< draft-crocker-email-arch-01.txt		draft-crocker-email-arch-02.txt >
	MARID / SMTP D. Crocker		SMTP D. Crocker
	Internet-Draft Brandenburg InternetWorking		Internet-Draft Brandenburg InternetWorking
	Expires: January 1, 2005 July 3, 2004		Expires: July 27, 2005 January 26, 2005
	Internet Mail Architecture		Internet Mail Architecture
	draft-crocker-email-arch-01		draft-crocker-email-arch-02
	Status of this Memo		Status of this Memo
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	Copyright Notice		Copyright Notice
	Copyright (C) The Internet Society (2004). All Rights Reserved.		Copyright (C) The Internet Society (2005).
	Abstract		Abstract
	Over its thirty year history, Internet mail has undergone significant		Over its thirty year history, Internet mail has undergone significant
	changes in scale and complexity. The first standardized architecture		changes in scale and complexity. The first standardized architecture
	for email specified a simple split between the user world and the		for email specified a simple split between the user world and the
	transmission world, in the form of Mail User Agents (MUA) and Mail		transmission world, in the form of Mail User Agents (MUA) and Mail
	Transfer Agents (MTA). Over time each of these has divided into		Transfer Agents (MTA). Over time each of these has divided into
	multiple, specialized modules. Public discussion and agreement about		multiple, specialized modules. Public discussion and agreement about
	the nature of the changes to Internet mail has not kept pace, and		the nature of the changes to Internet mail has not kept pace, and
	abuses of the Internet mail service have brought these issues into		abuses of the Internet mail service have brought these issues into
	stark relief. This draft offers clarifications and enhancements, to		stark relief. This draft offers clarifications and enhancements, to
	provide a more consistent base for community discussion of email		provide a more consistent base for community discussion of email
	service problems and proposed email service enhancements.		service problems and proposed email service enhancements.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . . 3		1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . . 4
	1.2 Document Changes . . . . . . . . . . . . . . . . . . . . . . . 4		1.2 Document Changes . . . . . . . . . . . . . . . . . . . . . . . 5
	1.3 Discussion venue . . . . . . . . . . . . . . . . . . . . . . . 4		1.3 Discussion venue . . . . . . . . . . . . . . . . . . . . . . . 5
	2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 5		2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 5
	2.1 User . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6		2.1 User-Level Actors . . . . . . . . . . . . . . . . . . . . . . 6
	2.2 Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8		2.2 Transfer-Level Actors . . . . . . . . . . . . . . . . . . . . 8
	2.3 Provider . . . . . . . . . . . . . . . . . . . . . . . . . . . 8		2.3 Administrative Actors . . . . . . . . . . . . . . . . . . . . 11
	3. Email Identities . . . . . . . . . . . . . . . . . . . . . . . 8		3. Email Identities . . . . . . . . . . . . . . . . . . . . . . . 11
	3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . . . 8		3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . . . 12
	3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . . 9		3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . . 13
	3.3 Message Identifers . . . . . . . . . . . . . . . . . . . . . . 10		3.3 Message Identifers . . . . . . . . . . . . . . . . . . . . . . 13
	3.4 Identity Reference Convention . . . . . . . . . . . . . . . . 10		3.4 Identity Referencing Convention . . . . . . . . . . . . . . . 13
	4. Email System Architecture . . . . . . . . . . . . . . . . . . 10		4. Protocols and Services . . . . . . . . . . . . . . . . . . . . 13
	4.1 Architectural Components . . . . . . . . . . . . . . . . . . . 11		4.1 Service Components . . . . . . . . . . . . . . . . . . . . . . 15
	4.2 Operational Configuration . . . . . . . . . . . . . . . . . . 19		4.2 Operational Configuration . . . . . . . . . . . . . . . . . . 21
	4.3 Layers of Identity References . . . . . . . . . . . . . . . . 20		4.3 Layers of Identity References . . . . . . . . . . . . . . . . 21
	5. Message Data . . . . . . . . . . . . . . . . . . . . . . . . . 20		5. Message Data . . . . . . . . . . . . . . . . . . . . . . . . . 22
	5.1 Envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . 20		5.1 Envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
	5.2 Message Headers . . . . . . . . . . . . . . . . . . . . . . . 21		5.2 Message Header Fields . . . . . . . . . . . . . . . . . . . . 22
	5.3 Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21		5.3 Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
	6. Two Levels of Store-And-Forward . . . . . . . . . . . . . . . 21		6. Two Levels of Store-And-Forward . . . . . . . . . . . . . . . 23
	6.1 MTA Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 21		6.1 MTA Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 23
	6.2 MUA Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 22		6.2 MUA Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 23
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 32		7. Security Considerations . . . . . . . . . . . . . . . . . . . 30
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32		8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . 34		Author's Address . . . . . . . . . . . . . . . . . . . . . . . 33
	A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34		A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
	Intellectual Property and Copyright Statements . . . . . . . . 35		Intellectual Property and Copyright Statements . . . . . . . . 34
	1. Introduction		1. Introduction
	Over its thirty year history, Internet mail has undergone significant		Over its thirty year history, Internet mail has undergone significant
	changes in scale and complexity. The first standardized architecture		changes in scale and complexity. The first standardized architecture
	for email specified a simple split between the user world and the		for email specified a simple split between the user world and the
	transmission world, in the form of Mail User Agents (MUA) and Mail		transmission world, in the form of Mail User Agents (MUA) and Mail
	Transfer Agents (MTA). Over time each of these has sub-divided into		Transfer Agents (MTA). Over time each of these has sub-divided into
	more specialized modules.		more specialized modules. However the basic style and use of names,
			addresses and message structure have remained remarkably constant.
	The basic style and use of names, addresses and message structure		There are two, basic categories of participants in Internet Mail.
	have remained remarkably constant. However each has benefited from		Users are customers of the Mail Handling Service (MHS). They
	significant elaborations. Public discussion and agreement about the		represent the sources and sinks of that service. The Mail Handling
	nature of these changes has not kept pace, and abuses of the Internet		Service is responsible for accepting a message from one user and
	mail service have brought these issues into stark relief.		delivering to one or more others.
			+--------+
			+---------------->| User |
			| +--------+
			| .
			+--------+ | +--------+ .
			| User +--+--------->| User | .
			+--------+ | +--------+ .
			| . .
			. | +--------+ . .
			. +-->| User | . .
			. +--------+ . .
			. . . .
			. . . .
			. . . .
			+--------------------------------------+
			| |
			| Mail Handling Service (MHS) |
			| |
			+--------------------------------------+
			Figure 1: Basic Email Service Model
			Public discussion and agreement about terms of reference have not
			kept pace with the changes, and abuses of the Internet mail service
			have brought this into stark relief. So, it is necessary to produce
			a revised architecture. However it is important that the original
			distinction between user-level concerns and transfer-level concerns
			be retained. This becomes challenging when the user-level exchange
			is, itself, a sequence, such as with group dialogue or organizational
			message flow, as occurs with a purchase approval process. It is easy
			to confuse this user-level activity with the underlying mail
			transmission service exchanges.
			For Internet mail, the term "end-to-end" usually refers to single
			posting and the set of deliveries resulting from a single transiting
			of the MHS. However, note that specialized uses of email consider
			the entire email service -- including Originator and Recipient -- as
			a subordinate component. For these services, "end-to-end" refers to
			points outside of the email service. Examples are voicemail over
			email and EDI over email.
	The current draft seeks to:		The current draft seeks to:
	1. Document changes that have taken place in refining the email		1. Document changes that have taken place in refining the email
	model		model
	2. Clarify functional roles for the architectural components		2. Clarify functional roles for the architectural components
	3. Clarify identity-related issues, across the email service		3. Clarify identity-related issues, across the email service
	skipping to change at page 4, line 5 ¶		skipping to change at page 4, line 45 ¶
	4. No prior arrangement between originator and recipient		4. No prior arrangement between originator and recipient
	5. No prior arrangement between point-to-point transfer services,		5. No prior arrangement between point-to-point transfer services,
	over the open Internet		over the open Internet
	The end-to-end portion of the service is the message. Broadly the		The end-to-end portion of the service is the message. Broadly the
	message, itself, is divided between handling control information and		message, itself, is divided between handling control information and
	user message payload.		user message payload.
	A precept to the design of Internet mail is to permit		A precept to the design of Internet mail is to permit user-to-user
	interoperability with no prior, direct administrative arrangement		and MTA-to-MTA interoperability with no prior, direct administrative
	between the participants. That is, all participants rely on having		arrangement. That is, all participants rely on having the core
	the core services be universally supported, either directly or		services be universally supported, either directly or through
	through gateways that translate between Internet mail standards and		gateways that translate between Internet mail standards and other
	other email conventions.		email conventions.
	For localized environments (edge networks) prior, administrative		For localized environments (edge networks) prior, administrative
	arrangement can include access control, routing and lookup service		arrangement can include access control, routing constraints and
	configuration. In recent years one change to local environments is		lookup service configuration. In recent years one change to local
	an increased requirement for authentication or, at least,		environments is an increased requirement for authentication or, at
	accountability. In these cases, the server performs explicit		least, accountability. In these cases, the server performs explicit
	validation of the client's identity.		validation of the client's identity.
	1.2 Document Changes		1.2 Document Changes
	The major changes from the previous version of this document are:		The major changes from the previous version of this document are:
	Actors: Addition of the User/Relay/Provider construct of actors.		Overall: Clarify roles and responsibilities
	Labeling of these roles has also been added to the tables showing
	architectural function. The distinction of Actors, versus
	architectural system components, is not typical for discussions of
	email. Therefore it is likely that the construct needs
	refinement. In particular, please review the table assignments.
	MDA/MS/MUA: The construct of the Message Store has been added. This		Diagrams: Revised diagrams and tightened things up
	change is intended to reflect the consensus view from online
	discussion, rather than being the editor's view, which has in any
	event changed... However it is likely that it will need
	significant revision or replacement. Please review it carefully!
	Message Identifiers: Discussion of message identifiers has been added		Distinct architectural 'sections': Added concept of ADMDs, as
	to the section on Email Identities.		operational layer, separate from functional or architectural
			layer. Added user "layer", as distinct from transfer. Introduced
			'mediator'.
	1.3 Discussion venue		1.3 Discussion venue
	NOTE: This document is the work of a single person, about a topic		NOTE: This document is the work of a single person, about a topic
	with considerable diversity of views. It is certain to be		with considerable diversity of views. It is certain to be
	incomplete and inaccurate. Some errors simply need to be		incomplete and inaccurate. Some errors simply need to be
	reported; they will get fixed. Others need to be discussed by the		reported; they will get fixed. Others need to be discussed by the
	community, because the real requirement is to develop common		community, because the real requirement is to develop common
	community views. To this end, please treat the draft as a		community views. To this end, please treat the draft as a
	touchstone for public discussion.		touchstone for public discussion.
