< draft-crocker-email-arch-11.txt   draft-crocker-email-arch-12.txt >
SMTP D. Crocker SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track October 31, 2008 Intended status: Standards Track April 12, 2009
Expires: May 4, 2009 Expires: October 14, 2009
Internet Mail Architecture Internet Mail Architecture
draft-crocker-email-arch-11 draft-crocker-email-arch-12
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Abstract Abstract
Over its thirty-five year history, Internet Mail has changed Over its thirty-five year history, Internet Mail has changed
significantly in scale and complexity, as it has become a global significantly in scale and complexity, as it has become a global
infrastructure service. These changes have been evolutionary, rather infrastructure service. These changes have been evolutionary, rather
than revolutionary, reflecting a strong desire to preserve both its than revolutionary, reflecting a strong desire to preserve both its
installed base and its usefulness. To collaborate productively on installed base and its usefulness. To collaborate productively on
this large and complex system, all participants must work from a this large and complex system, all participants must work from a
common view of it and use a common language to describe its common view of it and use a common language to describe its
components and the interactions among them. But the many differences components and the interactions among them. But the many differences
in perspective currently make it difficult to know exactly what in perspective currently make it difficult to know exactly what
another participant means. To serve as the necessary common frame of another participant means. To serve as the necessary common frame of
reference, this document describes the enhanced Internet Mail reference, this document describes the enhanced Internet Mail
architecture, reflecting the current service. architecture, reflecting the current service.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Document Conventions . . . . . . . . . . . . . . . . . . . 6 1.2. Document Conventions . . . . . . . . . . . . . . . . . . . 7
2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 7 2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 8
2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 10 2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 11
2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 13 2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 14
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2. Scope of Email Address Use . . . . . . . . . . . . . . . . 17 3.2. Scope of Email Address Use . . . . . . . . . . . . . . . . 18
3.3. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 17 3.3. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 19
3.4. Message Identifier . . . . . . . . . . . . . . . . . . . . 18 3.4. Message Identifier . . . . . . . . . . . . . . . . . . . . 19
4. Services and Standards . . . . . . . . . . . . . . . . . . . . 19 4. Services and Standards . . . . . . . . . . . . . . . . . . . . 21
4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 22 4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 24
4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 27 4.1.4. Identity References in a Message . . . . . . . . . . . 25
4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 29 4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 29
4.4. Transition Modes . . . . . . . . . . . . . . . . . . . . . 33 4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 31
4.5. Implementation and Operation . . . . . . . . . . . . . . . 33 4.4. Transition Modes . . . . . . . . . . . . . . . . . . . . . 35
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.5. Implementation and Operation . . . . . . . . . . . . . . . 35
5.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . . . 35 5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.2. ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 37 5.2. ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 39 5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 40
5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 40 5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 41
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 40 5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 42
6.1. Security Considerations . . . . . . . . . . . . . . . . . 40 6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 42
6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 41 6.1. Security Considerations . . . . . . . . . . . . . . . . . 43
6.3. Internationalization . . . . . . . . . . . . . . . . . . . 41 6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 44
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42 6.3. Internationalization . . . . . . . . . . . . . . . . . . . 44
7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 42 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 44 7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 45
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 45 7.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 47
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 49
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Intellectual Property and Copyright Statements . . . . . . . . . . 49 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction 1. Introduction
Over its thirty-five year history, Internet Mail has changed Over its thirty-five year history, Internet Mail has changed
significantly in scale and complexity, as it has become a global significantly in scale and complexity, as it has become a global
infrastructure service. These changes have been evolutionary, rather infrastructure service. These changes have been evolutionary, rather
than revolutionary, reflecting a strong desire to preserve both its than revolutionary, reflecting a strong desire to preserve both its
installed base and its usefulness. Today, Internet Mail is installed base and its usefulness. Today, Internet Mail is
distinguished by many independent operators, many different distinguished by many independent operators, many different
components for providing service to users, as well as many different components for providing service to users, as well as many different
components that transfer messages. components that transfer messages.
The underlying technical standards for Internet Mail comprise a rich
array of functional capabilities. The specifications form the core:
* Simple Mail Transfer Protocol (SMTP) [RFC0821], [RFC2821],
[RFC5321] moves a message through the Internet.
