., , S., and Isode Ltd., "Internet Email to Support
Diverse Service Environments (Lemonade) Profile", Diverse Service Environments (Lemonade) Profile",
June 2006. June 2006.
8.2. Descriptive 7.2. Descriptive
[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), Networks", draft-spamops-00 (work in progress),
March 2004. March 2004.
[RFC1733] Crispin, M., "Distributed Electronic Models in IMAP4", [RFC1733] Crispin, M., "Distributed Electronic Models in IMAP4",
December 1994. December 1994.
skipping to change at page 42, line 41 skipping to change at page 42, line 23
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-spamops] Discussion of the Source actor role was greatly [ID-spamops] Discussion of the Source actor role was greatly
clarified during discussions in the IETF's Marid working group. clarified 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 the original drafts. insight on the framework and details of the original drafts.
Later reviews and suggestions were provided by Nathaniel Borenstein, Later reviews and suggestions were provided by Eric Allman, Nathaniel
Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ned Freed, Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Willemien
Eric Hall, Brad Knowles, John Leslie, Bruce Lilly, Mark E. Mallett, Hoogendoorn, Tony Finch, Ned Freed, Eric Hall, Brad Knowles, John
David MacQuigg, Chris Newman, Daryl Odnert, Rahmat M. Samik-Ibrahim, Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg,
Marshall Rose, Hector Santos, Jochen Topf, Willemien Hoogendoorn, Alexey Melnikov, der Mouse, S. Moonesamy, Chris Newman, Daryl Odnert,
Valdis Kletnieks. Rahmat M. Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf,
Greg Vaudreuil.
Diligent proof-reading was performed by Bruce Lilly. Diligent proof-reading was performed by Bruce Lilly.
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
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