	Discussion about this document should be directed to the:		Discussion about this document should be directed to the:
			<mailto:ietf-smtp@imc.org> mailing list. The IETF-SMTP mailing list
	<mailto:ietf-smtp@imc.org>		<http://www.imc.org/ietf-smtp/index.htm> is the most active,
	mailing list. The <http://www.imc.org/ietf-smtp/index.htm> is the		long-standing venue for discussing email architecture. Although this
	most active, long-standing venue for discussing email architecture.		list is primarily for discussing only the SMTP protocol, it is
	Although this list is primarily for discussing only the SMTP		recommended that discussion of this draft take place on that mailing
	protocol, it is recommended that discussion of this draft take place		list. This list tends to attend to end-to-end infrastructure and
	on that mailing list. This list tends to attend to end-to-end		architecture issues more than other email-related mailing lists.
	infrastructure and architecture issues more than other email-related
	mailing lists.
	o The <mailto:ietf-822@imc.org> list also is pertinent
	<http://www.imc.org/ietf-822/index.html>. However it's focus is
	on the message, itself, so that transfer issues are typically
	excluded. In addition, this list has not be very active recently.
	o A currently active mailing list, likely to impact Internet mail
	architecture, is <mailto:ietf-mxcomp@imc.org>. This list is
	devoted to matters of spam control, so that underlying matters of
	Internet mail architecture are probably best deferred to a more
	general list, such as ietf-smtp.
	o Also currently active is the <mailto:lemonade@ietf.org>, which is
	considering enhancements for interaction between thin MUAs and
	MSAs.
	2. Email Actor Roles		2. Email Actor Roles
	Discussion of email architecture requires distinguishing different		Discussion of email architecture requires distinguishing different
	actors within the service, and being clear about the job each		actors within the service, and being clear about the job each
	performs in the overall handling of mail. For this level of		performs. The best way to maintain the distinction between user
	discussion "the service" has the task of performing a single,		activity and handling activities is to depict their details in
	end-to-end transfer. Protracted, iterative exchanges, such as those		separate diagrams. Current Internet mail provides only a small set
	used for collaboration over time, are beyond the scope of (this		of capabilities for supporting different kinds of ongoing, user-level
	version of) this document. Actors often will be associated with		exchanges.
	entirely independent organizations from other actors participating in
	an end-to-end email transfer.
	The following depicts the relationships among participants in		Although related to a technical architecture, the focus of a
	Internet Mail. Although related to a technical architecture, its		discussions on Actors is on participant responsibilities, rather than
	focus is on participant responsibilities, rather than functional		functional modules. Hence the labels used are different than for
	modules. Hence the labels used are different than for classic email		classic email architecture diagrams. The figures depict the
	architecture. This figure depicts the relationships among the		relationships among the Actors. Actors often will be associated with
	actors. It shows the Submitter as distinct from the Originator,		entirely independent organizations from other Actors who are
	although it is common for them to be the same actor. The figure also		participating in the email service.
	shows multiple Relays in the sequence. It is legal to have only one,
	and for intra-organization mail services, this is common.
	User (Originator, Author)		2.1 User-Level Actors
	| -+
	Submitter | Provider
	| -+
	| -+
	Relay |
	| | | Provider
	| Relay |
	| | -+
	| | -+
	| | -+ |
	| User (Forwarder) | |
	| [ Intermediate ] | |Provider
	| [ Recipient ] | |
	| [ Originator ] | Provider |
	| [ Submitter ] | |
	| | -+ |
	| Relay |
	| | -+
	| | -+
	Relay |
	| | Provider
	User (Recipient) |
	-+
	2.1 User		Users are the sources and sinks of messages. They may have an
			exchange that iterates and they may expand or contract the set of
			users participating in a set of exchanges.
	Users are customers of the email relaying service. They represent		In Internet Mail there are three, basic types of user-level Actors:
	the sources and sinks of that service.		Originators, Recipients, and Mediators. Fromhe t User-level
			perspective all mail transfer activities are performed by a
			monolithic, shared handling service. Users are customers of this
			service. The following depicts the relationships among them.
	Three types of users are distinguished:		+------------+
			| Originator |<--------------+
			+-+---+----+-+ |
			| | | |
			| | V |
			| | +-----------+ |
			| | | Recipient | |
			| | +-----------+ |
			| | |
			| | +----------+ |
			| | | | |
			| V V | |
			| +-----------+ +---+---+---+
			| | Mediator +--->| Recipient |
			| +-----------+ +-----------+
			|
			V
			+-----------+ +-----------+ +-----------+
			| Mediator +--->| Mediator +--->| Recipient |
			+-----------+ +-----------+ +-----------+
			The functions of these Actors are:
	2.1.1 Originator		2.1.1 Originator
	Also called "Author", this is a user-level participant responsible		Also called "Author", this is the user-level participant responsible
	for creating original content and requesting its transmission. The		for creating original content and requesting its transmission. The
	email service operates to send and deliver mail among Originators and		Mail Handling Service operates to send and deliver mail among
	Recipients.		Originators and Recipients.
	2.1.2 Submitter		2.1.2 Recipient
	The Submitter is responsible for ensuring that a message is valid for		The Recipient is a consumer of delivered content.
	posting and then submitting it into the mail transfer service. It
	primarily serves the Originator and often it is the same entity.
	The Submitter has the responsibility for any additional		A recipient may close the user-level communication loop by creating
			and submitting a new message that replies to an originator. An
			automated, or semi-automated form of reply informs the Originator
			about the Recipient's disposition of the message.
			2.1.3 Mediator
			A Mediator receives, aggregates, reformulates and distributes
			messages as part of a potentially-protracted, higher-level exchange
			among users. A Mediator is viewed by the Mail Handling Service, when
			the Mediator's address is specified in the envelope. When submitting
			messages, the Mediator is an Originator. What is distinctive is that
			a Mediator preserves Originator information of the message(s) it
			reformulates, but makes meaningful changes to the content. Hence the
			Mail Handling Service sees a new message, but Users receive a message
			that is interpreted as primarily being from the author of the
			original message. The role of a Mediator permits distinct, active
			creativity, rather than being limited the more passive job of merely
			connecting together other participants. Hence it is really the
			Mediator that is responsible for the new message.
			A Mediator's task may be complex, contingent and creative, such as by
			modifying and adding content or regulating which users may
			participate and when. The popular example of this role is a group
			mailing list. A sequence of mediators may even perform a series of
			formal steps, such as reviewing, modifying and approving a purchase
			request.
			Because a Mediator originates messages, it might also receive
			replies. That is, a Mediator is a full-fledged User.
			Specialized Mediators include:
			Forwarder: A new message encapsulates the original message and is
			seen as strictly "from" the Mediator. However the Mediator might
			add commentary and certainly has the opportunity to modify the
			original message content.
			Redirector: Redirection differs from Forwarding by virtue of having
			the Mediator "splice" communication between the Originator of the
			original message and the Recipient of the new message. Hence the
			new Recipient sees the message as being From the original
			Originator.
			Mailing List: This Actor performs a task that can be viewed as an
			elaboration of the Redirector role. In addition to sending the
			new message to a potentially large number of new Recipients,
			content might be modified, such as deletion of attachments,
			formatting conversion, and addition of list-specific comments. In
			additional, archival of list messages is common.
			Annotator: The integrity of the original message is preserved, but
			one or more comments about the message are added in a manner that
			distinguishes commentary from original text.
			Adaptor: {per Ned Freed}
			Security Filter: Organizations often enforce security boundaries by
			having message subjected to analysis for conformance with the
			organization's safety policies. Examples are detection of content
			classed as spam or a virus. A Security Filter might alter the
			content, to render it safe, such as by removing content deemed
			unacceptable. Typically these actions will result in the addition
			of content that records the actions.
			2.2 Transfer-Level Actors
			The Mail Handling Service has the task of performing a single,
			end-to-end transfer on behalf of the originator and reaching the
			recipient address(es) specified in the envelope. Protracted,
			iterative exchanges, such as those used for collaboration over time,
			are part of the User-level service, and are not part of this
			Transfer-level service.
			The following depicts the relationships among transfer participants
			in Internet Mail. It shows Source as distinct from the Originator,
			although it is common for them to be the same actor. The figure also
			shows multiple Relays in the sequence. It is legal to have only one,
			and for intra-organization mail services, this is common.
			+------------+ +-----------+
			| Originator | | Recipient |
			+-----+------+ +-----------+
			| ^
			| Mail Handling Service |
			+===================================================+
			|| | | ||
			|| | | ||
			V |
			+---------+ +--------+ +----+----+
			| | | |<------------+ |
			| Source +...>| Notice | | Dest |
			| | | |<---+ | |
			+----+----+ +--------+ | +---------+
			| | ^
			V | |
			+---------+ +----+----+ +----+----+
			| Relay +-->.......-->| Relay +-->| Relay |
			+---------+ +----+----+ +---------+
			|
			V
			+---------+
			| Gateway +-->...
			+---------+
			2.2.1 Source
			The Source role is responsible for ensuring that a message is valid
			for posting and then submitting it to a mail relay. Validity
			includes conformance with Internet mail standards, as well as local
			operational policies. Source may simply review the message for
			conformance, and reject it if there are errors, or it may create some
			or all of the necessary information.
			Source operates with dual allegiance. It serves the Originator and
			often it is the same entity. However its role in assuring validity
			means that it must represent the local operator of the Mail Handling
			Service.
			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.		delivery notices. Hence, Source is best held accountable for the
			message content, even when they did not create any or most of it.
	It may be helpful to think of the Submitter as more like the editor		2.2.2 Notices Handler
	or publisher of a periodical, rather than simply the administrative
	assistant for the Originator. Hence, the Submitter is best held
	accountable for the message content, even when they did not create
	any or most of it.
	2.1.3 Recipient		Transfer efforts might result in the generation of service reporting
			information about failures or completions. These Transfer or
			Delivery notification messages are sent to an address that is
			specified by the Source. A Notices handling address (also known as
			Bounce or Return address) might have no characteristics in common the
			with address of the Originator or Source.
	The Recipient is a consumer of delivered content. The recipient is		2.2.3 Relay
	specified as an addressee, in the envelope.
	2.1.4 Forwarder		A mail relay performs email transfer-service routing and
			store-and-forward. It adds envelope-related handling information and
			then (re-)transmits the message on towards its recipient(s). A Relay
			does not modify the message contents.