* Internet Mail Format (IMF) [RFC0733], [RFC0822], [RFC2822],
[RFC5321] defines a message object.
* Multipurpose Internet Mail Extensions (MIME) [RFC2045] defines
an enhancement to the message object that permits using multi-
media attachments.
Public collaboration on technical, operations, and policy activities Public collaboration on technical, operations, and policy activities
of email, including those that respond to the challenges of email of email, including those that respond to the challenges of email
abuse, has brought a much wider range of participants into the abuse, has brought a much wider range of participants into the
technical community. To collaborate productively on this large and technical community. To collaborate productively on this large and
complex system, all participants must work from a common view of it complex system, all participants must work from a common view of it
and use a common language to describe its components and the and use a common language to describe its components and the
interactions among them. But the many differences in perspective interactions among them. But the many differences in perspective
currently make it difficult to know exactly what another participant currently make it difficult to know exactly what another participant
means. means.
skipping to change at page 4, line 4 skipping to change at page 5, line 19
* Clarifying identity-related issues, across the email service * Clarifying identity-related issues, across the email service
* Defining terminology for architectural components and their * Defining terminology for architectural components and their
interactions interactions
1.1. History 1.1. History
The first standardized architecture for networked email specified a The first standardized architecture for networked email specified a
simple split between the user world, in the form of Mail User Agents 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 (MUA), and the transfer world, in the form of the Mail Handling
Service (MHS), which is composed of Mail Transfer Agents (MTA). The Service (MHS), which is composed of Mail Transfer Agents (MTA)
MHS accepts a message from one User and delivers it to one or more [RFC1506]. The MHS accepts a message from one User and delivers it
other users, creating a virtual MUA-to-MUA exchange environment. 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 As shown in Figure 1, this defines two logical layers of
interoperability. One is directly between Users. The other is among interoperability. One is directly between Users. The other is among
the components along the transfer path. In addition, there is the components along the transfer path. In addition, there is
interoperability between the layers, first when a message is posted 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 from the User to the MHS and later when it is delivered from the MHS
to the User. to the User.
The operational service has evolved, although core aspects of the The operational service has evolved, although core aspects of the
service, such as mailbox addressing and message format style, service, such as mailbox addressing and message format style,
remaining remarkably constant. The original distinction between the remaining remarkably constant. The original distinction between the
user level and transfer level remains, but with elaborations in each. user level and transfer level remains, but with elaborations in each.
The term "Internet Mail" is used to refer to the entire collection of The term "Internet Mail" is used to refer to the entire collection of
user and transfer components and services. user and transfer components and services.
For Internet Mail, the term "end-to-end" usually refers to a single For Internet Mail, the term "end-to-end" usually refers to a single
posting and the set of deliveries that result from a single transit posting and the set of deliveries that result from a single transit
of the MHS. A common exception is group dialogue that is mediated, of the MHS. A common exception is group dialogue that is mediated,
through a Mailing List; in this case, two postings occur before through a Mailing List; in this case, two postings occur before
intended Recipients receive an Author's message, as discussed in intended Recipients receive an Author's message, as discussed in
Section 2.1.3. In fact, some uses of email consider the entire email Section 2.1.4. In fact, some uses of email consider the entire email
service, including Author and Recipient, as a subordinate component. service, including Author and Recipient, as a subordinate component.
For these services, "end-to-end" refers to points outside the email For these services, "end-to-end" refers to points outside the email
service. Examples are voicemail over email "[RFC3801], EDI over service. Examples are voicemail over email "[RFC3801], EDI over
email [RFC1767] and facsimile over email [RFC4142]. email [RFC1767] and facsimile over email [RFC4142].
