SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track October 31, 2008
Expires: May 4, 2009
Internet Mail Architecture
draft-crocker-email-arch-11
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Abstract
Over its thirty-five year history, Internet Mail has changed
significantly in scale and complexity, as it has become a global
infrastructure service. These changes have been evolutionary, rather
than revolutionary, reflecting a strong desire to preserve both its
installed base and its usefulness. To collaborate productively on
this large and complex system, all participants must work from a
common view of it and use a common language to describe its
components and the interactions among them. But the many differences
in perspective currently make it difficult to know exactly what
another participant means. To serve as the necessary common frame of
reference, this document describes the enhanced Internet Mail
architecture, reflecting the current service.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Document Conventions . . . . . . . . . . . . . . . . . . . 6
2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 7
2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 10
2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 13
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2. Scope of Email Address Use . . . . . . . . . . . . . . . . 17
3.3. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 17
3.4. Message Identifier . . . . . . . . . . . . . . . . . . . . 18
4. Services and Standards . . . . . . . . . . . . . . . . . . . . 19
4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 22
4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 27
4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 29
4.4. Transition Modes . . . . . . . . . . . . . . . . . . . . . 33
4.5. Implementation and Operation . . . . . . . . . . . . . . . 33
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.2. ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 37
5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 40
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 40
6.1. Security Considerations . . . . . . . . . . . . . . . . . 40
6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 41
6.3. Internationalization . . . . . . . . . . . . . . . . . . . 41
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 42
7.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 44
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 45
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48
Intellectual Property and Copyright Statements . . . . . . . . . . 49
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1. Introduction
Over its thirty-five year history, Internet Mail has changed
significantly in scale and complexity, as it has become a global
infrastructure service. These changes have been evolutionary, rather
than revolutionary, reflecting a strong desire to preserve both its
installed base and its usefulness. Today, Internet Mail is
distinguished by many independent operators, many different
components for providing service to users, as well as many different
components that transfer messages.
Public collaboration on technical, operations, and policy activities
of email, including those that respond to the challenges of email
abuse, has brought a much wider range of participants into the
technical community. To collaborate productively on this large and
complex system, all participants must work from a common view of it
and use a common language to describe its components and the
interactions among them. But the many differences in perspective
currently make it difficult to know exactly what another participant
means.
It is the need to resolve these differences that motivates this
document, which describes the realities of the current system.
Internet Mail is the subject of ongoing technical, operations, and
policy work, and the discussions often are hindered by different
models of email service design and different meanings for the same
terms.
To serve as the necessary common frame of reference, this document
describes the enhanced Internet Mail architecture, reflecting the
current service. The document focuses on:
* Capturing refinements to the email model
* Clarifying functional roles for the architectural components
* Clarifying identity-related issues, across the email service
* Defining terminology for architectural components and their
interactions
1.1. History
The first standardized architecture for networked email specified a
simple split between the user world, in the form of Mail User Agents
(MUA), and the transfer world, in the form of the Mail Handling
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Service (MHS), which is composed of Mail Transfer Agents (MTA). The
MHS accepts a message from one User and delivers it to one or more
other users, creating a virtual MUA-to-MUA exchange environment.
As shown in Figure 1, this defines two logical layers of
interoperability. One is directly between Users. The other is among
the components along the transfer path. In addition, there is
interoperability between the layers, first when a message is posted
from the User to the MHS and later when it is delivered from the MHS
to the User.
The operational service has evolved, although core aspects of the
service, such as mailbox addressing and message format style,
remaining remarkably constant. The original distinction between the
user level and transfer level remains, but with elaborations in each.
The term "Internet Mail" is used to refer to the entire collection of
user and transfer components and services.
For Internet Mail, the term "end-to-end" usually refers to a single
posting and the set of deliveries that result from a single transit
of the MHS. A common exception is group dialogue that is mediated,
through a Mailing List; in this case, two postings occur before
intended Recipients receive an Author's message, as discussed in
Section 2.1.3. In fact, some uses of email consider the entire email
service, including Author and Recipient, as a subordinate component.
For these services, "end-to-end" refers to points outside the email
service. Examples are voicemail over email "[RFC3801], EDI over
email [RFC1767] and facsimile over email [RFC4142].
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+--------+
++================>| User |
|| +--------+
|| ^
+--------+ || +--------+ .
| User +==++=========>| User | .
+---+----+ || +--------+ .
. || ^ .
. || +--------+ . .
. ++==>| User | . .
. +--------+ . .
. ^ . .
. . . .
V . . .
+---+-----------------+------+------+---+
| . . . . |
| .................>. . . |
| . . . |
| ........................>. . |
| . . |
| ...............................>. |
| |
| Mail Handling Service (MHS) |
+---------------------------------------+
Figure 1: Basic Internet Mail Service Model
End-to-end Internet Mail exchange is accomplished by using a
standardized infrastructure with these components and
characteristics:
* An email object
* Global addressing
* An asynchronous sequence of point-to-point transfer mechanisms
* No prior arrangement between MTAs or between Authors and
Recipients
* No prior arrangement between point-to-point transfer services
over the open Internet
* No requirement for Author, Originator, or Recipients to be
online at the same time
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The end-to-end portion of the service is the email object, called a
"message." Broadly, the message itself distinguishes control
information, for handling, from the Author's content.
A precept to the design of mail over the open Internet is permitting
user-to-user and MTA-to-MTA interoperability without prior, direct
arrangement between the independent administrative authorities
responsible for handling a message. All participants rely on having
the core services universally supported and accessible, either
directly or through Gateways that act as translators between Internet
Mail and email environments conforming to other standards. Given the
importance of spontaneity and serendipity in interpersonal
communications, not requiring such prearrangement between
participants is a core benefit of Internet Mail and remains a core
requirement for it.
Within localized networks at the edge of the public Internet, prior
administrative arrangement often is required and can include access
control, routing constraints, and configuration of the information
query service. Although recipient authentication has usually been
required for message access since the beginning of Internet Mail, in
recent years it also has been required for message submission. In
these cases, a server validates the client's identity, whether by
explicit security protocols or by implicit infrastructure queries to
identify "local" participants.
1.2. Document Conventions
References to structured fields of a message use a two-part dotted
notation. The first part cites the document that contains the
specification for the field and the second is the name of the field.
Hence <RFC2822.From> is the From: header field in an email content
header and <RFC2821.MailFrom> is the address in the SMTP "Mail From"
command.
When occurring without the RFC2822 qualifier, header field names are
shown with a colon suffix. For example, From:.
References to labels for actors, functions or components have the
first letter capitalized.
Also, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[RFC2119] [RFC2119].
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RFC EDITOR: Remove the following paragraph before publication.
Discussion venue: Please direct discussion about this document
to the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>.
2. Responsible Actor Roles
Internet Mail is a highly distributed service, with a variety of
actors playing different roles. These actors fall into three basic
types:
* User
* Mail Handling Service (MHS)
* ADministrative Management Domain (ADMD)
Although related to a technical architecture, the focus on actors
concerns participant responsibilities, rather than functionality of
modules. For that reason, the labels used are different from those
used in classic email architecture diagrams.
2.1. User Actors
Users are the sources and sinks of messages. Users can be people,
organizations, or processes. They can have an exchange that
iterates, and they can expand or contract the set of users that
participate in a set of exchanges. In Internet Mail, there are four
types of Users:
* Authors
* Recipients
* Return Handlers
* Mediators
Figure 2 shows the primary and secondary flows of messages among
them.
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++==========++
|| Author ||<..................................<..
++=++=++=++=++ .
|| || || ++===========++ .
|| || ++====>|| Recipient || .
|| || ++=====+=====++ .
|| || . .
|| || ..........................>.+
|| || .
|| || ................... .
|| || . . .
|| || V . .
|| || +-----------+ ++=====+=====++ .
|| ++========>| Mediator +===>|| Recipient || .
|| +-----+-----+ ++=====+=====++ .
|| . . .
|| ..................+.......>.+
|| .
|| ..............+.................. .
|| . . . .
\/ V V ' .
+-----------+ +-----------+ ++=====+=====++ .
| Mediator +===>| Mediator +===>|| Recipient || .