	Email often transits intermediate, user-level points, called		A basic transfer operation is between a client and a server Relay. A
	Forwarders. The task of a Forwarder is to perform additional		set of Relays composes a Mail Handling Service network. This is
	processing, such as replacing one target address for one or more		above any underlying packet-switching network that they might be
	others, and then submitting the message for further transmission.		using.
	Examples are recipient-controlled aliasing and, of course, mailing
	list redistribution services. A Forwarder performs a natural
	sequence of email steps:
	o Service the mailbox specified in the envelope and accept arriving		Aborting message transfer results in having the Relay become an
	messages.		Originator and send an error message to the Notifications (Bounce)
			address. (The potential for looping is avoided by having this
			message, itself, contain no Bounce address.
	o Reformulate message content and addressing, according to the		2.2.4 Gateway
	policies of the administrator of the Forwarder. Request (further)
	message transmission. Note that an Intermediate Originator
	operates with dual allegiance, notably its operating authority,
	such as the mailing list administrator, as well as the "original"
	originator.
	o Perform the usual Submitter tasks.		A Gateway is a special form of Relay that interconnects heterogeneous
			mail services. Differences between the services can be as small as
			minor syntax variations, but usually encompass much more basic,
			semantic distinctions. For example, the concept of an email address
			might be as different as a hierarchical, machine-specific address
			versus a flat, global name space. Or between text-only and
			multi-media. Hence, the Relay function of a gateway is the minor
			component. The significant challenge is in the user-to-user
			functionality that matches syntax and semantics of independent email
			standards suites.
	2.2 Relay		The basic test of a gateway's adequacy is, of course, whether an
			originator can send a message to a recipient, without requiring any
			changes to the components in the originator's mail service or the
			recipient's mail service, other than adding the gateway. To each of
			these otherwise independent services, the gateway will appear to be a
			"native" participant. However the ultimate test of a gateway's
			adequacy is whether the originator and recipient can sustain a
			dialogue. In particular, can a recipient formulate a Reply?
	A mail relay performs email transfer-service routing and		2.3 Administrative Actors
	store-and-forward. It (re-)transmits the message on towards it
	recipient(s). A basic transfer operation is between a client and a
	server Relay. A set of Relays composes a mail handling service
	network. This is above any underlying packet-switching network that
	they might be using.
	2.3 Provider		Operation of Internet mail services is apportioned to different
			providers (or operators) each is composed of an independent
			Administrative Domain. Examples include an end-user operating their
			desktop client, a department operating a local relay, an IT
			department operating an enterprise relay, and an ISP operating a
			public, shared email service. These can be configured into many
			combinations of administrative and operational relationships, with
			each Administrative Domain potentially having a complex arrangement
			of functional components.
	Providers operate component services. As shown in the Figure, it is		The interactions between functional components within an
			Administrative Domain are subject to the policies of that domain.
			Policies can cover such things as reliability, access control,
			accountability and content evaluation and may be implemented in
			different functional components, according to the needs of the
			Administrative Domain.
			2.3.1 Provider
			Providers operate component services or sets of services. It is
	possible for Providers to host services for other Providers. Common		possible for Providers to host services for other Providers. Common
	examples are:		examples are:
	Enterprise Service Providers: Operating an organization's internal		Enterprise Service Providers: Operating an organization's internal
	data and/or mail operations.		data and/or mail operations.
	Internet Service Providers: Operating underlying data communication		Internet Service Providers: Operating underlying data communication
	services that, in turn, are used by one or more Relays and Users.		services that, in turn, are used by one or more Relays and Users.
	It is not their job to perform email functions, but to provide an		It is not their job to perform email functions, but to provide an
	environment in which those functions can be performed.		environment in which those functions can be performed.
	Mail Service Providers: Operate email services, such as for		Mail Service Providers: Operate email services, such as for
	end-users, or mailing lists.		end-users, or mailing lists.
	Operational pragmatics often dictate that Providers be involved in		Operational pragmatics often dictate that Providers be involved in
	detailed administration and enforcement issues, to help insure the		detailed administration and enforcement issues, to help insure the
	health of the overall Internet Mail service.		health of the overall Internet Mail Service.
	3. Email Identities		3. Email Identities
	Internet mail uses two forms of identity. The most common is the		Internet mail uses three forms of identity. The most common is the
	mailbox address <addr-spec> [RFC2822]. The other form is the <domain		mailbox address <addr-spec> [RFC2822]. The other two forms are the
	name> [RFC1034].		<domain name> [RFC1034] and message identifier [RFC2822].
	3.1 Mailbox Addresses		3.1 Mailbox Addresses
	An addr-spec has two distinct parts, divided by an at-sign ("@").		An addr-spec has two distinct parts, divided by an at-sign ("@").
	The right-hand side contains a globally interpreted name for an		The right-hand side contains a globally interpreted name for an
	administrative domain. This domain name might refer to an entire		administrative domain. This domain name might refer to an entire
	organization, or to a collection of machines integrated into a		organization, or to a collection of machines integrated into a
	homogeneous service, or to a single machine. Domain names are		homogeneous service, or to a single machine. Domain names are
	defined and operated through the DNS [RFC1034], [RFC1035].		defined and operated through the DNS [RFC1034], [RFC1035].
	The left-hand side of the at-sign contains a string that is globally		The left-hand side of the at-sign contains a string that is globally
	opaque and is called the <local-part>. It is to be interpreted only		opaque and is called the <local-part>. It is to be interpreted only
	by the entity specified in the address's right-hand side. All other		by the entity specified in the address's right-hand side. All other
	entities must treat the local-part as a uninterpreted, literal string		entities must treat the local-part as a uninterpreted, literal string
	and must preserve all of its original details. As such, its		and must preserve all of its original details. As such, its
	distribution is equivalent to sending a "cookie" that is only		distribution is equivalent to sending a "cookie" that is only
	interpreted upon being returned to its originator.		interpreted upon being returned to its originator.
	3.1.1 Global Standards for Local-Part		3.1.1 Global Standards for Local-Part
	It is common for sites to have local conventions for sub-structure to		It is common for sites to have local structuring conventions for the
	the left-hand side of an addr-spec. This permits sub-addressing,		left-hand side (local-part) of an addr-spec. This permits
	such as for distinguishing different discussion groups by the same		sub-addressing, such as for distinguishing different discussion
	participant. However it must be stressed that these conventions are		groups by the same participant. However it must be stressed that
	strictly private to the user's organization and must not be		these conventions are strictly private to the user's organization and
	interpreted by any domain except the one listed in the right-hand		must not be interpreted by any domain except the one listed in the
	side of the add-spec.		right-hand side of the add-spec.
	A small class of addresses have an elaboration on basic email		A small class of addresses have an elaboration on basic email
	addressing, with a standardized, global schema for the local-part.		addressing, with a standardized, global schema for the local-part.
	These are conventions between originating end-systems and recipient		These are conventions between originating end-systems and recipient
	gateways, and they are invisible to the public email transfer		gateways, and they are invisible to the public email transfer
	infrastructure. When an originator is explicitly sending via a		infrastructure. When an originator is explicitly sending via a
	gateway out of the Internet, there are coding conventions for the		gateway out of the Internet, there are coding conventions for the
	local-part, so that the originator can formulate instructions for the		local-part, so that the originator can formulate instructions for the
	gateway. Standardized examples of this are the telephone numbering		gateway. Standardized examples of this are the telephone numbering
	formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com",		formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com",
	skipping to change at page 10, line 20 ¶		skipping to change at page 13, line 34 ¶
	the administrative domain assigning the identifier. The left-hand		the administrative domain assigning the identifier. The left-hand
	side of the at-sign contains a string that is globally opaque and		side of the at-sign contains a string that is globally opaque and
	serves to uniquely identify the message within the domain referenced		serves to uniquely identify the message within the domain referenced
	on the right-hand side. The duration of uniqueness for the message		on the right-hand side. The duration of uniqueness for the message
	identifier is undefined.		identifier is undefined.
	The identifier may be assigned by the user or by any component of the		The identifier may be assigned by the user or by any component of the
	system along the path. Although Internet mail standards provide for		system along the path. Although Internet mail standards provide for
	a single identifier, more than one is sometimes assigned.		a single identifier, more than one is sometimes assigned.
	3.4 Identity Reference Convention		3.4 Identity Referencing Convention
	In this document, fields references to identities are labeled in a		In this document, fields references to identities are labeled in a
	two-part, dotted notation. The first part cites the document		two-part, dotted notation. The first part cites the document
	defining the identity and the second defines the name of the		defining the identity and the second defines the name of the
	identity. Hence, <RFC2822.From> is the From field in an email		identity. Hence, <RFC2822.From> is the From field in an email
	content header, and <RFC2821.MailFrom> is the address in the SMTP		content header, and <RFC2821.MailFrom> is the address in the SMTP
	"Mail From" command.		"Mail From" command.
	4. Email System Architecture		4. Protocols and Services
	NOTE: A discussion about any interesting system architecture is often		NOTE: A discussion about any interesting system architecture is
	complicated by confusion between architecture versus		often complicated by confusion between architecture versus
	implementation. An architecture defines the conceptual functions		implementation. An architecture defines the conceptual functions
	of a service, divided into discrete conceptual modules. An		of a service, divided into discrete conceptual modules. An
	implementation of that architecture may combine or separate		implementation of that architecture may combine or separate
	architectural components, as needed for a particular operational		architectural components, as needed for a particular operational
	environment. It is important not to confuse the engineering		environment. It is important not to confuse the engineering
	decisions that are made to implement a product, with the		decisions that are made to implement a product, with the
	architectural abstractions used to define conceptual functions.		architectural abstractions used to define conceptual functions.
	Modern Internet email architecture distinguishes four types of		Modern Internet email architecture distinguishes four types of
	components, arranged to support a store-and-forward service		functional components, arranged to support a store-and-forward
	architecture:		service architecture:
	+-----------<-oMUA <-------------------------------+		+------+
	| | < smtp, |		.............+ oMUA |<------------------------------+
	(envelope) V submission |		. +--+---+ |
	RFC2822 MSA <-------------------------+ |		. | { smtp,submission |
	MIME | < smtp | |		. V |
	| MTA dsn |		. +------+ |
	| | < smtp | |		. | MSA |<--------------------+ |
	| MTA ->----------------------->+ |		. +--+---+ | |
	| | < smtp |		. | { smtp | |
	| . |		. V | |
	| . |		. +------+ +====+====+ |
	| . |		. | MTA | || dsn || |
	| V |		+============+ +--+---+ +=========+ |
	| MTA |		|| MESSAGE || . { smtp ^ ^ |
	| | < local, smtp, lmtp |		|| || . | | |
	| MDA <-------------------------+ |		||(envelope)|| . | | |
	| | | | |		|| || V | | |
	| MS-1 | < local, | |		|| RFC2822 || +------+ | | +===+===+
	| | | | < pop, sieve |		|| || | MTA +-------------------+ | || mdn ||
	| | | | < imap, | |		|| MIME || +--+---+ | +=======+
	| | MS-2 < smtp, or | mdn		+============+ | { local, smtp, lmtp | |
	| | | < web | |		. V | |
	V | V | |		. +------+ | |
	+---------> rMUA ->--------------------------+-----+		. | +-----------------------+ |
			. | MDA | |
			. | |<--------------------+ |
			. +-+--+-+ | |
			. local } | | | |
			. V | | |
			. +------+ | +====+====+ |
			. | MS-1 | | || sieve || |
			. +-+--+-+ | +=========+ |
			. | | | { pop, imap ^ |
			. | V V | |
			. | +------+ | |
			. | | MS-2 | | |
			. | +--+---+ | |
			. | | { pop, imap, local | |
			. V V | |
			. +------+ | |
			.........>| rMUA +------------------------+---------+
			+------+
	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 architecture are becoming more extensive. So, their separation		the service are becoming more extensive. So, their separation is
	is increasingly common.		increasingly common.