+--------+ +--------+
++================>| User | ++================>| User |
|| +--------+ || +--------+
|| ^ || ^
+--------+ || +--------+ . +--------+ || +--------+ .
skipping to change at page 5, line 29 skipping to change at page 6, line 29
+---+-----------------+------+------+---+ +---+-----------------+------+------+---+
| . . . . | | . . . . |
| .................>. . . | | .................>. . . |
| . . . | | . . . |
| ........................>. . | | ........................>. . |
| . . | | . . |
| ...............................>. | | ...............................>. |
| | | |
| Mail Handling Service (MHS) | | Mail Handling Service (MHS) |
+---------------------------------------+ +---------------------------------------+
Legend: == lines indicate primary (possibly indirect) transfers; ...
lines indicate supporting transfers
Figure 1: Basic Internet Mail Service Model Figure 1: Basic Internet Mail Service Model
End-to-end Internet Mail exchange is accomplished by using a End-to-end Internet Mail exchange is accomplished by using a
standardized infrastructure with these components and standardized infrastructure with these components and
characteristics: characteristics:
* An email object * An email object
* Global addressing * Global addressing
* An asynchronous sequence of point-to-point transfer mechanisms * An asynchronous sequence of point-to-point transfer mechanisms
skipping to change at page 6, line 36 skipping to change at page 7, line 39
recent years it also has been required for message submission. In recent years it also has been required for message submission. In
these cases, a server validates the client's identity, whether by these cases, a server validates the client's identity, whether by
explicit security protocols or by implicit infrastructure queries to explicit security protocols or by implicit infrastructure queries to
identify "local" participants. identify "local" participants.
1.2. Document Conventions 1.2. Document Conventions
References to structured fields of a message use a two-part dotted References to structured fields of a message use a two-part dotted
notation. The first part cites the document that contains the notation. The first part cites the document that contains the
specification for the field and the second is the name of the field. 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 Hence <RFC5322.From> is the IMF From: header field in an email
header and <RFC2821.MailFrom> is the address in the SMTP "Mail From" content header and <RFC5321.MailFrom> is the address in the SMTP
command. "Mail From" command.
When occurring without the RFC2822 qualifier, header field names are When occurring without the IMF (rfc5322) qualifier, header field
shown with a colon suffix. For example, From:. names are shown with a colon suffix. For example, From:.
References to labels for actors, functions or components have the References to labels for actors, functions or components have the
first letter capitalized. first letter capitalized.
Also, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL Also, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 this document are to be interpreted as described in RFC 2119
[RFC2119] [RFC2119]. [RFC2119] [RFC2119].
RFC EDITOR: Remove the following paragraph before publication. RFC EDITOR: Remove the following paragraph before publication.
skipping to change at page 8, line 31 skipping to change at page 9, line 31
|| ..................+.......>.+ || ..................+.......>.+
|| . || .
|| ..............+.................. . || ..............+.................. .
|| . . . . || . . . .
\/ V V ' . \/ V V ' .
+-----------+ +-----------+ ++=====+=====++ . +-----------+ +-----------+ ++=====+=====++ .
| Mediator +===>| Mediator +===>|| Recipient || . | Mediator +===>| Mediator +===>|| Recipient || .
+-----+-----+ +-----+-----+ ++=====+=====++ . +-----+-----+ +-----+-----+ ++=====+=====++ .
. . . . . . . .
.................+.................+.......>.. .................+.................+.......>..
Legend: == lines indicate primary (possibly indirect) transfers; ...
lines indicate supporting transfers
Figure 2: Relationships Among User Actors Figure 2: Relationships Among User Actors
From the user perspective, all mail transfer activities are performed From the user perspective, all mail transfer activities are performed
by a monolithic Mail Handling Service (MHS), even though the actual by a monolithic Mail Handling Service (MHS), even though the actual
service can be provided by many independent organizations. Users are service can be provided by many independent organizations. Users are
customers of this unified service. customers of this unified service.
Whenever any MHS actor sends information to back to an Author or Whenever any MHS actor sends information to back to an Author or
Originator in the sequence of handling a message, that actor is a Originator in the sequence of handling a message, that actor is a
skipping to change at page 9, line 8 skipping to change at page 10, line 8
2.1.1. Author 2.1.1. Author
The Author is responsible for creating the message, its contents, and The Author is responsible for creating the message, its contents, and
its list of recipient addresses. The MHS transfers the message from its list of recipient addresses. The MHS transfers the message from
the Author and delivers it to the Recipients. The MHS has an the Author and delivers it to the Recipients. The MHS has an
Originator role (Section 2.2.1) that correlates with the Author role. Originator role (Section 2.2.1) that correlates with the Author role.