+-----+-----+ +-----+-----+ ++=====+=====++ .
. . . .
.................+.................+.......>..
Figure 2: Relationships Among User Actors
From the user perspective, all mail transfer activities are performed
by a monolithic Mail Handling Service (MHS), even though the actual
service can be provided by many independent organizations. Users are
customers of this unified service.
Whenever any MHS actor sends information to back to an Author or
Originator in the sequence of handling a message, that actor is a
User.
2.1.1. Author
The Author is responsible for creating the message, its contents, and
its list of recipient addresses. The MHS transfers the message from
the Author and delivers it to the Recipients. The MHS has an
Originator role (Section 2.2.1) that correlates with the Author role.
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2.1.2. Recipient
The Recipient is a consumer of the delivered message. The MHS has a
Receiver role (Section 2.2.4)correlates with the Recipient role.
This is labeled Recv in Figure 3.
Any Recipient can close the user communication loop by creating and
submitting a new message that replies to the Author. An example of
an automated form of reply is the Message Disposition Notification
(MDN), which informs the Author about the Recipient's handling of the
message. (See Section 4.1.)
The Return Handler, also called "Bounce Handler," receives and
services notifications that the MHS generates, as it transfers or
delivers the message. These notices can be about failures or
completions and are sent to an address that is specified by the
Originator<<initial def>> . This Return handling address (also known
as a Return address) might have no visible characteristics in common
with the address of the Author or Originator.
2.1.3. Mediator
A Mediator receives, aggregates, reformulates, and redistributes
messages among Authors and Recipients who are the principals in
protracted exchanges. This activity is easily confused with the
underlying MHS transfer exchanges. However, each serves very
different purposes and operates in very different ways.
When mail is delivered to the Mediator specified in the
RFC2821.RcptTo command, the MHS handles it the same way as for any
other Recipient. The MHS sees each posting and delivery activity
between sources and sinks as independent; it does not see subsequent
re-posting as a continuation of a process. Because the Mediator
originates messages, it can receive replies. Hence, when submitting
messages, the Mediator is an Author. So a Mediator really is a full-
fledged User. Mediators are considered extensively in Section 5.
The distinctive aspects of a Mediator are outside the MHS. A
Mediator preserves the Author information of the message it
reformulates and is permitted to make meaningful changes to the
message content or envelope. The MHS sees a new message, but users
receive a message that they interpret as being from, or at least
initiated by, the Author of the original message. The role of a
Mediator is not limited to merely connecting other participants; the
Mediator is responsible for the new message.
A Mediator's role is complex and contingent, for example, modifying
and adding content or regulating which users are allowed to
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participate and when. The common example of this role is a group
Mailing List. In a more complex use, a sequence of Mediators could
perform a sequence of formal steps, such as reviewing, modifying, and
approving a purchase request.
A Gateway is a particularly interesting form of Mediator. It is a
hybrid of User and Relay that connects heterogeneous mail services.
Its purpose is to emulate a Relay. For a detailed discussion, see
Section 2.2.3. .
2.2. Mail Handling Service (MHS) Actors
The Mail Handling Service (MHS) performs a single end-to-end transfer
on behalf of the Author to reach the Recipient addresses specified in
the original RFC2821.RcptTo commands. Exchanges that are either
mediated or iterative and protracted, such as those used for
collaboration over time are handled by the User actors, not by the
MHS actors.
Figure 3 shows the relationships among transfer participants in
Internet Mail. Although it shows the Originator (labeled Origin) as
distinct from the Author and Receiver (labeled Recv) as distinct from
Recipient, each pair of roles usually has the same actor.
Transfers typically entail one or more Relays. However direct
delivery from the Originator to Receiver is possible. Intra-
organization mail services usually have only one Relay.
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++==========++ ++===========++
|| Author || || Recipient ||
++====++====++ +--------+ ++===========++
|| | Return | /\
|| +-+------+ ||
\/ . ^ ||
+---------+ . . +---++---+
| | . . | |
//==+=========+============================+========+===\\
|| | | . . MHS | | ||
|| | Origin +<...... .................+ Recv | ||
| | ^ | |
+---++----+ . +--------+
|| . /\
|| ..............+.................. ||
\/ . . . ||
+-------+-+ +--+------+ +-+--++---+
| Relay +=======>| Relay +=======>| Relay |
+---------+ +----++---+ +---------+
||
||
\/
+---------+
| Gateway +-->...
+---------+
Figure 3: Relationships Among MHS Actors
2.2.1. Originator
The Originator ensures that a message is valid for posting and then
submits it to a Relay. A message is valid if it conforms to both
Internet Mail standards and local operational policies. The
Originator can simply review the message for conformance and reject
it if it finds errors, or it can create some or all of the necessary
information. In effect, the Originator is responsible for the
functions of the Mail Submission Agent.
The Originator operates with dual allegiance. It serves the Author
and can be the same entity. But its role in assuring validity means
that it MUST also represent the local operator of the MHS, that is,
the local ADministrative Management Domain (ADMD).
The Originator also performs any post-submission, Author-related
administrative tasks associated with message transfer and delivery.
Notably, these tasks pertain to sending error and delivery notices,
enforcing local policies, and dealing with messages from the Author
that prove to be problematic for the Internet. The Originator is
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accountable for the message content, even when it is not responsible
for it. The Author creates the message, but the Originator handles
any transmission issues with it.
2.2.2. Relay
The Relay performs MHS-level transfer-service routing and store-and-
forward, by transmitting or retransmitting the message to its
Recipients. The Relay adds trace information [RFC2505] but does not
modify the envelope information or the message content semantics. It
can modify message content representation, such as changing the form
of transfer encoding from binary to text, but only as required to
meet the capabilities of the next hop in the MHS.
A Mail Handling Service (MHS) network consists of a set of Relays.
This network is above any underlying packet-switching network that
might be used and below any Gateways or other Mediators.
In other words, email scenarios can involve three distinct
architectural layers, each providing its own type of data of store-
and-forward service:
* User Mediators
* MHS Relays
* Packet Switches
The bottom layer is the Internet's IP service. The most basic email
scenarios involve Relays and Switches.
Aborting a message transfer makes the Relay an Author because it must
send an error message to the Return address. The potential for
looping is avoided by omitting a Return address from this message.
2.2.3. Gateway
A Gateway is a hybrid of User and Relay that connects heterogeneous
mail services. Its purpose is to emulate a Relay and the closer it
comes to this, the better. A Gateway operates as a User when it
needs the ability to modify message content.
Differences between mail services can be as small as minor syntax
variations, but they usually encompass significant, semantic
distinctions. One difference could be email addresses that are
hierarchical and machine-specific rather than a flat, global
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namespace. Another difference could be support for text-only content
or multi-media. Hence the Relay function in a Gateway presents a
significant design challenge, if the resulting performance is to be
seen as nearly seamless. The challenge is to ensure user-to-user
functionality between the services, despite differences in their
syntax and semantics.
The basic test of Gateway design is whether an Author on one side of
a Gateway can send a useful message to a Recipient on the other side,
without requiring changes to any components in the Author's or
Recipient's mail services other than adding the Gateway. To each of
these otherwise independent services, the Gateway appears to be a
native participant. But the ultimate test of Gateway design is
whether the Author and Recipient can sustain a dialogue. In
particular, can a Recipient's MUA automatically formulate a valid
Reply that will reach the Author?
2.2.4. Receiver
The Receiver performs final delivery or sends the message to an
alternate address. It can also perform filtering and other policy
enforcement immediately before or after delivery.
2.3. Administrative Actors
Administrative actors can be associated with different organizations,
each with its own administrative authority. This operational
independence, coupled with the need for interaction between groups,
provides the motivation to distinguish among ADministrative
Management Domains (ADMDs ). Each ADMD can have vastly different
operating policies and trust-based decision-making. One obvious
example is the distinction between mail that is exchanged within an
organization and mail that is exchanged between independent
organizations. The rules for handling both types of traffic tend to
be quite different. That difference requires defining the boundaries
of each, and this requires the ADMD construct.