	4.1 Architectural Components		4.1 Service Components
	4.1.1 Mail User Agent (MUA)		4.1.1 Mail User Agent (MUA)
	An <MUA> works on behalf of end-users and end-user applications. It		A Mail User Agent (MUA) works on behalf of end-users and end-user
	is their "representative" within the email service.		applications. It is their "representative" within the email service.
	At the origination side of the service, the <oMUA> is used to create		At the origination side of the service, the oMUA is used to create a
	a message and perform initial "submission" into the transfer		message and perform initial "submission" into the transfer
	infrastructure, via an <MSA>. It may also perform any creation- and		infrastructure, via a Mail Submission Agent (MSA). It may also
	posting-time archival. An MUA outbox is part of the origination-side		perform any creation- and posting-time archival. An MUA outbox is
	MUA.		part of the origination-side MUA.
	The recipient-side <rMUA> works on behalf of the end-user to process		The recipient-side rMUA works on behalf of the end-user recipient to
	received mail. This includes generating user-level return control		process received mail. This includes generating user-level return
	messages, display and disposition of the received message, and		control messages, display and disposition of the received message,
	closing or expanding the user communication loop, by initiating		and closing or expanding the user communication loop, by initiating
	replies and forwarding new messages.		replies and forwarding new messages.
	An MUA may, itself, have a distributed architecture, such as		An MUA may, itself, have a distributed architecture, such as
	implementing a "thin" user interface module on a limited end-user		implementing a "thin" user interface module on a limited end-user
	device, with the bulk of the MUA functionality operated remotely on a		device, with the bulk of the MUA functionality operated remotely on a
	more capable server. An example of such an architecture might use		more capable server. An example of such an architecture might use
	IMAP [RFC3501] for most of the interactions between an MUA client and		IMAP [RFC3501] for most of the interactions between an MUA client and
	an MUA server.		an MUA server.
	A special class of MUA functions perform message forwarding, as		A special class of MUA performs message re-posting, as discussed in
	discussed in the [2] section.		the <Mediator> section.
	Identity fields set by the MUA include:		Identity fields set by the MUA include:
	+----------------------+----------------------+---------------------+		RFC2822.From
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2822.From | Originator | Names and addresses |
	| | | for author(s) of |
	| | | the message content |
	| | | are listed in the |
	| | | From header |
	| RFC2822.Reply-To | Originator | If a message |
	| | | recipient sends a |
	| | | message that would |
	| | | otherwise use the |
	| | | RFC2822.From field |
	| | | information in the |
	| | | original message, |
	| | | they are to use the |
	| | | contents of the |
	| | | RFC2822.Reply-To |
	| | | field instead. In |
	| | | other words, this |
	| | | field is a direct |
	| | | override of the |
	| | | From field, for |
	| | | responses from |
	| | | recipients. |
	| RFC2822.Sender | Submitter | This specifies the |
	| | | address responsible |
	| | | for submission into |
	| | | the transfer |
	| | | service. For |
	| | | efficiency, this |
	| | | field should be |
	| | | omitted if it |
	| | | contains the same |
	| | | address as |
	| | | RFC2822.From. |
	| | | However this does |
	| | | not mean there is |
	| | | no Sender |
	| | | specified. Rather, |
	| | | it means that that |
	| | | header is virtual |
	| | | and that the |
	| | | address in the From |
	| | | field must be used. |
	| | | Specification of |
	| | | the error return |
	| | | addresses (the |
	| | | "bounces" address, |
	| | | contained in |
	| | | RFC2821.MailFrom) |
	| | | is made by the |
	| | | Sender. Typically |
	| | | the bounce address |
	| | | is the same as the |
	| | | Sender address. |
	| | | However some usage |
	| | | scenarios require |
	| | | it to be different. |
	| RFC2822.To, | Recipient | These specify MUA |
	| RFC2822.CC | | recipient |
	| | | addresses. The |
	| | | distinction between |
	| | | To and CC is |
	| | | subjective. |
	| | | Generally, a To |
	| | | addressee is |
	| | | considered primary |
	| | | and is expected to |
	| | | take action on the |
	| | | message. A CC |
	| | | addressee typically |
	| | | receives a copy |
	| | | only for their |
	| | | information. |
	| RFC2822.BCC | Recipient | A message might be |
	| | | copied to an |
	| | | addressee who is |
	| | | not to be disclosed |
	| | | to the RFC2822.TO |
	| | | or RFC2822.CC |
	| | | recipients. The BCC |
	| | | header indicates a |
	| | | message copy to |
	| | | such a recipient. |
	| | | Typically, the |
	| | | field lists no |
	| | | addresses or only |
	| | | lists the address |
	| | | of the single |
	| | | recipient receiving |
	| | | the copy. This |
	| | | ensures that even |
	| | | other BCC |
	| | | recipients do not |
	| | | know of each other. |
	| | | An MUA will |
	| | | typically make |
	| | | separate postings |
	| | | for TO and CC |
	| | | recipients, versus |
	| | | BCC recipients. The |
	| | | former will see no |
	| | | indication that any |
	| | | BCCs were sent, |
	| | | whereas the latter |
	| | | have a BCC field |
	| | | present. It might |
	| | | be empty, contain a |
	| | | comment, or contain |
	| | | one or more BCC |
	| | | addresses, |
	| | | depending upon the |
	| | | preferences or the |
	| | | Originator. |
	+----------------------+----------------------+---------------------+
	Table 1: Message Identities		Actor: Originator
			Names and addresses for author(s) of the message content are
			listed in the From header field
			RFC2822.Reply-To
			Actor: Originator
			If a message recipient sends a reply message that would otherwise
			use RFC2822.From field address(es) contained in the original
			message, then they are instead to use the address(es) in the
			RFC2822.Reply-To field. In other words, this field is a direct
			override of the From field, for responses from recipients.
			RFC2822.Sender
			Actor: Source
			This specifies the address responsible for submission into the
			transfer service. For efficiency, this field should be omitted if
			it contains the same address as RFC2822.From. However this does
			not mean there is no Sender specified. Rather, it means that that
			header field is virtual and that the address in the From field
			must be used. Specification of the error return addresses -- the
			"notifications" (or "bounces") address, contained in
			RFC2821.MailFrom -- is made by the RFC2822.Sender. Typically the
			notifications address is the same as the Sender address. However
			some usage scenarios require it to be different.
			RFC2822.To, RFC2822.CC
			Actor: Recipient
			These specify MUA recipient addresses. The distinction between To
			and CC is subjective. Generally, a To addressee is considered
			primary and is expected to take action on the message. A CC
			addressee typically receives a copy only for their information.
			RFC2822.BCC
			Actor: Recipient
			A message might be copied to an addressee whose participation is
			not to be disclosed to the RFC2822.To or RFC2822.CC recipients.
			The BCC header field indicates a message copy to such a recipient.
			Typically, the field lists no addresses or only lists the address
			of the single recipient receiving the copy. This usually ensures
			that even other BCC recipients do not know of each other. An MUA
			will typically make separate postings for TO and CC recipients,
			versus BCC recipients. The former will see no indication that any
			BCCs were sent, whereas the latter have a BCC field present. It
			might be empty, contain a comment, or contain one or more BCC
			addresses, depending upon the preferences or the Originator.
	4.1.2 Mail Submission Agent (MSA)		4.1.2 Mail Submission Agent (MSA)
	An <MSA> accepts the message submission from the oMUA and conditions		A Mail Submission Agent (MSA) accepts the message submission from the
	it for insertion into the global email transfer network, according to		oMUA and enforces the policies of the hosting network and the
	the policies of the hosting network and the requirements of Internet		requirements of Internet standards. Enforcement might be passive,
	standards. It implements a server function to MUAs and a client		involving review and approval or rejection, or it might be active,
	function to MTAs (or MDAs).		involving direct modification of the message. An MSA implements a
			server function to MUAs and a client function to MTAs (or MDAs).
	Examples of MSA-styled functions, in the world of paper mail, might		Examples of MSA-styled functions, in the world of paper mail, might
	range across the very different capabilities of administrative		range across the very different capabilities of administrative
	assistants, postal drop boxes, and post office front-counter		assistants, postal drop boxes, and post office front-counter
	employees.		employees.
	The MUA/MSA interface can be implemented within single host and use		The MUA/MSA interface can be implemented within single host and use
	private conventions for their interactions. Historically,		private conventions for their interactions. Historically,
	standards-based MUA/MSA interactions have used SMTP [RFC2821].		standards-based MUA/MSA interactions have used SMTP [RFC2821].
	However a recent alternative is SUBMISSION [RFC2476]. Although		However a recent alternative is SUBMISSION [RFC2476]. Although
	SUBMISSION derives from SMTP, it operates on a separate TCP port, and		SUBMISSION derives from SMTP, it operates on a separate TCP port, and
	will typically impose distinct requirements, such as access		will typically impose distinct requirements, such as access
	authorization.		authorization.