2.1.2. Recipient 2.1.2. Recipient
The Recipient is a consumer of the delivered message. The MHS has a The Recipient is a consumer of the delivered message. The MHS has a
Receiver role (Section 2.2.4)correlates with the Recipient role. Receiver role (Section 2.2.4) that correlates with the Recipient
This is labeled Recv in Figure 3. role. This is labeled Recv in Figure 3.
Any Recipient can close the user communication loop by creating and Any Recipient can close the user communication loop by creating and
submitting a new message that replies to the Author. An example of submitting a new message that replies to the Author. An example of
an automated form of reply is the Message Disposition Notification an automated form of reply is the Message Disposition Notification
(MDN), which informs the Author about the Recipient's handling of the (MDN), which informs the Author about the Recipient's handling of the
message. (See Section 4.1.) message. (See Section 4.1.)
The Return Handler, also called "Bounce Handler," receives and 2.1.3. Return Handler
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 Also called "Bounce Handler", the Return Handler is a special form of
Recipient tasked with servicing 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. 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.
2.1.4. Mediator
A Mediator receives, aggregates, reformulates, and redistributes A Mediator receives, aggregates, reformulates, and redistributes
messages among Authors and Recipients who are the principals in messages among Authors and Recipients who are the principals in
protracted exchanges. This activity is easily confused with the (potentially) protracted exchanges. This activity is easily confused
underlying MHS transfer exchanges. However, each serves very with the underlying MHS transfer exchanges. However, each serves
different purposes and operates in very different ways. very different purposes and operates in very different ways.
When mail is delivered to the Mediator specified in the When mail is delivered to the Mediator specified in the
RFC2821.RcptTo command, the MHS handles it the same way as for any RFC5321.RcptTo command for the original message, the MHS handles it
other Recipient. The MHS sees each posting and delivery activity the same way as for any other Recipient. In particular, the MHS sees
between sources and sinks as independent; it does not see subsequent each posting and delivery activity between sources and sinks as
re-posting as a continuation of a process. Because the Mediator independent; it does not see subsequent re-posting as a continuation
originates messages, it can receive replies. Hence, when submitting of a process. Because the Mediator originates messages, it can
messages, the Mediator is an Author. So a Mediator really is a full- receive replies. Hence, when submitting a reformulated message, the
fledged User. Mediators are considered extensively in Section 5. Mediator is an Author, albeit an author actually serving as an agent
of one or more other authors. 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 A Mediator attempts to preserve the original Author's information in
Mediator preserves the Author information of the message it the message it reformulates but is permitted to make meaningful
reformulates and is permitted to make meaningful changes to the changes to the message content or envelope. The MHS sees a new
message content or envelope. The MHS sees a new message, but users message, but users receive a message that they interpret as being
receive a message that they interpret as being from, or at least from, or at least initiated by, the Author of the original message.
initiated by, the Author of the original message. The role of a The role of a Mediator is not limited to merely connecting other
Mediator is not limited to merely connecting other participants; the participants; the Mediator is responsible for the new message.
Mediator is responsible for the new message.
A Mediator's role is complex and contingent, for example, modifying A Mediator's role is complex and contingent, for example, modifying
and adding content or regulating which users are allowed to and adding content or regulating which users are allowed to
participate and when. The common example of this role is a group participate and when. The common example of this role is a group
Mailing List. In a more complex use, a sequence of Mediators could Mailing List. In a more complex use, a sequence of Mediators could
perform a sequence of formal steps, such as reviewing, modifying, and perform a sequence of formal steps, such as reviewing, modifying, and
approving a purchase request. approving a purchase request.
A Gateway is a particularly interesting form of Mediator. It is a A Gateway is a particularly interesting form of Mediator. It is a
hybrid of User and Relay that connects heterogeneous mail services. hybrid of User and Relay that connects heterogeneous mail services.
Its purpose is to emulate a Relay. For a detailed discussion, see Its purpose is to emulate a Relay. For a detailed discussion, see
Section 2.2.3. . Section 2.2.3.