Operation of Internet Mail services is carried out by different
providers (or operators). Each can be an independent ADMD. This
independence of administrative decision-making defines boundaries
that distinguish different portions of the Internet Mail service. A
department that operates a local Relay, an IT department that
operates an enterprise Relay, and an ISP that operates a public
shared email service can be configured into many combinations of
administrative and operational relationships. Each is a distinct
ADMD, potentially having a complex arrangement of functional
components. Figure 4 depicts relationships among ADMDs. The benefit
of the ADMD construct is to facilitate discussion about designs,
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policies and operations that need to distinguish between internal
issues and external ones.
The architectural impact of the need for boundaries between ADMDs is
discussed in [Tussle]. Most significant is that the entities
communicating across ADMD boundaries typically have the added burden
of enforcing organizational policies concerning external
communications. At a more mundane level, routing mail between ADMDs
can be an issue, such as needing to route mail for partners over
specially trusted paths.
These are the basic types of ADMDs:
Edge: Independent transfer services in networks at the edge of
the open Internet Mail service.
Consumer: This might be a type of Edge service, as is common
for web-based email access.
Transit: Mail Service Providers (MSP) that offer value-added
capabilities for Edge ADMDs, such as aggregation and filtering.
The mail-level transit service is different from packet-level
switching. End-to-end packet transfers usually go through
intermediate routers; email exchange across the open Internet can be
directly between the Boundary MTAs of Edge ADMDs. This distinction
between direct and indirect interaction highlights the differences
discussed in Section 2.2.2
+--------+ +---------+ +-------+ +-----------+
| ADMD1 |<===>| ADMD2 |<===>| ADMD3 |<===>| ADMD4 |
| ----- | | ----- | | ----- | | ----- |
| | | | | | | |
| Author | | | | | | |
| . | | | | | | |
| V | | | | | | |
| Edge..+....>|.Transit.+....>|-Edge..+....>|.Recipient |
| | | | | | | |
+--------+ +---------+ +-------+ +-----------+
Figure 4: Administrative Domain (ADMD) Example
Edge networks can use proprietary email standards internally.
However the distinction between Transit network and Edge network
transfer services is significant because it highlights the need for
concern over interaction and protection between independent
administrations. In particular, this distinction calls for
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additional care in assessing the transitions of responsibility and
the accountability and authorization relationships among participants
in message transfer.
The interactions of ADMD components are subject to the policies of
that domain, which cover concerns such as these:
* Reliability
* Access control
* Accountability
* Content evaluation and modification
These policies can be implemented in different functional components,
according to the needs of the ADMD. For example, see [RFC5068].
Consumer, Edge, and Transit services can be offered by providers that
operate component services or sets of services. Further, it is
possible for one ADMD to host services for other ADMDs.
These are common examples of ADMDs:
Enterprise Service Providers:
These ADMDs operate the internal data and/or the mail services
within an organization.
Internet Service Providers (ISP):
These ADMDs operate the underlying data communication services,
which are used by one or more Relay and User. ISPs are not
responsible for performing email functions, but they can
provide an environment in which those functions can be
performed.
Mail Service Providers:
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These ADMDs operate email services, such as for consumers or
client companies.
Practical operational concerns demand that providers be involved in
administration and enforcement issues. This involvement can extend
to operators of lower-level packet services.
3. Identities
Internet Mail uses three forms of identity: mailbox, domain name, and
message-ID. Each must be globally unique.
3.1. Mailbox
"A mailbox sends and receives mail. It is a conceptual entity
which does not necessarily pertain to file storage." [RFC2822]
A mailbox is specified as an Internet Mail address <addr-spec>. It
has two distinct parts, separated by an at-sign (@). The right side
is a globally interpreted domain name associated with an ADMD.
Domain names are discussed in Section 3.3. Formal Internet Mail
addressing syntax can support source routes, to indicate the path
through which a message ought to be sent. The use of source routes
is not common and has been deprecated in [RFC2821].
The portion to the left of the at-sign contains a string that is
globally opaque and is called the <local-part>. It is to be
interpreted only by the entity specified by the address's domain
name. Except as noted later in this section all other entities MUST
treat the <local-part> as an uninterpreted literal string and MUST
preserve all of its original details. As such its public
distribution is equivalent to sending a Web browser "cookie" that is
only interpreted upon being returned to its creator.
Some local-part values have been standardized, for contacting
personnel at an organization. These names cover common operations
and business functions. [RFC2142]
It is common for sites to have local structuring conventions for the
left-hand side <local-part> of an <addr-spec>. This permits sub-
addressing, such as for distinguishing different discussion groups
used by the same participant. However it is worth stressing that
these conventions are strictly private to the user's organization and
MUST NOT be interpreted by any domain except the one listed in the
right side of the <addr-spec>. The exceptions are those specialized
services that conform to public, standardized conventions, as noted
below.
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A few types of addresses elaborate on basic email addressing, with a
standardized, global schema for the <local-part>, Include are
conventions between authoring systems and Gateways. They are
invisible to the public email transfer infrastructure. When an
Author is explicitly sending through a Gateway out of the Internet,
coding conventions for the <local-part> allow the Author to formulate
instructions for the Gateway. Standardized examples of such
conventions are the telephone numbering formats for VPIM [RFC3801],
such as:
+16137637582@vpim.example.com
and iFax [RFC3192], such as:
FAX=+12027653000/T33S=1387@ifax.example.com
3.2. Scope of Email Address Use
Email addresses are being used far beyond their original role in
email transfer and delivery. In practical terms, an email address
string has become the common identifier for representing online
identity. Hence, it is essential to be clear about both the nature
and role of an identity string in a particular context and the entity
responsible for setting that string. For example, see Section 4.1.4,
Section 4.3.3 and Section 5.
3.3. Domain Names
A domain name is a global reference to an Internet resource, such as
a host, a service, or a network. A domain name usually maps to one
or more IP Addresses. Conceptually, the name can encompass an
organization, a collection of machines integrated into a homogeneous
service, or a single machine. A domain name can be administered to
refer to individual users, but this is not common practice. The name
is structured as a hierarchical sequence of names, separated by dots
(.), with the top of the hierarchy being on the right end of the
sequence. Domain names are defined and operated through the Domain
Name System (DNS) [RFC1034], [RFC1035], [RFC2181].
When not part of a mailbox address, a domain name is used in Internet
Mail to refer to the ADMD or to the host that took action upon the
message, such as providing the administrative scope for a message
identifier or performing transfer processing.
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3.4. Message Identifier
There are two standardized tags for identifying messages: Message-ID:
and ENVID. A Message-ID: pertains to content, and an ENVID pertains
to transfer.
3.4.1. Message-ID
Internet Mail standards provide for, at most, a single Message-ID:.
The Message-ID: for a single message, which is a user-level tag, has
a variety of uses including threading, aiding identification of
duplicates, and DSN tracking. [RFC2822]. The Originator assigns the
Message-ID:. The Recipient's ADMD is the intended consumer of the
Message-ID:, although any actor along the transfer path can use it.
Message-ID: MUST be globally unique. Its format is similar to that
of a mailbox, with two distinct parts, separated by an at-sign (@).
Typically, the right side specifies the ADMD or host that assigns the
identifier, and the left side contains a string that is globally
opaque and serves to uniquely identify the message within the domain
referenced on the right side. The duration of uniqueness for the
message identifier is undefined.
When a message is revised in any way, the decision whether to assign
a new Message-ID: requires a subjective assessment to determine
whether the editorial content has been changed enough to constitute a
new message. [RFC2822] states that "a message identifier pertains to
exactly one instantiation of a particular message; subsequent
revisions to the message each receive new message identifiers." Yet
experience suggests that some flexibility is needed. An impossible
test is whether the recipient will consider the new message to be
equivalent to the old one. For most components of Internet Mail,
there is no way to predict a specific recipient's preferences on this
matter. Both creating and failing to create a new Message-ID: have
their downsides.
Here are some guidelines and examples:
* If a message is changed only in form, such as character-
encoding, it is still the same message.
* If a message has minor additions to the content, such as a
mailing list tag at the beginning of the RFC2822.Subject header
field, or some mailing list administrative information added to
the end of the primary body-part text, it is probably the same
message.