	Identities set by the MSA include:		Identities set by the MSA include:
	+----------------------+----------------------+---------------------+		RFC2821.HELO or RFC2821.EHLO
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2821.HELO or | Submitter | The MSA may specify |
	| RFC2821.EHLO | | its hosting domain |
	| | | identity for the |
	| | | SMTP HELO or EHLO |
	| | | command operation. |
	| RFC2821.MailFrom | Submitter | This is an |
	| | | end-to-end string |
	| | | that specifies an |
	| | | email address for |
	| | | receiving return |
	| | | control |
	| | | information, such |
	| | | as "bounces". The |
	| | | name of this field |
	| | | is misleading, |
	| | | because it is not |
	| | | required to specify |
	| | | either the author |
	| | | or the agent |
	| | | responsible for |
	| | | submitting the |
	| | | message. Rather, |
	| | | the agent |
	| | | responsible for |
	| | | submission |
	| | | specifies the |
	| | | RFC2821.MailFrom |
	| | | address. Ultimately |
	| | | the simple basis |
	| | | for deciding what |
	| | | address needs to be |
	| | | in the |
	| | | RFC2821.MailFrom is |
	| | | to determine what |
	| | | address needs to be |
	| | | informed about |
	| | | transmission-level |
	| | | problems (and, |
	| | | possibly, |
	| | | successes.) |
	| RFC2821.Rcpt-To | Recipient | This specifies the |
	| | | MUA inbox address |
	| | | of a recipient. The |
	| | | string might not be |
	| | | visible in the |
	| | | message content |
	| | | headers. For |
	| | | example, the |
	| | | message destination |
	| | | address headers, |
	| | | such as RFC2822.To, |
	| | | might specify a |
	| | | mailing list |
	| | | address, while the |
	| | | RFC2821.Rcpt-To |
	| | | address specifies a |
	| | | member of that |
	| | | list. |
	| RFC2821.Received | Submitter | An MSA may record a |
	| | | Received header, to |
	| | | indicate initial |
	| | | submission trace |
	| | | information, |
	| | | including |
	| | | originating host |
	| | | and MSA host domain |
	| | | names and/or IP |
	| | | Addresses. |
	+----------------------+----------------------+---------------------+
	Table 2: MSA Identities		Actor: Source
			The MSA may specify its hosting domain identity for the SMTP HELO
			or EHLO command operation.
			RFC2821.MailFrom
			Actor: Source
			This is an end-to-end string that specifies an email address for
			receiving return control information, such as "bounces". The name
			of this field is misleading, because it is not required to specify
			either the author or the agent responsible for submitting the
			message. Rather, the agent responsible for submission specifies
			the RFC2821.MailFrom address. Ultimately the simple basis for
			deciding what address needs to be in the RFC2821.MailFrom is to
			determine what address needs to be informed about
			transmission-level problems (and, possibly, successes.
			RFC2821.Rcpt-To
			Actor: Recipient
			This specifies the MUA inbox address of a recipient. The string
			might not be visible in the message content header. For example,
			the message destination address header fields, such as RFC2822.To,
			might specify a mailing list address, while the RFC2821.Rcpt-To
			address specifies a member of that list.
			RFC2821.Received
			Actor: Source
			An MSA may record a Received header field, to indicate initial
			submission trace information, including originating host and MSA
			host domain names and/or IP Addresses.
	4.1.3 Mail Transfer Agent (MTA)		4.1.3 Mail Transfer Agent (MTA)
	An <MTA> relays a message to another other MTA or to an <MDA>, in a		An <MTA> relays a message to another other MTA or to an <MDA>, in a
	point-to-point exchange. Relaying is performed by a sequence of		point-to-point exchange. Relaying is performed by a sequence of
	MTAs, until the message reaches its destination MDA. Hence an MTA		MTAs, until the message reaches its destination MDA. Hence an MTA
	implements both client and server MTA functionality.		implements both client and server MTA functionality.
	The basic functionality of an MTA is similar to that of a packet		The basic functionality of an MTA is similar to that of a packet
	switch or IP router. That is, it does email store-and-forward email,		switch or IP router. That is, it does email store-and-forward email,
	skipping to change at page 17, line 35 ¶		skipping to change at page 19, line 12 ¶
	An important characteristic of MTA-MTA communications, over the open		An important characteristic of MTA-MTA communications, over the open
	Internet, is that they do not require prior arrangement between the		Internet, is that they do not require prior arrangement between the
	independent administrations operating the different MTAs. Given the		independent administrations operating the different MTAs. Given the
	importance of spontaneity and serendipity in the world of human		importance of spontaneity and serendipity in the world of human
	communications, this lack of prearrangement, between the		communications, this lack of prearrangement, between the
	participants, is a core benefit of Internet mail and remains a core		participants, is a core benefit of Internet mail and remains a core
	requirement for it.		requirement for it.
	Identities set by the MTA include:		Identities set by the MTA include:
	+----------------------+----------------------+---------------------+		RFC2821.HELO
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2821.HELO | Relay | The MTA may specify |
	| | | its hosting domain |
	| | | identity for the |
	| | | SMTP HELO or EHLO |
	| | | command operation. |
	| RFC2821.Return-Path | Originator | The MDA records the |
	| | | RFC2821.MailFrom |
	| | | address into an |
	| | | RFC2822 header |
	| | | named Return-Path. |
	| RFC2822.Received | Relay | An MTA must record |
	| | | a Received header, |
	| | | to indicate trace |
	| | | information, |
	| | | including source |
	| | | host and receiving |
	| | | host domain names |
	| | | and/or IP |
	| | | Addresses. |
	+----------------------+----------------------+---------------------+
	Table 3: MTA Identities		Actor: Relay
			The MTA may specify its hosting domain identity for the SMTP HELO
			or EHLO command operation.
			RFC2821.Return-Path
			Actor: Source
			The MDA records the RFC2821.MailFrom address into an RFC2822
			header field named Return-Path.
			RFC2822.Received
			Actor: Relay
			An MTA must record a Received header field, to indicate trace
			information, including source host and receiving host domain names
			and/or IP Addresses.
	4.1.4 Mail Delivery Agent (MDA)		4.1.4 Mail Delivery Agent (MDA)
	The <MDA> delivers email to the recipient's inbox.		The <MDA> delivers email to the recipient's inbox.
	An MDA can provide distinctive, address-based functionality, made		A Mail Delivery Agent (MDA) can provide distinctive, address-based
	possible by its detailed knowledge of the properties of the		functionality, made possible by its detailed knowledge of the
	destination address. This knowledge might also be present earlier in		properties of the destination address. This knowledge might also be
	an MTA relaying sequence that ends with the MDA, such as at an		present elsewhere in the recipient's Administrative Domain, such as
	organizational gateway. However it is required for the MDA, if only		at an organizational border gateway. However it is required for the
	because the MDA must know where to store the message. This knowledge		MDA, if only because the MDA must know where to store the message.
	is used to achieve differential handling of messages.		This knowledge is used to achieve differential handling of messages.
	Using Internet protocols, delivery is effected with POP [RFC1939],		Using Internet protocols, delivery is effected with POP [RFC1939] or
	IMAP [RFC3501]. SMTP permits "push" delivery to the recipient		IMAP [RFC3501]. When coupled with an internal, local mechanism, SMTP
	system, at the imitative of the upstream email service. POP is used		permits "push" delivery to the recipient system, at the initative of
	for "pull" delivery at the initiative of the recipient system.		the upstream email service. POP is used for "pull" delivery at the
	Notably, SMTP and POP effect a transfer of message control from the		initiative of the recipient system. Notably, SMTP and POP effect a
	email service to the recipient host. In contrast, IMAP provides		transfer of message control from the email service to the recipient
	on-going, interactive access to a message store, and does not effect		host. In contrast, IMAP provides on-going, interactive access to a
	a transfer of message control to the end-user host. Instead, control		message store, and does not effect a transfer of message control to
	stays with the message store host that is being access by the user.		the end-user host. Instead, control stays with the message store
			host that is being access by the user.
	Identities set by the MDA include:		Identities set by the MDA include:
	+----------------------+----------------------+---------------------+		RFC2821.HELO or RFC2821.EHLO
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2821.HELO or | Relay | The MDA may specify |
	| RFC2821.EHLO | | its hosting domain |
	| | | identity for the |
	| | | SMTP HELO or EHLO |
	| | | command operation. |
	| RFC2822.Received | h | An MTA must record |
	| | | a Received header, |
	| | | to indicate trace |
	| | | information, |
	| | | including source |
	| | | host and receiving |
	| | | host domain names |
	| | | and/or IP |
	| | | Addresses. |
	+----------------------+----------------------+---------------------+
	Table 4: MDA Identities		Actor: Relay
			The MDA may specify its hosting domain identity for the SMTP HELO
			or EHLO command operation.
			RFC2822.Received
			Actors: Source, Relay, Dest
			An MTA must record a Received header field, to indicate trace
			information, including source host and receiving host domain names
			and/or IP Addresses.
	4.1.5 Message Store		4.1.5 Message Store
	An MUA's uses a long-term Message Store (MS). A rich set of choices		An MUA's uses a long-term Message Store (MS). A rich set of choices
	for the use of that store derives from permitting more than one to be		for the use of that store derives from permitting more than one to be
	associated with a single user, demonstrated as MS-1 and MS-2 in the		associated with a single user, demonstrated as MS-1 and MS-2 in the
	Figure. MS-1 is shown as being remote from the MUA and MS-2 as being		Figure. MS-1 is shown as being remote from the MUA and MS-2 as being
	local. Further the relationship between two message store may vary.		local. Further the relationship between two message store may vary.
	Between the MDA and the MUA, these choices are supported by a wide		Between the MDA and the MUA, these choices are supported by a wide
	variety of protocol options.		variety of protocol options.
	The operational relationship among two MSs can be:		The operational relationship among two MSs can 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 fetches		only when the MUA is attached to the MS, and the MUA repeatedly
	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: The MS is local to the user, and messages are moved from any		Offline: The MS is local to the user, and messages are moved from
	remote store, rather than (also) being retained there.		any remote store, rather than (also) being retained there.
	Disconnected: A remote MS and a local MS synchronize all or parts of		Disconnected: A remote MS and a local MS synchronize all or parts of
	their contents, while connected. The user may make changes while		their contents, while connected. The user may make changes while
	disconnected, and the two stores are re-synchronized upon		disconnected, and the two stores are re-synchronized upon
	reconnection.		reconnection.