2.2. Mail Handling Service (MHS) Actors 2.2. Mail Handling Service (MHS) Actors
The Mail Handling Service (MHS) performs a single end-to-end transfer The Mail Handling Service (MHS) performs a single end-to-end transfer
on behalf of the Author to reach the Recipient addresses specified in on behalf of the Author to reach the Recipient addresses specified in
the original RFC2821.RcptTo commands. Exchanges that are either the original RFC5321.RcptTo commands. Exchanges that are either
mediated or iterative and protracted, such as those used for mediated or iterative and protracted, such as those used for
collaboration over time are handled by the User actors, not by the collaboration over time are handled by the User actors, not by the
MHS actors. MHS actors.
Figure 3 shows the relationships among transfer participants in Figure 3 shows the relationships among transfer participants in
Internet Mail. Although it shows the Originator (labeled Origin) as Internet Mail. Although it shows the Originator (labeled Origin) as
distinct from the Author and Receiver (labeled Recv) as distinct from distinct from the Author and Receiver (labeled Recv) as distinct from
Recipient, each pair of roles usually has the same actor. Recipient, each pair of roles usually has the same actor.
Transfers typically entail one or more Relays. However direct Transfers typically entail one or more Relays. However direct
delivery from the Originator to Receiver is possible. Intra- delivery from the Originator to Receiver is possible. Intra-
skipping to change at page 11, line 4 skipping to change at page 12, line 12
collaboration over time are handled by the User actors, not by the collaboration over time are handled by the User actors, not by the
MHS actors. MHS actors.
Figure 3 shows the relationships among transfer participants in Figure 3 shows the relationships among transfer participants in
Internet Mail. Although it shows the Originator (labeled Origin) as Internet Mail. Although it shows the Originator (labeled Origin) as
distinct from the Author and Receiver (labeled Recv) as distinct from distinct from the Author and Receiver (labeled Recv) as distinct from
Recipient, each pair of roles usually has the same actor. Recipient, each pair of roles usually has the same actor.
Transfers typically entail one or more Relays. However direct Transfers typically entail one or more Relays. However direct
delivery from the Originator to Receiver is possible. Intra- delivery from the Originator to Receiver is possible. Intra-
organization mail services usually have only one Relay. organization mail services usually have only one Relay.
++==========++ ++===========++ ++==========++ ++===========++
|| Author || || Recipient || || Author || || Recipient ||
++====++====++ +--------+ ++===========++ ++====++====++ +--------+ ++===========++
|| | Return | /\ || | Return | /\
|| +-+------+ || || +-+------+ ||
\/ . ^ || \/ . ^ ||
+---------+ . . +---++---+ +---------+ . . +---++---+
| | . . | | | | . . | |
//==+=========+============================+========+===\\ /--+---------+----------------------------+--------+----\
|| | | . . MHS | | || | | | . . MHS | | |
|| | Origin +<...... .................+ Recv | || | | Origin +<...... .................+ Recv | |
| | ^ | | | | | ^ | | |
+---++----+ . +--------+ | +---++----+ . +--------+ |
|| . /\ | || . /\ |
|| ..............+.................. || | || ..............+.................. || |
\/ . . . || | \/ . . . || |
+-------+-+ +--+------+ +-+--++---+ | +-------+-+ +--+------+ +-+--++---+ |
| Relay +=======>| Relay +=======>| Relay | | | Relay +=======>| Relay +=======>| Relay | |
+---------+ +----++---+ +---------+ | +---------+ +----++---+ +---------+ |
|| | || |
|| | || |
\/ | \/ |
+---------+ | +---------+ |
| Gateway +-->... | | Gateway +-->... |
+---------+ | +---------+ |
\-------------------------------------------------------/
Legend: == lines indicate primary (possibly indirect) transfers or
roles; ... lines indicate supporting transfers or roles
Figure 3: Relationships Among MHS Actors Figure 3: Relationships Among MHS Actors
2.2.1. Originator 2.2.1. Originator
The Originator ensures that a message is valid for posting and then 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 submits it to a Relay. A message is valid if it conforms to both
Internet Mail standards and local operational policies. The Internet Mail standards and local operational policies. The
Originator can simply review the message for conformance and reject 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 it if it finds errors, or it can create some or all of the necessary
skipping to change at page 12, line 35 skipping to change at page 13, line 46
* User Mediators * User Mediators
* MHS Relays * MHS Relays
* Packet Switches * Packet Switches
The bottom layer is the Internet's IP service. The most basic email The bottom layer is the Internet's IP service. The most basic email
scenarios involve Relays and Switches. scenarios involve Relays and Switches.