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* If a message has viruses deleted from it, it is probably the
same message.
* If a message has offensive words deleted from it, some
recipients will consider it the same message, but some will
not.
* If a message is translated into a different language, some
recipients will consider it the same message, but some will
not.
* If a message is included in a digest of messages, the digest
constitutes a new message.
* If a message is forwarded by a recipient, what is forwarded is
a new message.
* If a message is "redirected", such as using RFC2822 "Resent-*"
header fields, some recipients will consider it the same
message, but some will not.
The absence of both objective, precise criteria for re-generating a
Message-ID: and strong protection associated with the string means
that the presence of an ID can permit an assessment that is
marginally better than a heuristic, but the ID certainly has no value
on its own for strict formal reference or comparison. For that
reason, the Message-ID: SHOULD NOT be used for any function that has
security implications.
3.4.2. ENVID
The ENVID (envelope identifier) can be used for message-tracking
purposes [RFC3885] concerning a single posting/delivery transfer.
The ENVID labels a single transit of the MHS by a specific message.
So, the ENVID is used for one message posting, until that message is
delivered. A re-posting of the message, such as by a Mediator, does
not re-use that ENVID, but can use a new one, even though the message
might legitimately retain its original Message-ID:.
The format of an ENVID is free form. Although its creator might
choose to impose structure on the string, none is imposed by Internet
standards. By implication, the scope of the string is defined by the
domain name of the Return Address.
4. Services and Standards
The Internet Mail architecture comprises six basic types of
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functionality, which are arranged to support a store-and-forward
service. As shown in Figure 5, each type can have multiple
instances, some of which represent specialized roles. This section
considers the activities and relationships among these components,
and the Internet Mail standards that apply to them.
Message
Mail User Agent (MUA)
Author MUA (aMUA)
Recipient MUA (rMUA)
Message Submission Agent (MSA)
Author-focused MSA functions (aMSA)
MHS-focused MSA functions (hMSA)
Message Transfer Agent (MTA)
Message Delivery Agent (MDA)
Recipient-focused MDA functions (rMDA)
MHS-focused MDA functions (hMDA)
Message Store (MS)
Author MS (aMS)
Recipient MS (rMS)
This figure shows function modules and the standardized protocols
used between them.
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++========++
|| || +-------+
...........++ aMUA ||<............................+ Disp |
. || || +-------+
. ++=+==+===++ ^
. local,imap}| |{smtp,submission .
. +-----+ | | +--------+ .
. | aMS |<---+ | ........................>| Return | .
. +-----+ | . +--------+ .
. | . ***************** ^ .
. +-----V-.----*------------+ * . .
. MSA | +-------+ * +------+ | * . .
. | | aMSA +-(S)->| hMSA | | * . .
. | +-------+ * +--+---+ | * . .
V +------------*------+-----+ * . .
//==========\\ * V {smtp * . .
|| MESSAGE || * +------+ * //===+===\\ .
||----------|| MHS * | MTA | * || dsn || .
|| Envelope || * +--+---+ * \\=======// .
|| SMTP || * V {smtp * ^ ^ .
|| Content || * +------+ * . . //==+==\\
|| RFC2822 || * | MTA +....*...... . || mdn ||
|| MIME || * +--+---+ * . \\=====//
\\==========// * smtp}| {local * . ^
. MDA * | {lmtp * . .
. +----------------+------V-----+ * . .
. | +----------+ * +------+ | * . .
. | | | * | | +..*.......... .
. | | rMDA |<-(D)--+ hMDA | | * .
. | | | * | | |<.*........ .
. | +-+------+-+ * +------+ | * . .
. +------+---------*------------+ * . .
. | ***************** . .
. V{smtp,imap,pop,local . .
. +-----+ //===+===\\ .
. | rMS | || sieve || .
. +--+--+ \\=======// .
. |{imap,pop,local ^ .
. V . .
. ++==========++ . .
. || || . .
.......>|| rMUA ++........................... .
|| ++...................................
++==========++
Figure 5: Protocols and Services
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4.1. Message Data
The purpose of the Mail Handling Service (MHS) is to exchange a
message object among participants [RFC2822], [RFC0822]. All of its
underlying mechanisms serve to deliver that message from its Author
to its Recipients. A message can be explicitly labeled as to its
nature [RFC3458].
A message comprises a transit-handling envelope and the message
content. The envelope contains information used by the MHS. The
content is divided into a structured header and the body. The header
comprises transit handling trace information and structured fields
that are part of the Author's message content. The body can be
unstructured lines of text or a tree of multi-media subordinate
objects, called "body-parts" or attachments [RFC2045], [RFC2046],
[RFC2047], [RFC4288], [RFC4289], [RFC2049].
In addition, Internet Mail has a few conventions for special control
data, notably:
Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) is a message that can be
generated by the MHS (MSA, MTA, or MDA) and sent to the
RFC2821.MailFrom address. An MDA and MTA are shown as sources
of DSNs in Figure 5, and the destination is shown as Returns.
DSNs provide information about message transit, such as
transfer errors or successful delivery. [RFC3461]
Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) is a message that
provides information about post-delivery processing, such as
indicating that the message has been displayed [RFC3798] or the
form of content that can be supported [RFC3297]. It can be
generated by an rMUA and is sent to the Disposition-
Notification-To addresses. The mailbox for this is shown as
Disp in Figure 5.
Message Filtering (SIEVE):
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Sieve is a scripting language used to specify conditions for
differential handling of mail, typically at the time of
delivery [RFC5228]. Scripts can be conveyed in a variety of
ways, as a MIME part. Figure 5 shows a Sieve script going
from the rMUA to the MDA. However, filtering can be done at
many different points along the transit path, and any one or
more of them might be subject to Sieve directives, especially
within a single ADMD. the Figure 5 shows only one relationship,
for (relative) simplicity.
4.1.1. Envelope
Internet Mail has a fragmented framework for transit-related handling
information. Information that is used directly by the MHS is called
the "envelope." It directs handling activities by the transfer
service and is carried in transfer service commands. That is, the
envelope exists in the transfer protocol SMTP. [RFC2821]
Trace information, such as RFC2822.Received, is recorded in the
message header and is not subsequently altered. [RFC2822]
4.1.2. Header Fields
Header fields are attribute name/value pairs that cover an extensible
range of email service parameters, structured user content, and user
transaction meta-information. The core set of header fields is
defined in [RFC2822], [RFC0822]. It is common practice to extend
this set for different applications. Procedures for registering
header fields are defined in [RFC3864]. An extensive set of existing
header field registrations is provided in [RFC4021].
One danger of placing additional information in header fields is that
Gateways often alter or delete them.
4.1.3. Body
The body of a message might be lines of ASCII text or a
hierarchically structured composition of multi-media body-part
attachments, using MIME. [RFC2045], [RFC2046], [RFC2047], [RFC4288],
[RFC2049]
4.1.4. Identity References in a Message
Table 1 lists the core identifiers present in a message during
transit.
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+----------------------+----------------+---------------------------+
| Layer | Field | Set By |
+----------------------+----------------+---------------------------+
| Message Body | MIME Header | Author |
| Message header | From: | Author |
| fields | | |
| | Sender: | Originator |
| | Reply-To: | Author |
| | To:, CC:, BCC: | Author |
| | Message-ID: | Originator |
| | Received: | Originator, Relay, |
| | | Receiver |
| | Return-Path: | MDA, from MailFrom |
| | Resent-*: | Mediator |
| | List-Id: | Mediator |
| | List-*: | Mediator |
| SMTP | HELO/EHLO | Latest Relay Client |
| | ENVID | Originator |
| | MailFrom | Originator |
| | RcptTo | Author |
| | ORCPT | Author |
| IP | Source Address | Latest Relay Client |
+----------------------+----------------+---------------------------+
Table 1: Layered Identities
These are the most common address-related fields:
RFC2822.From: Set by - Author
Names and addresses for authors of the message content are
listed in the From: field.