	4.2 Operational Configuration		4.2 Operational Configuration
	Mail service components can be arranged into numerous organizational		Mail service components can be arranged into numerous organizational
	structures, each with independent software and administration. One		structures, each with independent software and administration. One
	common arrangement is to distinguish:		common arrangement is to distinguish:
	skipping to change at page 20, line 21 ¶		skipping to change at page 21, line 38 ¶
	organizational firewalls to the Internet. It should be noted that		organizational firewalls to the Internet. It should be noted that
	the current Internet Mail architecture offers no special constructs		the current Internet Mail architecture offers no special constructs
	for these configuration choices. The current design of Internet mail		for these configuration choices. The current design of Internet mail
	is for a seamless, end-to-end store-and-forward sequence. It is		is for a seamless, end-to-end store-and-forward sequence. It is
	possible that the architectural enhancement will not require new		possible that the architectural enhancement will not require new
	protocols, but rather will require clarification of best practises,		protocols, but rather will require clarification of best practises,
	as exemplified by a recent effort [ID-spamops]		as exemplified by a recent effort [ID-spamops]
	4.3 Layers of Identity References		4.3 Layers of Identity References
	For a message in transit, the core identity fields combine into		For a message in transit, the core identity fields combine into:
	+-----------------+--------------+-----------------------------+		+-----------------------+-------------+---------------------+
	| Layer | Field | Set By |		| Layer | Field | Set By |
	+-----------------+--------------+-----------------------------+		+-----------------------+-------------+---------------------+
	| Message Content | MIME Headers | Originator |		| Message Content | MIME Header | Originator |
	| Message Headers | From | Originator |		| Message header fields | From | Originator |
	| | Sender | Submitter |		| | Sender | Source |
	| | Reply-To | Originator |		| | Reply-To | Originator |
	| | To, CC, BCC | Originator |		| | To, CC, BCC | Originator |
	| | Received | Submitter, Relay, Recipient |		| | Received | Source, Relay, Dest |
	| | Return-Path | MDA from MailFrom |		| | Return-Path | MDA, from MailFrom |
	| SMTP | HELO | Latest Relay Client |		| SMTP | HELO | Latest Relay Client |
	| | MailFrom | Submitter |		| | MailFrom | Source |
	| | RCPT-TO | Submitter |		| | RCPT-TO | Originator |
	| IP | IP Address | Latest Relay Client |		| IP | IP Address | Latest Relay Client |
	+-----------------+--------------+-----------------------------+		+-----------------------+-------------+---------------------+
	5. Message Data		5. Message Data
	5.1 Envelope		5.1 Envelope
	Information that is directly used or produced by the email transfer		Information that is directly used or produced by the email transfer
	service is called the "envelope". It controls and records handling		service is called the "envelope". It controls and records handling
	activities by the transfer service. Internet mail has a fragmented		activities by the transfer service. Internet mail has a fragmented
	framework for handling this "handling" information. The envelope		framework for handling this "handling" information. The envelope
	exists partly in the transfer protocol SMTP [RFC2821] and partly in		exists partly in the transfer protocol SMTP [RFC2821] and partly in
	the message object [RFC2822].		the message object [RFC2822].
	Direct envelope addressing information, as well as optional transfer		Direct envelope addressing information, as well as optional transfer
	directives, are carried in-band by MTAs. All other envelope		directives, are carried in-band by MTAs. All other envelope
	information, such as trace records, is carried within the content		information, such as trace records, is carried within the content
	headers. Upon delivery, SMTP-level envelope information is typically		header fields. Upon delivery, SMTP-level envelope information is
	encoded within additional content headers, such as Return-Path and		typically encoded within additional content header fields, such as
	Received (From and For).		Return-Path and Received (From and For).
	5.2 Message Headers		5.2 Message Header Fields
	Headers are attribute/value pairs covering an extensible range of		Header fields are attribute/value pairs covering an extensible range
	email service, user content and user transaction meta-information.		of email service, user content and user transaction meta-information.
	The core set of headers is defined in [RFC2822], [RFC0822]. It is		The core set of header fields is defined in [RFC2822], [RFC0822]. It
	common to extend this set, for different applications. A complete		is common to extend this set, for different applications. A complete
	set of registered headers is being developed through [ID-hdr-reg].		set of registered header fields is being developed through
			[ID-hdr-reg].
	One danger with placing additional information in headers is that		One danger with placing additional information in header fields is
	gateways often alter or delete them.		that gateways often alter or delete them.
	5.3 Body		5.3 Body
	The body of a message might simply be lines of ASCII text or it might		The body of a message might simply be lines of ASCII text or it might
	be structured into a composition of multi-media, body-part		be structured into a composition of multi-media, body-part
	attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],		attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],
	and [RFC2049]. It should be noted that MIME structures each		and [RFC2049]. It should be noted that MIME structures each
	body-part into a recursive set of MIME Header meta-data and MIME		body-part into a recursive set of MIME header field meta-data and
	Content sections.		MIME Content sections.
	6. Two Levels of Store-And-Forward		6. Two Levels of Store-And-Forward
	Basic email transfer is accomplished with an asynchronous		Basic email transfer is accomplished with an asynchronous
	store-and-forward communication infrastructure. This means that		store-and-forward communication infrastructure. This means that
	moving a message from an originator to a recipient involves a		moving a message from an originator to a recipient involves a
	sequence of independent transmissions through some number of		sequence of independent transmissions through some number of
	intermediaries, called MTAs. A very different task is the user-level		intermediaries, called MTAs. A very different task is the user-level
	process of re-posting a message through a new submission process,		process of re-posting a message through a new submission process,
	after final delivery for an earlier transfer sequence. Such		after final delivery for an earlier transfer sequence. Such
	skipping to change at page 22, line 10 ¶		skipping to change at page 23, line 42 ¶
	6.1 MTA Relaying		6.1 MTA Relaying
	MTAs relay mail. They are like packet-switches and IP routers.		MTAs relay mail. They are like packet-switches and IP routers.
	Their job is to make routing assessments and to move the message		Their job is to make routing assessments and to move the message
	payload data closer to the recipient. It is not their job to		payload data closer to the recipient. It is not their job to
	reformulate the payload or to change addresses in the envelope or the		reformulate the payload or to change addresses in the envelope or the
	content.		content.
	6.2 MUA Forwarding		6.2 MUA Forwarding
	As discussed in <Forwarder> section, forwarding is performed by MUAs		As discussed in <Mediator> section, forwarding is performed by MUAs
	that take a received message and submit it back to the transfer		that take a received message and submit it back to the transfer
	service, for delivery to one or more different addresses. A		service, for delivery to one or more different addresses. A
	forwarded message may appear identical to a relayed message, such as		forwarded message may appear identical to a relayed message, such as
	for Alias forwarders, or it may have minimal similarity, as with a		for Alias forwarders, or it may have minimal similarity, as with a
	Reply.		Reply.
	6.2.1 MUA Basic Forwarding		6.2.1 MUA Basic Forwarding
	The simplest type of forwarding involves creating an entirely new		The simplest type of forwarding involves creating an entirely new
	message, with new content, that includes the original message between		message, with new content, that includes the original message between
	skipping to change at page 23, line 7 ¶		skipping to change at page 24, line 24 ¶
	the original communication. They do this through a user-level		the original communication. They do this through a user-level
	forwarding function, called re-sending. The act of re-sending, or		forwarding function, called re-sending. The act of re-sending, or
	re-directing, splices a communication between Originator-1 and		re-directing, splices a communication between Originator-1 and
	Recipient-1, to become a communication between Originator-1 and new		Recipient-1, to become a communication between Originator-1 and new
	Recipient-2. In this case, the content of the new message is the old		Recipient-2. In this case, the content of the new message is the old
	message, including preservation of the essential aspects of the		message, including preservation of the essential aspects of the
	original message's origination information.		original message's origination information.
	Identities specified in a resent message include		Identities specified in a resent message include
	+----------------------+----------------------+---------------------+		RFC2822.From
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2822.From | Originator | Names and email |
	| | | addresses for the |
	| | | original author(s) |
	| | | of the message |
	| | | content are |
	| | | retained. The |
	| | | free-form |
	| | | (display-name) |
	| | | portion of the |
	| | | address might be |
	| | | modified to provide |
	| | | informal reference |
	| | | to the agent |
	| | | responsible for the |
	| | | redirection. |
	| RFC2822.Reply-To | Originator | If this field is |
	| | | present in the |
	| | | original message, |
	| | | it should be |
	| | | retained in the |
	| | | Re-sent message. |
	| RFC2822.Sender | Submitter | This field is |
	| | | expected to contain |
	| | | the original Sender |
	| | | value. |
	| RFC2822.TO, | Recipient | These specify the |
	| RFC2822.CC, | | original message |
	| RFC2822.BCC | | recipients. |
	| RFC2822.Resent-From | Intermediate | The address of the |
	| | Originator | original recipient |
	| | | who is redirecting |
	| | | the message. |
	| | | Otherwise, the same |
	| | | rules apply for the |
	| | | Resent-From field |
	| | | as for an original |
	| | | RFC2822.From field |
	| RFC2822.Resent-Sende | Intermediate | The address of the |
	| r | Submitter | agent responsible |
	| | | for re-submitting |
	| | | the message. For |
	| | | efficiency, this |
	| | | field should be |
	| | | omitted if it |
	| | | contains the same |
	| | | address as |
	| | | RFC2822.Resent-From |
	| | | . However this does |
	| | | not mean there is |
	| | | no Resend-Sender |
	| | | specified. Rather, |
	| | | it means that that |
	| | | header is virtual |
	| | | and that the |
	| | | address in the |
	| | | Resent-From field |
	| | | must be used. |
	| | | Specification of |
	| | | the error return |
	| | | addresses (the |
	| | | "bounces" address, |
	| | | contained in |
	| | | RFC2821.MailFrom) |
	| | | is made by the |
	| | | Resent-Sender. |
	| | | Typically the |
	| | | bounce address is |
	| | | the same as the |
	| | | Resent-Sender |
	| | | address. However |
	| | | some usage |
	| | | scenarios require |
	| | | it to be different. |
	| RFC2822.Resent-To, | Recipient | The addresses of |
	| RFC2822.Resent-cc, | | the new recipients |
	| RFC2822.Resent-bcc | | who will now be |
	| | | able to reply to |
	| | | the original |
	| | | author. |
	| RFC2821.MailFrom | Intermediate | The agent |
	| | Submitter | responsible for |
	| | | re-submission |
	| | | (RFC2822.Resent-Sen |
	| | | der) is also |
	| | | responsible for |
	| | | specifying the new |
	| | | RFC2821.MailFrom |
	| | | address. |
	| RFC2821.Rcpt-to | Recipient | This will contain |
	| | | the address of a |
	| | | new recipient |
	| RFC2822.Received | Intermediate | When re-sending a |
	| | Submitter | message, the |
	| | | submission agent |
	| | | may record a |
	| | | Received header, to |
	| | | indicate the |
	| | | transition from |
	| | | original posting to |
	| | | resubmission. |
	+----------------------+----------------------+---------------------+
	Table 6: ReSent Identities		Actor: Originator
			Names and email addresses for the original author(s) of the
			message content are retained. The free-form (display-name)
			portion of the address might be modified to provide informal
			reference to the agent responsible for the redirection.
			RFC2822.Reply-To
			Actor: Originator
			If this field is present in the original message, it should be
			retained in the Re-sent message.
			RFC2822.Sender
			Actor: Source
			This field is expected to contain the original Sender value.