Aborting a message transfer makes the Relay an Author because it must When a Relay stops attempting to transfer a message, it becomes an
send an error message to the Return address. The potential for Author because it must send an error message to the Return address.
looping is avoided by omitting a Return address from this message. The potential for looping is avoided by omitting a Return address
from this message.
2.2.3. Gateway 2.2.3. Gateway
A Gateway is a hybrid of User and Relay that connects heterogeneous 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 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 comes to this, the better. A Gateway operates as a User when it
needs the ability to modify message content. needs the ability to modify message content.
Differences between mail services can be as small as minor syntax Differences between mail services can be as small as minor syntax
variations, but they usually encompass significant, semantic variations, but they usually encompass significant, semantic
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Operation of Internet Mail services is carried out by different Operation of Internet Mail services is carried out by different
providers (or operators). Each can be an independent ADMD. This providers (or operators). Each can be an independent ADMD. This
independence of administrative decision-making defines boundaries independence of administrative decision-making defines boundaries
that distinguish different portions of the Internet Mail service. A that distinguish different portions of the Internet Mail service. A
department that operates a local Relay, an IT department that department that operates a local Relay, an IT department that
operates an enterprise Relay, and an ISP that operates a public operates an enterprise Relay, and an ISP that operates a public
shared email service can be configured into many combinations of shared email service can be configured into many combinations of
administrative and operational relationships. Each is a distinct administrative and operational relationships. Each is a distinct
ADMD, potentially having a complex arrangement of functional ADMD, potentially having a complex arrangement of functional
components. Figure 4 depicts relationships among ADMDs. The benefit components. Figure 4 depicts relationships among ADMDs. The
of the ADMD construct is to facilitate discussion about designs, benefit of the ADMD construct is to facilitate discussion about
policies and operations that need to distinguish between internal designs, policies and operations that need to distinguish between
issues and external ones. internal issues and external ones.
The architectural impact of the need for boundaries between ADMDs is The architectural impact of the need for boundaries between ADMDs is
discussed in [Tussle]. Most significant is that the entities discussed in [Tussle]. Most significant is that the entities
communicating across ADMD boundaries typically have the added burden communicating across ADMD boundaries typically have the added burden
of enforcing organizational policies concerning external of enforcing organizational policies concerning external
communications. At a more mundane level, routing mail between ADMDs communications. At a more mundane level, routing mail between ADMDs
can be an issue, such as needing to route mail for partners over can be an issue, such as needing to route mail between organizational
specially trusted paths. partners over specially trusted paths.
These are the basic types of ADMDs: These are three basic types of ADMDs:
Edge: Independent transfer services in networks at the edge of Edge: Independent transfer services in networks at the edge of
the open Internet Mail service. the open Internet Mail service.
Consumer: This might be a type of Edge service, as is common Consumer: This might be a type of Edge service, as is common
for web-based email access. for web-based email access.
Transit: Mail Service Providers (MSP) that offer value-added Transit: Mail Service Providers (MSP) that offer value-added
capabilities for Edge ADMDs, such as aggregation and filtering. capabilities for Edge ADMDs, such as aggregation and filtering.