RFC2822.Reply-To: Set by - Author
If a Recipient sends a reply message that would otherwise use
the RFC2822.From field addresses in the original message, the
addresses in the RFC2822.Reply-To field are used instead. In
other words, this field overrides the From: field for responses
from Recipients.
RFC2822.Sender: Set by - Originator
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This field specifies the address responsible for submitting the
message to the transfer service. This field can be omitted if
it contains the same address as RFC2822.From. However,
omitting this field does not mean that no Sender is specified;
it means that that header field is virtual and that the address
in the From: field MUST be used.
Specification of the notifications Return addresses, which are
contained in RFC2821.MailFrom, is made by the RFC2822.Sender.
Typically the Return address is the same as the Sender address.
However, some usage scenarios require it to be different.
RFC2822.To/.CC: Set by - Author
These fields specify MUA Recipient addresses. However, some or
all of the addresses in these fields might not be present in
the RFC2821.RcptTo commands.
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 as a courtesy.
RFC2822.BCC: Set by - Author
A copy of the message might be sent to an addressee whose
participation is not to be disclosed to the RFC2822.To or
RFC2822.CC Recipients and, usually, not to the other BCC
Recipients. The BCC: header field indicates a message copy to
such a Recipient. Use of this field is discussed in [RFC2822].
RFC2821.HELO/.EHLO: Set by - Originator, MSA, MTA
Any SMTP client -- including Originator, MSA, or MTA -- can
specify its hosting domain identity for the SMTP HELO or EHLO
command operation.
RFC3461.ENVID: Set by - Originator
The MSA can specify an opaque string, to be included in a DSN,
as a means of assisting the Return address recipient in
identifying the message that produced a DSN or message
tracking.
RFC2821.MailFrom: Set by - Originator
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This field is an end-to-end string that specifies an email
address for receiving return control information, such as
returned messages. The name of this field is misleading,
because it is not required to specify either the Author or the
actor responsible for submitting the message. Rather, the
actor responsible for submission specifies the RFC2821.MailFrom
address. Ultimately, the simple basis for deciding which
address needs to be in the RFC2821.MailFrom field is to
determine which address must be informed about transfer-level
problems (and possibly successes.)
RFC2821.RcptTo: Set by - Author, Final MTA, MDA.
This field specifies the MUA mailbox address of a Recipient.
The string might not be visible in the message content header.
For example, the message destination address header fields,
such as RFC2822.To, might specify a mailing list mailbox, while
the RFC2821.RcptTo address specifies a member of that list.
RFC2821.ORCPT: Set by - Author.
This is an optional parameter to the RCPT command, indicating
the original address to which the current RCPT TO address
corresponds, after a mapping was performed during transit. An
ORCPT is the only reliable way to correlate a DSN from a multi-
recipient message transfer with the intended recipient.
RFC2821.Received: Set by - Originator, Relay, Mediator, Dest
This field contains trace information, including originating
host, Relays, Mediators, and MSA host domain names and/or IP
Addresses.
RFC2821.Return-Path: Set by - Originator
The MDA records the RFC2821.MailFrom address into the
RFC2822.Return-Path field.
RFC2919.List-Id: Set by - Mediator Author
This field provides a globally unique mailing list naming
framework that is independent of particular hosts. [RFC2919]
The identifier is in the form of a domain name; however, the
string usually is constructed by combining the two parts of an
email address. The result is rarely a true domain name, listed
in the domain name service, although it can be.
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RFC2369.List-*: Set by - Mediator Author
[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. [RFC2369]
RFC0791.SourceAddr: Set by - The Client SMTP sending host
immediately preceding the current receiving SMTP server.
[RFC0791] defines the basic unit of data transfer for the
Internet: the IP Datagram. It contains a Source Address field
that specifies the IP Address for the host (interface) from
which the datagram was sent. This information is set and
provided by the IP layer, which makes it independent of mail-
level mechanisms. As such, it is often taken to be
authoritative, although it is possible to provide false
addresses.
4.2. User-Level Services
Interactions at the user level entail protocol exchanges, distinct
from those that occur at lower layers of the Internet Mail MHS
architecture that is, in turn, above the Internet Transport layer.
Because the motivation for email, and much of its use, is for
interaction among people, the nature and details of these protocol
exchanges often are determined by the needs of interpersonal and
group communication. To accommodate the idiosyncratic behavior
inherent in such communication, only subjective guidelines, rather
than strict rules, can be offered for some aspects of system
behavior. Mailing Lists provide particularly salient examples.
4.2.1. Mail User Agent (MUA)
A Mail User Agent (MUA) works on behalf of User actors and User
applications. It is their representative within the email service.
The Author MUA (aMUA) creates a message and performs initial
submission into the transfer infrastructure via a Mail Submission
Agent (MSA). It can also perform any creation- and posting-time
archival in its Message Store (aMS). An MUA aMS can organize
messages in many different ways. A common model uses aggregations,
called "folders". This model allows a folder for messages under
development (Drafts), a folder for messages waiting to be sent
(Queued or Unsent), and a folder for messages that have been
successfully posted for transfer (Sent). But none of these folders
is required. For example, IMAP allows drafts to be stored in any
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folder; so no Drafts folder is present.
The Recipient MUA (rMUA) works on behalf of the Recipient to process
received mail. This processing includes generating user-level
disposition control messages, displaying and disposing of the
received message, and closing or expanding the user communication
loop by initiating replies and forwarding new messages.
NOTE: Although not shown in Figure 5, an MUA itself can have a
distributed implementation, such as a "thin" user interface
module on a constrained device such as a smartphone, with most
of the MUA functionality running remotely on a more capable
server. An example of such an architecture might use IMAP
[RFC3501] for most of the interactions between an MUA client
and an MUA server. An approach for such scenarios is defined
by [RFC4550].
A Mediator is special class of MUA. It performs message re-posting,
as discussed in Section 2.1.
An MUA can be automated, on behalf of a user who is not present at
the time the MUA is active. One example is a bulk sending service
that has a timed-initiation feature. These services are not to be
confused with a mailing list Mediator, since there is no incoming
message triggering the activity of the automated service.
A popular and problematic MUA is an automatic responder, such as one
that sends out-of-office notices. This behavior might be confused
with that of a Mediator, but this MUA is generating a new message.
Automatic responders can annoy users of mailing lists unless they
follow [RFC3834]. ****** The recommendations in RFC 3834 are an
important consequence of the addressing architecture of Internet Mail
so they do help illustrate the architecture. *****
These identity fields are relevant to a typical MUA:
RFC2822.From
RFC2822.Reply-To
RFC2822.Sender
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RFC2822.To, RFC2822.CC
RFC2822.BCC
4.2.2. Message Store (MS)
An MUA can employ a long-term Message Store (MS). Figure 5 depicts
an Author's MS (aMS) and a Recipient's MS (rMS). An MS can be
located on a remote server or on the same machine as the MUA.
An MS acquires messages from an MDA either by a local mechanism or by
using POP or IMAP. The MUA accesses the MS either by a local
mechanism or by using POP or IMAP. Using POP for message access,
rather than bulk transfer, is rare, awkward, and largely non-
standard.
4.3. MHS-Level Services
4.3.1. Mail Submission Agent (MSA)
A Mail Submission Agent (MSA) accepts the message submitted by the
aMUA and enforces the policies of the hosting ADMD and the
requirements of Internet standards. An MSA represents an unusual
functional dichotomy. It represents the interests of the Author
(aMUA) during message posting, to facilitate posting success; it also
represents the interests of the MHS. In the architecture, these
responsibilities are modeled, as shown in Figure 5, by dividing the
MSA into two sub-components, aMSA and hMSA, respectively. Transfer
of responsibility for a single message, from an Author's environment
to the MHS, is called "posting". In Figure 5 it is marked as the (S)
transition, within the MSA.
The hMSA takes transit responsibility for a message that conforms to
the relevant Internet standards and to local site policies. It
rejects messages that are not in conformance. The MSA performs final
message preparation for submission and effects the transfer of
responsibility to the MHS, via the hMSA. The amount of preparation
depends upon the local implementations. Examples of oMSA tasks
include adding header fields, such as Date: and Message-ID:, and
modifying portions of the message from local notations to Internet
standards, such as expanding an address to its formal RFC2822
representation.