			RFC2822.TO, RFC2822.CC, RFC2822.BCC
			Actor: Recipient
			These specify the original message recipients.
			RFC2822.Resent-From
			Actor: Mediating Originator
			The address of the original recipient who is redirecting the
			message. Otherwise, the same rules apply for the Resent-From
			field as for an original RFC2822.From field
			RFC2822.Resent-Sender
			Actor: Mediating Source
			The address of the agent responsible for re-submitting the
			message. For efficiency, this field should be omitted if it
			contains the same address as RFC2822.Resent-From. However this
			does not mean there is no Resend-Sender specified. Rather, it
			means that that header field is virtual and that the address in
			the Resent-From field must be used. Specification of the error
			return addresses (the "bounces" address, contained in
			RFC2821.MailFrom) is made by the Resent-Sender. Typically the
			bounce address is the same as the Resent-Sender address. However
			some usage scenarios require it to be different.
			RFC2822.Resent-To, RFC2822.Resent-cc, RFC2822.Resent-bcc: Actor:
			Recipient
			The addresses of the new recipients who will now be able to reply
			to the original author.
			RFC2821.MailFrom
			Actor: Mediating Source
			The agent responsible for re-submission (RFC2822.Resent-Sender) is
			also responsible for specifying the new RFC2821.MailFrom address.
			RFC2821.Rcpt-to
			Actor: Recipient
			This will contain the address of a new recipient
			RFC2822.Received
			Actor: Mediating Source
			When re-sending a message, the submission agent may record a
			Received header field, to indicate the transition from original
			posting to resubmission.
	6.2.3 MUA Reply		6.2.3 MUA Reply
	When a recipient formulates a response to a message, the new message		When a recipient formulates a response to a message, the new message
	is not typically viewed as being a "forwarding" of the original.		is not typically viewed as being a "forwarding" of the original.
	6.2.4 MUA Gateways		6.2.4 MUA Gateways
	Gateways perform the basic routing and transfer work of message		Gateways perform the basic routing and transfer work of message
	relaying, but they also make any message or address modifications		relaying, but they also make any message or address modifications
	that are needed to send the message into the next messaging		that are needed to send the message into the next messaging
	environment. When a gateway connects two differing messaging		environment. When a gateway connects two differing messaging
	services, its role is easy to identify and understand. When it		services, its role is easy to identify and understand. When it
	connects environments that have technical similarity, but may have		connects environments that have technical similarity, but may have
	significant administrative differences, it is easy to think that a		significant administrative differences, it is easy to think that a
	gateway is merely an MTA. The critical distinguish between an MTA		gateway is merely an MTA. The critical distinguish between an MTA
	and a gateway is that the latter modifies addresses and/or message		and a gateway is that the latter modifies addresses and/or message
	content.		content.
	A gateway may set any identity field available to a regular MUA.		A gateway may set any identity field available to a regular MUA.
	Identities typically set by gateways include		Identities typically set by gateways include:
	+----------------------+----------------------+---------------------+		RFC2822.From
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2822.From | Originator | Names and email |
	| | | addresses for the |
	| | | original author(s) |
	| | | of the message |
	| | | content are |
	| | | retained. As for |
	| | | all original |
	| | | addressing |
	| | | information in the |
	| | | message, the |
	| | | gateway may |
	| | | translate addresses |
	| | | in whatever way |
	| | | will allow them |
	| | | continue to be |
	| | | useful in the |
	| | | target environment. |
	| RFC2822.Reply-To | Originator | The gateway should |
	| | | retain this |
	| | | information, if it |
	| | | is originally |
	| | | present. The |
	| | | ability to perform |
	| | | a successful reply |
	| | | by a gatewayed |
	| | | recipient is a |
	| | | typical test of |
	| | | gateway |
	| | | functionality. |
	| RFC2822.Sender | Submitter | This may retain the |
	| | | original value or |
	| | | may be set to a new |
	| | | address |
	| RFC2822.TO, | Recipient | These usually |
	| RFC2822.CC, | | retain their |
	| RFC2822.BCC | | original addresses. |
	| RFC2821.MailFrom | Submitter | The agent |
	| | | responsible for |
	| | | gatewaying the |
	| | | message may choose |
	| | | to specify a new |
	| | | address to receive |
	| | | handling notices. |
	| RFC2822.Received | Forwarder | The gateway may |
	| | | record a Received |
	| | | header, to indicate |
	| | | the transition from |
	| | | original posting to |
	| | | the new messaging |
	| | | environment. |
	+----------------------+----------------------+---------------------+
	Table 7: Gateway Identities		Actor: Originator
			Names and email addresses for the original author(s) of the
			message content are retained. As for all original addressing
			information in the message, the gateway may translate addresses in
			whatever way will allow them continue to be useful in the target
			environment.
			RFC2822.Reply-To
			Actor: Originator
			The gateway should retain this information, if it is originally
			present. The ability to perform a successful reply by a gatewayed
			recipient is a typical test of gateway functionality.
			RFC2822.Sender
			Actor: Source
			This may retain the original value or may be set to a new address
			RFC2822.TO, RFC2822.CC, RFC2822.BCC
			Actor: Recipient
			These usually retain their original addresses.
			RFC2821.MailFrom
			Actor: Source
			The agent responsible for gatewaying the message may choose to
			specify a new address to receive handling notices.
			RFC2822.Receive
			Actors - Source, Relay, Dest
			The gateway may record a Received header field, to indicate the
			transition from original posting to the new messaging environment.
	6.2.5 MUA Alias Handling		6.2.5 MUA Alias Handling
	A simple re-addressing facility that is available in most MDA		A simple re-addressing facility that is available in most MDA
	implementations is called Aliasing. It is performed just before		implementations is called Aliasing. It is performed just before
	placing a message into the specified recipient's inbox. Instead, the		placing a message into the specified recipient's inbox. Instead, the
	message is submitted back to the transfer service, for delivery to		message is submitted back to the transfer service, for delivery to
	one or more alternate addresses. Although implemented as part of the		one or more alternate addresses. Although implemented as part of the
	message delivery service, this facility is strictly a recipient user		message delivery service, this facility is strictly a recipient user
	function. In effect it resubmits the message to a new address, on		function. In effect it resubmits the message to a new address, on
	behalf of the listed recipient.		behalf of the listed recipient.
	What is most distinctive about this forwarding mechanism is how		What is most distinctive about this forwarding mechanism is how
	closely it compares to normal MTA store-and-forward. In reality its		closely it compares to normal MTA store-and-forward. In reality its
	only interesting difference is that it changes the RFC2821.RCPT-TO		only interesting difference is that it changes the RFC2821.RCPT-TO
	value. Notably it does not typically change the RFC2821.Mailfrom		value. Notably it does not typically change the RFC2821.Mailfrom
	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
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2822.TO, | Recipient | These retain their |
	| RFC2822.CC, | | original addresses. |
	| RFC2822.BCC | | |
	| RFC2821.Rcpt-To | Recipient | This field contains |
	| | | an alias address. |
	| RFC2821.MailFrom | Intermediate | The agent |
	| | Submitter | responsible for |
	| | | submission to an |
	| | | alias address will |
	| | | usually retain the |
	| | | original address to |
	| | | receive handling |
	| | | notifications. The |
	| | | benefit of |
	| | | retaining the |
	| | | original MailFrom |
	| | | value is to ensure |
	| | | that the |
	| | | origination-side |
	| | | agent knows of that |
	| | | there has been a |
	| | | delivery problem. |
	| | | On the other hand, |
	| | | the responsibility |
	| | | for the problem |
	| | | usually lies with |
	| | | the recipient, |
	| | | since the Alias |
	| | | mechanism is |
	| | | strictly under the |
	| | | recipient's |
	| | | control. |
	| RFC2821.Received | Intermediate | The agent should |
	| | Recipient | record Received |
	| | | information, to |
	| | | indicate the |
	| | | delivery to the |
	| | | original address |
	| | | and submission to |
	| | | the alias address. |
	| | | The trace of |
	| | | Received headers |
	| | | should include |
	| | | everything from |
	| | | original posting |
	| | | through final |
	| | | delivery to the |
	| | | alias. |
	+----------------------+----------------------+---------------------+
	Table 8: Alias Identities		Actor: Recipient
			These retain their original addresses.
			RFC2821.Rcpt-To
			Actor: Recipient
			This field contains an alias address.
			RFC2821.MailFrom
			Actor: Mediating Source
			The agent responsible for submission to an alias address will
			usually retain the original address to receive handling
			notifications. The benefit of retaining the original MailFrom
			value is to ensure that the origination-side agent knows of that
			there has been a delivery problem. On the other hand, the
			responsibility for the problem usually lies with the recipient,
			since the Alias mechanism is strictly under the recipient's
			control.
			RFC2821.Received
			Actor: Mediating Recipient
			The agent should record Received information, to indicate the
			delivery to the original address and submission to the alias
			address. The trace of Received header fields should include
			everything from original posting through final delivery to the
			alias.
	6.2.6 MUA Mailing Lists		6.2.6 MUA Mailing Lists
	Mailing lists have explicit email addresses and forward messages to a		Mailing lists have explicit email addresses and they forward messages
	list of subscribed members. Mailing list processing is a user-level		to a list of subscribed members. Mailing list processing is a
	activity, outside of the core email transfer service. The mailing		user-level activity, outside of the core email transfer service. The
	list address is, therefore, associated with a distinct user-level		mailing list address is, therefore, associated with a distinct
	entity that can perform arbitrary actions upon the original message,		user-level entity that can perform arbitrary actions upon the
	before submitting it to the mailing list membership. Hence, mailing		original message, before submitting it to the mailing list
	lists are similar to gateways.		membership. Hence, mailing lists are similar to gateways.