The mail-level transit service is different from packet-level The mail-level transit service is different from packet-level
switching. End-to-end packet transfers usually go through switching. End-to-end packet transfers usually go through
intermediate routers; email exchange across the open Internet can be intermediate routers; email exchange across the open Internet can be
directly between the Boundary MTAs of Edge ADMDs. This distinction directly between the Boundary MTAs of Edge ADMDs. This distinction
between direct and indirect interaction highlights the differences between direct and indirect interaction highlights the differences
discussed in Section 2.2.2 discussed in Section 2.2.2
+--------+ +---------+ +-------+ +-----------+ +--------+ +---------+ +-------+ +-----------+
| ADMD1 |<===>| ADMD2 |<===>| ADMD3 |<===>| ADMD4 | | ADMD1 |<===>| ADMD2 |<===>| ADMD3 |<===>| ADMD4 |
| ----- | | ----- | | ----- | | ----- | | ----- | | ----- | | ----- | | ----- |
| | | | | | | | | | | | | | | |
| Author | | | | | | | | Author | | | | | | Recipient |
| . | | | | | | | | . | | | | | | ^ |
| V | | | | | | | | V | | | | | | . |
| Edge..+....>|.Transit.+....>|-Edge..+....>|.Recipient | | Edge..+....>|.Transit.+....>|-Edge..+....>|..Consumer |
| | | | | | | | | | | | | | | |
+--------+ +---------+ +-------+ +-----------+ +--------+ +---------+ +-------+ +-----------+
Legend: == lines indicate primary (possibly indirect) transfers or
roles; ... lines indicate supporting transfers or roles
Figure 4: Administrative Domain (ADMD) Example Figure 4: Administrative Domain (ADMD) Example
Edge networks can use proprietary email standards internally. Edge networks can use proprietary email standards internally.
However the distinction between Transit network and Edge network However the distinction between Transit network and Edge network
transfer services is significant because it highlights the need for transfer services is significant because it highlights the need for
concern over interaction and protection between independent concern over interaction and protection between independent
administrations. In particular, this distinction calls for administrations. In particular, this distinction calls for
additional care in assessing the transitions of responsibility and additional care in assessing the transitions of responsibility and
the accountability and authorization relationships among participants the accountability and authorization relationships among participants
skipping to change at page 16, line 14 skipping to change at page 17, line 29
These ADMDs operate email services, such as for consumers or These ADMDs operate email services, such as for consumers or
client companies. client companies.
Practical operational concerns demand that providers be involved in Practical operational concerns demand that providers be involved in
administration and enforcement issues. This involvement can extend administration and enforcement issues. This involvement can extend
to operators of lower-level packet services. to operators of lower-level packet services.
3. Identities 3. Identities
Internet Mail uses three forms of identity: mailbox, domain name, and Internet Mail uses three forms of identity: mailbox, domain name,
message-ID. Each must be globally unique. message-ID and ENVID. Each must be globally unique.
3.1. Mailbox 3.1. Mailbox
"A mailbox sends and receives mail. It is a conceptual entity "A mailbox sends and receives mail. It is a conceptual entity
which does not necessarily pertain to file storage." [RFC2822] which does not necessarily pertain to file storage." [RFC5322]
A mailbox is specified as an Internet Mail address <addr-spec>. It A mailbox is specified as an Internet Mail address <addr-spec>. It
has two distinct parts, separated by an at-sign (@). The right side has two distinct parts, separated by an at-sign (@). The right side
is a globally interpreted domain name associated with an ADMD. is a globally interpreted domain name associated with an ADMD.
Domain names are discussed in Section 3.3. Formal Internet Mail Domain names are discussed in Section 3.3. Formal Internet Mail
addressing syntax can support source routes, to indicate the path addressing syntax can support source routes, to indicate the path
through which a message ought to be sent. The use of source routes through which a message ought to be sent. The use of source routes
is not common and has been deprecated in [RFC2821]. is not common and has been deprecated in [RFC5321].
The portion to the left of the at-sign contains a string that is The portion to the left of the at-sign contains a string that is
globally opaque and is called the <local-part>. It is to be globally opaque and is called the <local-part>. It is to be
interpreted only by the entity specified by the address's domain interpreted only by the entity specified by the address's domain
name. Except as noted later in this section all other entities MUST name. Except as noted later in this section all other entities MUST
treat the <local-part> as an uninterpreted literal string and MUST treat the <local-part> as an uninterpreted literal string and MUST
preserve all of its original details. As such its public preserve all of its original details. As such its public
distribution is equivalent to sending a Web browser "cookie" that is distribution is equivalent to sending a Web browser "cookie" that is
only interpreted upon being returned to its creator. only interpreted upon being returned to its creator.