Historically, standards-based MUA/MSA message postings have used
SMTP. [RFC2821] The standard currently preferred is SUBMISSION.
[RFC4409] Although SUBMISSION derives from SMTP, it uses a separate
TCP port and imposes distinct requirements, such as access
authorization.
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These identities are relevant to the MSA:
RFC2821.HELO/.EHLO
RFC3461.ENVID
RFC2821.MailFrom
RFC2821.RcptTo
RFC2821.Received
RFC0791.SourceAddr
4.3.2. Mail Transfer Agent (MTA)
A Mail Transfer Agent (MTA) relays mail for one application-level
"hop." It is like a packet-switch or IP router in that its job is to
make routing assessments and to move the message closer to the
Recipients. Of course, email objects are typically much larger than
the payload of a packet or datagram, and the end-to-end latencies are
typically much higher. Relaying is performed by a sequence of MTAs,
until the message reaches a destination MDA. Hence, an MTA
implements both client and server MTA functionality; it does not
change addresses in the envelope or reformulate the editorial
content. A change in data form, such as to MIME Content-Transfer-
Encoding, is within the purview of an MTA, but removal or replacement
of body content is not. An MTA also adds trace information.
[RFC2505]
NOTE: Within a destination ADMD, email relaying modules can
make a variety of changes to the message, prior to delivery.
In such cases, these modules are acting as Gateways, rather
than MTAs.
Internet Mail uses SMTP [RFC2821], [RFC0821] primarily to effect
point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]. As with
most network layer mechanisms, the Internet Mail SMTP supports a
basic level of reliability, by virtue of providing for retransmission
after a temporary transfer failure. Unlike typical packet switches
(and Instant Messaging services), Internet Mail MTAs are expected to
store messages in a manner that allows recovery across service
interruptions, such as host system shutdown. The degree of such
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robustness and persistence by an MTA can vary. The base SMTP
specification provides a framework for protocol response codes. An
extensible enhancement to this framework is defined in [RFC5248]
The primary routing mechanism for Internet Mail is the DNS MX record
[RFC1035], which specifies an MTA through which the queried domain
can be reached. This mechanism presumes a public, or at least a
common, backbone that permits any attached MTA to connect to any
other.
MTAs can perform any of these well-established roles:
Boundary MTA: An MTA that is part of an ADMD and interacts with
MTAs in other ADMDs. This is also called a Border MTA. There
can be different Boundary MTAs, according to the direction of
mail-flow.
Outbound MTA: An MTA that relays messages to other ADMDs.
Inbound MTA: An MTA that receives inbound SMTP messages from
MTA Relays in other ADMDs, for example, an MTA running on
the host listed as the target of an MX record.
Final MTA: The MTA that transfers a message to the MDA.
These identities are relevant to the MTA:
RFC2821.HELO/.EHLO
RFC3461.ENVID
RFC2821.MailFrom
RFC2821.RcptTo
RFC2822.Received: Set by - Relay Server
RFC0791.SourceAddr
4.3.3. Mail Delivery Agent (MDA)
A transfer of responsibility from the MHS to a Recipient's
environment (mailbox) is called "delivery." In the architecture, as
depicted in Figure 5, delivery takes place within a Mail Delivery
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Agent (MDA) and is shown as the (D) transition from the MHS-oriented
MDA component (hMDA) to the Recipient-oriented MDA component (rMDA).
An MDA can provide distinctive, address-based functionality, made
possible by its detailed information about the properties of the
destination address. This information might also be present
elsewhere in the Recipient's ADMD, such as at an organizational
border (Boundary) Relay. However, it is required for the MDA, if
only because the MDA is required to know where to deliver the
message.
Like an MSA, an MDA serves two roles, as depicted in Figure 5.
Formal transfer of responsibility, called "delivery", is effected
between the two components that embody these roles as shows as "(D)"
in Figure 5. The MHS portion (hMDA) primarily functions as a server
SMTP engine. A common additional role is to re-direct the message to
an alternative address, as specified by the recipient addressee's
preferences. The job of the recipient portion of the MDA (rMDA) is
to perform any delivery actions that the Recipient specifies.
Transfer into the MDA is accomplished by a normal MTA transfer
mechanism. Transfer from an MDA to an MS uses an access protocol,
such as POP or IMAP.
NOTE: The term "delivery" can refer to the formal, MHS function
specified here or to the first time a message is displayed to a
Recipient. A simple, practical test for whether the MHS-based
definition applies is whether a DSN can be generated.
These identities are relevant to the MDA:
RFC2821.Return-Path: Set by - Author Originator or Mediator
Originator
The MDA records the RFC2821.MailFrom address into the
RFC2822.Return-Path field.
RFC2822.Received: Set by - MDA server
An MDA can record a Received: header field to indicate trace
information, including source host and receiving host domain
names and/or IP Addresses.
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4.4. Transition Modes
From the origination site to the point of delivery, Internet Mail
usually follows a "push" model. That is, the actor that holds the
message initiates transfer to the next venue, typically with SMTP
[RFC2821] or LMTP [RFC2033]. With a "pull" model, the actor that
holds the message waits for the actor in the next venue to initiate a
request for transfer. Standardized mechanisms for pull-based MHS
transfer are ETRN [RFC1985] and ODMR [RFC2645].
After delivery, the Recipient's MUA (or MS) can gain access by having
the message pushed to it or by having the receiver of access pull the
message, such as by using POP [RFC1939] and IMAP [RFC3501].
4.5. Implementation and Operation
A discussion of any interesting system architecture often bogs down
when architecture and implementation are confused. An architecture
defines the conceptual functions of a service, divided into discrete
conceptual modules. An implementation of that architecture can
combine or separate architectural components, as needed for a
particular operational environment. For example, a software system
that primarily performs message relaying is an MTA, yet it might
also include MDA functionality. That same MTA system might be able
to interface with non-Internet email services and thus perform both
as an MTA and as a Gateway.
Similarly, implemented modules might be configured to form
elaborations of the architecture. An interesting example is a
distributed MS. One portion might be a remote server and another
might be local to the MUA. As discussed in [RFC1733], there are
three operational relationships among such MSs:
Online: The MS is remote, and messages are accessible only when
the MUA is attached to the MS so that the MUA will re-fetch all
or part of a message, from one session to the next.
Offline: The MS is local to the user, and messages are
completely moved from any remote store, rather than (also)
being retained there.
Disconnected: An rMS and a uMS are kept synchronized, for all or
part of their contents, while they are connected. When they
are disconnected, mail can arrive at the rMS and the user can
make changes to the uMS. The two stores are re-synchronized
when they are reconnected.
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5. Mediators
Basic message transfer from Author to Recipients is accomplished by
using an asynchronous store-and-forward communication infrastructure
in a sequence of independent transmissions through some number of
MTAs. A very different task is a sequence of postings and deliveries
through Mediators. A Mediator forwards a message, through a re-
posting process. The Mediator shares some functionality with basic
MTA relaying, but has greater flexibility in both addressing and
content than is available to MTAs.
This is the core set of message information that is commonly set by
all types of Mediators:
RFC2821.HELO/.EHLO: Set by - Mediator Originator
RFC3461.ENVID: Set by - Mediator Originator
RFC2821.RcptTo: Set by - Mediator Author
RFC2821.Received: Set by - Mediator Dest
The Mediator can record received information, to indicate the
delivery to the original address and submission to the alias
address. The trace of Received: header fields can include
everything from original posting, through relaying, to final
delivery.
The aspect of a Mediator that distinguishes it from any other MUA
creating a message is that a Mediator preserves the integrity and
tone of the original message, including the essential aspects of its
origination information. The Mediator might also add commentary.
Examples of MUA messages that a Mediator does not create include:
New message that forwards an existing message:
Although this action provides a basic template for a class of
Mediators, its typical occurrence is not, itself, an example of
a Mediator. The new message is viewed as being from the actor
that is doing the forwarding, rather than from the original
Author.
A new message encapsulates the original message and is seen as
from the new Originator. This Mediator Originator might add
commentary and can modify the original message content.