	Identities set by a mailing list processor, when submitting a		Identities set by a mailing list processor, when submitting a
	message, include:		message, include:
	+----------------------+----------------------+---------------------+		RFC2919.List-id
	| Identity | Actor | Description |
	+----------------------+----------------------+---------------------+
	| RFC2919.List-id | -- | This provides a |
	| | | global mailing list |
	| | | naming framework |
	| | | that is independent |
	| | | of particular |
	| | | hosts. Although |
	| | | [RFC2919] is a |
	| | | standards-track |
	| | | specification, it |
	| | | has not gained |
	| | | significant |
	| | | adoption. |
	| RFC2369.List-* | Recipient | [RFC2369] defines a |
	| | | collection of |
	| | | message headers for |
	| | | use by mailing |
	| | | lists. In effect, |
	| | | they supply |
	| | | list-specific |
	| | | parameters for |
	| | | common mailing list |
	| | | user operations. |
	| | | The identifiers for |
	| | | these operations |
	| | | are for the list, |
	| | | itself, and the |
	| | | user-as-subscriber. |
	| | | |
	| RFC2822.From | Originator | Names and email |
	| | | addresses for the |
	| | | original author(s) |
	| | | of the message |
	| | | content are |
	| | | specified. |
	| RFC2822.Reply-To | Originator | Mailing lists have |
	| | | introduced an |
	| | | ambiguity for the |
	| | | Reply-To field. |
	| | | Some List |
	| | | operations choose |
	| | | to force all |
	| | | replies to go to |
	| | | all list members. |
	| | | They achieve this |
	| | | by placing the list |
	| | | address into the |
	| | | RFC2822.Reply-To |
	| | | field. Hence, |
	| | | direct, "private" |
	| | | replies only to the |
	| | | original author |
	| | | cannot be achieved |
	| | | by using the MUA's |
	| | | typical "reply to |
	| | | author" function. |
	| | | If the author |
	| | | created a Reply-To |
	| | | field, its |
	| | | information is |
	| | | lost. |
	| RFC2822.Sender | Submitter | This will usually |
	| | | specify the address |
	| | | of the agent |
	| | | responsible for |
	| | | mailing list |
	| | | operations. |
	| | | However, some |
	| | | mailing lists |
	| | | operate in a manner |
	| | | very similar to a |
	| | | simple MTA relay, |
	| | | so that they |
	| | | preserve as much of |
	| | | the original |
	| | | handling |
	| | | information as |
	| | | possible, including |
	| | | the original |
	| | | RFC2822.Sender |
	| | | field. |
	| RFC2822.TO, | Intermediate | These will usually |
	| RFC2822.CC | Recipient | contain the |
	| | | original list of |
	| | | recipient |
	| | | addresses. |
	| RFC2821.MailFrom | Intermediate | This may contain |
	| | Submitter | the original |
	| | | address to be |
	| | | notified of |
	| | | transmission |
	| | | issues, or the |
	| | | mailing list agent |
	| | | may set it to |
	| | | contain a new |
	| | | notification |
	| | | address. Typically, |
	| | | the value is set to |
	| | | a new address, so |
	| | | that mailing list |
	| | | members and posters |
	| | | are not burdened |
	| | | with |
	| | | transmission-relate |
	| | | d notifications. |
	| RFC2821.Rcpt-To | Recipient | This contain the |
	| | | address of a |
	| | | mailing list |
	| | | member. |
	| RFC2821.Received | Intermediate | An Mailing List |
	| | Recipient | Agent should record |
	| | | a Received header, |
	| | | to indicate the |
	| | | transition from |
	| | | original posting to |
	| | | mailing list |
	| | | forwarding. The |
	| | | Agent may choose to |
	| | | have the message |
	| | | retain the original |
	| | | set of Received |
	| | | headers or may |
	| | | choose to remove |
	| | | them. In the latter |
	| | | case, it should |
	| | | ensure that the |
	| | | original Received |
	| | | headers are |
	| | | otherwise |
	| | | available, to |
	| | | ensure later |
	| | | accountability and |
	| | | diagnostic access |
	| | | to it. |
	+----------------------+----------------------+---------------------+
	Table 9: Mailing List Identities		Actor: Mediating Originator
			This provides a global mailing list naming framework that is
			independent of particular hosts. Although [RFC2919] is a
			standards-track specification, it has not gained significant
			adoption.
			RFC2369.List-*
			Actor: Mediating Recipient
			[RFC2369] defines a collection of message header fields for use by
			mailing lists. In effect, they supply list-specific parameters
			for common mailing list user operations. The identifiers for
			these operations are for the list, itself, and the
			user-as-subscriber.
			RFC2822.From
			Actor: Originator
			Names and email addresses for the original author(s) of the
			message content are specified.
			RFC2822.Reply-To
			Actor: Originator
			Mailing lists have introduced an ambiguity for the Reply-To field.
			Some List operations choose to force all replies to go to all list
			members. They achieve this by placing the list address into the
			RFC2822.Reply-To field. Hence, direct, "private" replies only to
			the original author cannot be achieved by using the MUA's typical
			"reply to author" function. If the author created a Reply-To
			field, its information is lost.
			RFC2822.Sender
			Actor: Source
			This will usually specify the address of the agent responsible for
			mailing list operations. However, some mailing lists operate in a
			manner very similar to a simple MTA relay, so that they preserve
			as much of the original handling information as possible,
			including the original RFC2822.Sender field.
			RFC2822.TO, RFC2822.CC
			Actor: Mediating Recipient
			These will usually contain the original list of recipient
			addresses.
			RFC2821.MailFrom
			Actor: Mediating Source
			This may contain the original address to be notified of
			transmission issues, or the mailing list agent may set it to
			contain a new notification address. Typically, the value is set
			to a new address, so that mailing list members and posters are not
			burdened with transmission-related notifications.
			RFC2821.Rcpt-To
			Actor: Recipient
			This contains the address of a mailing list member.
			RFC2821.Received
			Actor: Mediating Recipient
			An Mailing List Agent should record a Received header field, to
			indicate the transition from original posting to mailing list
			forwarding. The Agent may choose to have the message retain the
			original set of Received header fields or may choose to remove
			them. In the latter case, it should ensure that the original
			Received header fields are otherwise available, to ensure later
			accountability and diagnostic access to it.
	7. Security Considerations		7. Security Considerations
	This document does not specify any new Internet mail functionality.		This document does not specify any new Internet mail functionality.
	Consequently it should introduce no new security considerations.		Consequently it should introduce no new security considerations.
	However its discussion of the roles and responsibilities for		However its discussion of the roles and responsibilities for
	different mail service modules, and the information they create,		different mail service modules, and the information they create,
	highlights the considerable security considerations that must be		highlights the considerable security considerations that must be
	present when implementing any component of the Internet mail service.		present when implementing any component of the Internet mail service.
	8 References		8 References
	[ID-hdr-reg]		[ID-hdr-reg]
	"Registration of mail and MIME header fields",		"Registration of mail and MIME header fields",
	draft-klyne-hdrreg-mail-04.txt (work in progress), Apr		draft-klyne-hdrreg-mail-04.txt (work in progress), Apr
	2004.		2004.
	[ID-marid-core]
	Lyon, J. and M. Wong, "MTA Authentication Records in DNS",
	draft-ietf-marid-core-01.txt (work in progress), June
	2004.
	[ID-spamops]		[ID-spamops]
	Hutzler, C., Crocker, D., Resnick, P., Sanderson, R. and		Hutzler, C., Crocker, D., Resnick, P., Sanderson, R. and
	E. Allman, "Email Submission Between Independent		E. Allman, "Email Submission Between Independent
	Networks", draft-spamops-00 (work in progress), March		Networks", draft-spamops-00 (work in progress), March
	2004.		2004.
	[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC		[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
	821, August 1982.		821, August 1982.
	[RFC0822] Crocker, D., "Standard for the format of ARPA Internet		[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
	skipping to change at page 34, line 25 ¶		skipping to change at page 33, line 10 ¶
	[RFC3028] Showalter, T., "Sieve: A Mail Filtering Language", RFC		[RFC3028] Showalter, T., "Sieve: A Mail Filtering Language", RFC
	3028, January 2001.		3028, January 2001.
	[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service		[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
	Extension for Delivery Status Notifications (DSNs)", RFC		Extension for Delivery Status Notifications (DSNs)", RFC
	3461, January 2003.		3461, January 2003.
	[RFC3501] Crispin, M., "Internet Message Access Protocol - Version		[RFC3501] Crispin, M., "Internet Message Access Protocol - Version
	4rev1", RFC 3501, March 2003.		4rev1", RFC 3501, March 2003.
	[2] <Forwarder>
	Author's Address		Author's Address
	Dave Crocker		Dave Crocker
	Brandenburg InternetWorking		Brandenburg InternetWorking
	675 Spruce Drive		675 Spruce Drive
	Sunnyvale, CA 94086		Sunnyvale, CA 94086
	USA		USA
	Phone: +1.408.246.8253		Phone: +1.408.246.8253
	EMail: dcrocker@brandenburg.com		EMail: dcrocker@bbiw.net
	Appendix A. Acknowledgements		Appendix A. Acknowledgements
	The Email Architecture section derives from draft-hutzler-spamops		The Email Architecture section derives from draft-hutzler-spamops
	[ID-spamops]. The text has been further elaborated.		[ID-spamops]. The text has been further elaborated.
	Discussion of the Submitter actor role was greatly clarified by		Discussion of the Source actor role was greatly clarified during
	[ID-marid-core]. Reference to this role has been written to align		discussions in the IETF's Marid working group.
	with that document's label and discussion.
	Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful		Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
	insight on the framework and details of early drafts. Additional		insight on the framework and details of early drafts. Additional
	review and suggestions have been provided by Nathaniel Borenstein,		review and suggestions were provided by Nathaniel Borenstein, Ed
	Chris Newman, Eric Hall, Tony Finch, Ed Bradford, Cyrus Daboo, Ned		Bradford, Cyrus Daboo, Tony Finch, Ned Freed, Eric Hall, Bruce Lilly,
	Freed.		Eric Hall, Chris Newman, Jochen Topf.
	Intellectual Property Statement		Intellectual Property Statement
	The IETF takes no position regarding the validity or scope of any		The IETF takes no position regarding the validity or scope of any
	Intellectual Property Rights or other rights that might be claimed to		Intellectual Property Rights or other rights that might be claimed to
	pertain to the implementation or use of the technology described in		pertain to the implementation or use of the technology described in
	this document or the extent to which any license under such rights		this document or the extent to which any license under such rights
	might or might not be available; nor does it represent that it has		might or might not be available; nor does it represent that it has
	made any independent effort to identify any such rights. Information		made any independent effort to identify any such rights. Information
	on the procedures with respect to rights in RFC documents can be		on the procedures with respect to rights in RFC documents can be
	skipping to change at page 35, line 41 ¶		skipping to change at page 34, line 41 ¶
	This document and the information contained herein are provided on an		This document and the information contained herein are provided on an
	"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS		"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
	OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET		OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
	ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,		ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
	INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE		INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
	INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED		INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
	WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.		WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
	Copyright Statement		Copyright Statement
	Copyright (C) The Internet Society (2004). This document is subject		Copyright (C) The Internet Society (2005). This document is subject
	to the rights, licenses and restrictions contained in BCP 78, and		to the rights, licenses and restrictions contained in BCP 78, and
	except as set forth therein, the authors retain all their rights.		except as set forth therein, the authors retain all their rights.
	Acknowledgment		Acknowledgment
	Funding for the RFC Editor function is currently provided by the		Funding for the RFC Editor function is currently provided by the
	Internet Society.		Internet Society.
End of changes. 93 change blocks.
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