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Because the forwarded message is a component of the message
sent by the new Originator, the new message creates a new
dialogue. However the final Recipient still sees the contained
message as from the original Author.
Reply:
When a Recipient responds to the Author of a message, the new
message is not typically viewed as a forwarding of the
original. Its focus is the new content, although it might
contain all or part of the material from the original message.
The earlier material is merely contextual and secondary. This
includes automated replies, such as vacation out-of-office
notices, as discussed in Section 4.2.1.
Annotation:
The integrity of the original message is usually preserved, but
one or more comments about the message are added in a manner
that distinguishes commentary from original text. The primary
purpose of the new message is to provide commentary from a new
Author, similar to a Reply.
The remainder of this section describes common examples of
Mediators.
5.1. Alias
One function of an MDA is to determine the internal location of a
mailbox in order to perform delivery. An Alias is a simple re-
addressing facility that provides one or more new Internet Mail
addresses, rather than a single, internal one; the message continues
through the transfer service, for delivery to one or more alternate
addresses. Although typically implemented as part of an MDA, this
facility is a Recipient function. It resubmits the message, although
all handling information except the envelope recipient
(rfc2821.RcptTo) address is retained. In particular, the Return
address (rfc2821.MailFrom) is unchanged.
What is distinctive about this forwarding mechanism is how closely it
resembles normal MTA store-and-forward relaying. Its only
significant difference is that it changes the RFC2821.RcptTo value.
Because this change is so small, aliasing can be viewed as a part of
the lower-level mail relaying activity. However, this small change
has a large semantic impact: The designated recipient has chosen a
new recipient.
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NOTE: When the replacement list includes more than one address,
the alias is increasingly likely to have delivery problems.
Any problem reports go to the original Author, not the
administrator of the alias entry. This makes it more difficult
to resolve the problem, because the original Author has no
knowledge of the Alias mechanism.
Alias typically changes only envelope information:
RFC2822.To/.CC/.BCC: Set by - Author
These fields retain their original addresses.
RFC2821.MailFrom: Set by - Author
The benefit of retaining the original MailFrom value is to
ensure that an actor related to the originating ADMD knows
there has been a delivery problem. On the other hand, the
responsibility for handling problems, when transiting from the
original recipient mailbox to the alias mailbox usually lies
with that original Recipient, because the Alias mechanism is
strictly under that Recipient's control. Retaining the
original MailFrom address prevents this.
5.2. ReSender
Also called the ReDirector, the ReSender's actions differ from
forwarding because the Mediator "splices" a message's addressing
information to connect the Author of the original message with the
Recipient of the new message. This connection permits them to have
direct exchange, using their normal MUA Reply functions, while also
recording full reference information about the Recipient who served
as a Mediator. Hence, the new Recipient sees the message as being
from the original Author, even if the Mediator adds commentary.
These identities are relevant to a resent message:
RFC2822.From: Set by - original Author
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Names and addresses for the original Author of the message
content are retained. The free-form (display-name) portion of
the address might be modified to provide informal reference to
the ReSender.
RFC2822.Reply-To: Set by - original Author
If this field is present in the original message, it is
retained in the resent message.
RFC2822.Sender: Set by - Author's Originator or Mediator
Originator.
RFC2822.To/.CC/.BCC: Set by - original Author
These fields specify the original message Recipients.
RFC2822.Resent-From: Set by - Mediator Author
This address is of the original Recipient who is redirecting
the message. Otherwise, the same rules apply to the Resent-
From: field as to an original RFC2822.From field.
RFC2822.Resent-Sender: Set by - Mediator Originator
The address of the actor responsible for resubmitting the
message. As with RFC2822.Sender, this field can be omitted
when it contains the same address as RFC2822.Resent-From.
RFC2822.Resent-To/-CC/-BCC: Set by: Mediator Author
The addresses of the new Recipients who are now able to reply
to the original author.
RFC2821.MailFrom: Set by - Mediator Originator
The actor responsible for resubmission (RFC2822.Resent-Sender)
is also responsible for specifying the new MailFrom address.
5.3. Mailing Lists
A Mailing List receives messages as an explicit addressee and then
re-posts them to a list of subscribed members. The Mailing List
performs a task that can be viewed as an elaboration of the ReSender.
In addition to sending the new message to a potentially large number
of new Recipients, the Mailing List can modify content, for example,
by deleting attachments, converting the format, and adding list-
specific comments. Mailing Lists also archive messages posted by
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Authors. Still the message retains characteristics of being from the
original Author.
These identities are relevant to a mailing list processor, when
submitting a message:
RFC2919.List-Id: Set by - Mediator Author
RFC2369.List-*: Set by - Mediator Author
RFC2822.From: Set by - original Author
Names and email addresses for the original Author of the
message content are retained.
RFC2822.Reply-To: Set by - Mediator or original Author
Although problematic, it is common for a Mailing List to assign
its own addresses to the Reply-To: header field of messages
that it posts. This assignment is intended to ensure that
replies go to all list members, rather than to only the
original Author. As a User actor, a Mailing List is the Author
of the new message and can legitimately set the Reply-To:
value. As a Mediator attempting to represent the message on
behalf of its original Author, creating or modifying a
Reply-To: field can be viewed as violating that Author's
intent. Modifying the field to include the list address can
send to the entire list replies that are meant only for the
original Author. When the Mailing List does not set the field,
a reply meant for the entire list can instead go only to the
original Author. At best, either choice is a matter of group
culture for the particular list.
RFC2822.Sender: Set by - Author Originator or Mediator
Originator
This field usually specifies the address of the actor
responsible for Mailing List operations. Mailing Lists that
operate in a manner similar to a simple MTA Relay preserve as
much of the original handling information as possible,
including the original RFC2822.Sender field. (Note that this
mode of operation causes the Mailing List to behave much like
an Alias, with a possible difference in number of new
addressees.)
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RFC2822.To/.CC: Set by - original Author
These fields usually contain the original list of Recipient
addresses.
RFC2821.MailFrom: Set by - Mediator Originator
Because a Mailing List can modify the content of a message in
any way, it is responsible for that content; that is, it is an
Author. As such, the Return Address is specified by the
Mailing List. Although it is plausible for the Mailing List to
re-use the Return Address employed by the original Originator,
notifications sent to that address after a message has been
processed by a Mailing List could be problematic.
5.4. Gateways
A Gateway performs the basic routing and transfer work of message
relaying, but it also is permitted to modify content, structure,
address, or attributes as needed to send the message into a messaging
environment that operates under different standards or potentially
incompatible policies. When a Gateway connects two differing
messaging services, its role is easy to identify and understand.
When it connects environments that follow similar technical
standards, but significantly different administrative policies, it is
easy to view a Gateway as merely an MTA.
The critical distinction between an MTA and a Gateway is that a
Gateway can make substantive changes to a message to map between the
standards. In virtually all cases, this mapping results in some
degree of semantic loss. The challenge of Gateway design is to
minimize this loss. Standardized gateways to Internet Mail are
facsimile [RFC4143], voicemail [RFC3801], and MMS [RFC4356]
A Gateway can set any identity field available to an MUA. These
identities are typically relevant to Gateways:
RFC2822.From: Set by - original Author
Names and addresses for the original Author of the message
content are retained. As for all original addressing
information in the message, the Gateway can translate addresses
as required to continue to be useful in the target environment.
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RFC2822.Reply-To: Set by - original Author
The Gateway SHOULD retain this information, if it is present.
The ability to perform a successful reply by a Recipient is a
typical test of Gateway functionality.
RFC2822.Sender: Set by - Author Originator or Mediator
Originator
This field can retain the original value or can be set to a new
address.
RFC2822.To/.CC/.BCC: Set by - original Recipient
These fields usually retain their original addresses.
RFC2821.MailFrom: Set by - Author Originator or Mediator
Originator
The actor responsible for handling the message can specify a
new address to receive handling notices.
5.5. Boundary Filter
To enforce security boundaries, organizations can subject messages to
analysis, for conformance with its safety policies. An example is
detection of content classed as spam or a virus. A filter might
alter the content, to render it safe, such as by removing content
deemed unacceptable. Typically, these actions add content to the
message that records the actions.
6. Considerations
6.1. Security Considerations
This document describes the existing Internet Mail architecture. It
introduces no new capabilities. The security considerations of this
deployed architecture are documented extensively in the technical
specifications referenced by this document. These specifications
cover classic security topics, such as authentication and privacy.
For example, email transfer protocols can use standardized mechanisms
for operation over authenticated and/or encrypted links, and message
content has similar protection standards available. Examples of such
mechanisms include SMTP-TLS [RFC3207], SMTP-Auth [RFC2554], OpenPGP
[RFC4880], and S/MIME [RFC3851].
The core of the Internet Mail architecture does not impose any
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security requirements or functions on the end-to-end or hop-by-hop
components. For example, it does not require participant
authentication and does not attempt to prevent data disclosure.
Particular message attributes might expose specific security
considerations. For example, the blind carbon copy feature of the
architecture invites disclosure concerns, as discussed in section 7.2
of [RFC2821] and section 5 of [RFC2822]. Transport of text or non-
text content in this architecture has security considerations that
are discussed in [RFC2822], [RFC2045], [RFC2046], and [RFC4288] as
well as the security considerations present in the IANA media types
registry for the respective types.
Agents that automatically respond to email raise significant security
considerations, as discussed in [RFC3834]. Gateway behaviors affect
end-to-end security services, as discussed in [RFC2480]. Security
considerations for boundary filters are discussed in [RFC5228].
See section 7.1 of [RFC2821] for a discussion of the topic of
origination validation. As mentioned in Section 4.1.4, it is common
practice for components of this architecture to use the
[RFC0791].SourceAddr to make policy decisions [RFC2505], although the
address can be "spoofed". It is possible to use it without
authorization. SMTP and Submission authentication [RFC2554],
[RFC4409] provide more secure alternatives.
The discussion of trust boundaries, ADMDs, actors, roles, and
responsibilities in this document highlights the relevance and
potential complexity of security factors for operation of an Internet
mail service. The core design of Internet Mail to encourage open and
casual exchange of messages has met with scaling challenges, as the
population of email participants has grown to include those with
problematic practices. For example, spam, as defined in [RFC2505],
is a by-product of this architecture. A number of standards track or
BCP documents on the subject have been issued. [RFC2505], [RFC5068],
[RFC3685]
6.2. IANA Considerations
This document has no actions for IANA.
6.3. Internationalization
Because its origins date back to the use of ASCII, Internet Mail has
had an ongoing challenge to support the wide range of necessary
international data representations. For a discussion of this topic,
see [MAIL-I18N].
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7. References
7.1. Normative
[RFC0791] Postel, J., "Internet Protocol", RFC 791, 1981 September.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.
[RFC2645] "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
Addresses", RFC 2645, August 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
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April 2001.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC3192] Allocchio, C., "Minimal FAX address format in Internet
Mail", RFC 2304, October 2001.
[RFC3297] Klyne, G., Iwazaki, R., and D. Crocker, "Content
Negotiation for Messaging Services based on Email",
RFC 3297, July 2002.
[RFC3458] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
[RFC3501] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[RFC3798] Hansen, T. and G. Vaudreuil, "Message Disposition
Notification", RFC 3798, May 2004.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", RFC 3864,
September 2004.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, March 2005.
[RFC4288] Freed, N., Klensin, J., and J. Postel, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 4288, December 2005.
[RFC4289] Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 4289, December 2005.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
[RFC4550] Maes, S., , S., and Isode Ltd., "Internet Email to Support
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Diverse Service Environments (Lemonade) Profile",
June 2006.
[RFC5228] Showalter, T., "Sieve: A Mail Filtering Language",
RFC 5228.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", RFC 5248, June 2008.
7.2. Informative
[MAIL-I18N]
Internet Mail Consortium, "Using International Characters
in Internet Mail", IMC IMCR-010, August 1998.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1733] Crispin, M., "Distributed Electronic Models in IMAP4",
December 1994.
[RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects",
RFC 1767, March 1995.
[RFC1985] De Winter, J., "SMTP Service Extension for Remote
Message Queue Starting", August 1996.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[RFC2142] Crocker, D., "Mailbox Names for Common services, Roles and
Functions", RFC 2142, May 1997.
[RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998.
[RFC2480] Freed, N., "Gateways and MIME Security Multiparts",
RFC 2480, January 1999.
[RFC2505] Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs",
RFC 2505, February 1999.
[RFC2554] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
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Transport Layer Security", RFC 3207, February 2002.
[RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
Extensions", RFC 3685, February 2004.
[RFC3801] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
Mail - version 2 (VPIMv2)", RFC 3801, June 2004.
[RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC3885] Allman, E. and T. Hansen, "SMTP Service Extension for
Message Tracking", RFC 3885, September 2004.
[RFC4142] Crocker, D. and G. Klyne, "Full-mode Fax Profile for
Internet Mail: FFPIM", December 2005.
[RFC4143] Toyoda, K. and D. Crocker, "Facsimile Using Internet Mail
(IFAX) Service of ENUM", RFC 4143, November 2005.
[RFC4356] Gellens, R., "Mapping Between the Multimedia Messaging
Service (MMS) and Internet Mail", RFC 4356, January 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
E. Allman, "Email Submission Operations: Access and
Accountability Requirements", RFC 5068, BCP 134, Nov 2007.
[Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
"Tussle in Cyberspace: Defining Tomorrow's Internet",
ACM SIGCOMM, 2002.
Appendix A. Acknowledgements
This work derives from a section in an early version of [RFC5068].
Discussion of the Originator actor role was greatly clarified during
discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
insight on the framework and details of the original drafts, as did
Chris Newman for the final versions, while also serving as cognizant
Area Director for the document. Tony Hansen served as document
shepherd, through the IETF process.
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Later reviews and suggestions were provided by Eric Allman, Nathaniel
Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch,
Ned Freed, Eric Hall, Willemien Hoogendoorn, Brad Knowles, John
Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg,
Alexey Melnikov, der Mouse, S. Moonesamy, Daryl Odnert, Rahmat M.
Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, Greg
Vaudreuil.
Diligent early proof-reading was performed by Bruce Lilly. Diligent
technical editing was provided by Susan Hunziker
Index
10
A
accountability 11
accountable 12-13
Actor
Administrative 13
Author 8
Consumer 14
Edge 14
Gateway 12
Originator 11
Recipient 9
Return Handler 9
Transit 14
Actors
MHS 10
ADMD 11, 13-14, 18, 23, 29, 36
Administrative Actors 13
Administrative Management Domain 11
aMSA 29
Author 8, 10
author 33
B
body-parts 22
bounce handler 9
boundary 14
C
Consumer Actor 14
content 10, 12-13, 18, 22, 30
D
delivery 4, 9-10, 12-13, 17, 22-23, 33, 35-36
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Discussion of document 7
E
Edge Actor 14
end-to-end 4
envelope 9, 12, 19, 22-23, 30, 35-36
ETRN 33
G
Gateway 10, 12
H
header 22
hMSA 29
I
Internet Mail 4
L
LMTP 33
local-part 16
M
Mail 4
Mail User Agent 4
Mail From 35
Mail Handling Service 4, 10
Mail Submission Agent 11
Mail Transfer Agent 4
mailbox 35
MDA 35
MDN 9
message 6, 22
Message Disposition Notification 9
MHS 4, 9-12, 19-20, 22-23
Actors 10
MSA 11, 29
MTA 4, 14
boundary 14
MUA 4, 13, 28-29
O
ODMR 33
Originator 10-11
P
posting 4, 9, 11, 19, 28-29, 33, 36
pull 33
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push 33
R
RcptTo 10
Receiver 10
Recipient 9-10, 35
recipient 33
relay 10
responsibility 29
responsible 12-13
Return address 35
Return Handler 9
role 9, 17
Author 8
Originator 11
Recipient 9
S
SIEVE 22
SMTP 33
T
transfer 10, 12-13
Transit Actor 14
transition 29
U
UA 4
User Agent 4
Author's Address
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Crocker Expires May 4, 2009 [Page 49]