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It is common for sites to have local structuring conventions for the It is common for sites to have local structuring conventions for the
left-hand side <local-part> of an <addr-spec>. This permits sub- left-hand side <local-part> of an <addr-spec>. This permits sub-
addressing, such as for distinguishing different discussion groups addressing, such as for distinguishing different discussion groups
used by the same participant. However it is worth stressing that used by the same participant. However it is worth stressing that
these conventions are strictly private to the user's organization and these conventions are strictly private to the user's organization and
MUST NOT be interpreted by any domain except the one listed in the MUST NOT be interpreted by any domain except the one listed in the
right side of the <addr-spec>. The exceptions are those specialized right side of the <addr-spec>. The exceptions are those specialized
services that conform to public, standardized conventions, as noted services that conform to public, standardized conventions, as noted
below. below.
A few types of addresses elaborate on basic email addressing, with a Basic email addressing defines the <local-part> as being globally
standardized, global schema for the <local-part>, Include are opaque. However there are some uses of email that add a
conventions between authoring systems and Gateways. They are standardized, global schema to the value, such as between an author
invisible to the public email transfer infrastructure. When an and a Gateway. The <local-part> details remain invisible to the
Author is explicitly sending through a Gateway out of the Internet, public email transfer infrastructure, but provide addressing and
coding conventions for the <local-part> allow the Author to formulate handling instructions for further processing by the Gateway.
instructions for the Gateway. Standardized examples of such Standardized examples of such conventions are the telephone numbering
conventions are the telephone numbering formats for VPIM [RFC3801], formats for VPIM [RFC3801] such as:
such as:
+16137637582@vpim.example.com +16137637582@vpim.example.com
and iFax [RFC3192], such as: and iFax [RFC3192], such as:
FAX=+12027653000/T33S=1387@ifax.example.com FAX=+12027653000/T33S=1387@ifax.example.com
3.2. Scope of Email Address Use 3.2. Scope of Email Address Use
Email addresses are being used far beyond their original role in Email addresses are being used far beyond their original role in
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responsible for setting that string. For example, see Section 4.1.4, responsible for setting that string. For example, see Section 4.1.4,
Section 4.3.3 and Section 5. Section 4.3.3 and Section 5.
3.3. Domain Names 3.3. Domain Names
A domain name is a global reference to an Internet resource, such as A domain name is a global reference to an Internet resource, such as
a host, a service, or a network. A domain name usually maps to one a host, a service, or a network. A domain name usually maps to one
or more IP Addresses. Conceptually, the name can encompass an or more IP Addresses. Conceptually, the name can encompass an
organization, a collection of machines integrated into a homogeneous organization, a collection of machines integrated into a homogeneous
service, or a single machine. A domain name can be administered to service, or a single machine. A domain name can be administered to
refer to individual users, but this is not common practice. The name refer to an individual user, but this is not common practice. The
is structured as a hierarchical sequence of names, separated by dots name is structured as a hierarchical sequence of names, separated by
(.), with the top of the hierarchy being on the right end of the dots (.), with the top of the hierarchy being on the right end of the
sequence. Domain names are defined and operated through the Domain sequence. There can be many names in the sequence -- that is, the
Name System (DNS) [RFC1034], [RFC1035], [RFC2181]. depth of the hierarchy can be substantial. 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 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 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 message, such as providing the administrative scope for a message
identifier or performing transfer processing. identifier or performing transfer processing.
3.4. Message Identifier 3.4. Message Identifier
There are two standardized tags for identifying messages: Message-ID: There are two standardized tags for identifying messages: Message-ID:
and ENVID. A Message-ID: pertains to content, and an ENVID pertains and ENVID. A Message-ID: pertains to content, and an ENVID pertains
to transfer. to transfer.
3.4.1. Message-ID 3.4.1. Message-ID
Internet Mail standards provide for, at most, a single 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 The Message-ID: for a single message, which is a user-level IMF tag,
a variety of uses including threading, aiding identification of has a variety of uses including threading, aiding identification of
duplicates, and DSN tracking. [RFC2822]. The Originator assigns the duplicates, and DSN tracking. [RFC5322]. The Originator assigns the
Message-ID:. The Recipient's ADMD is the intended consumer of 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:, although any actor along the transfer path can use it.
Message-ID: MUST be globally unique. Its format is similar to that Message-ID: MUST be globally unique. Its format is similar to that
of a mailbox, with two distinct parts, separated by an at-sign (@). of a mailbox, with two distinct parts, separated by an at-sign (@).
Typically, the right side specifies the ADMD or host that assigns the Typically, the right side specifies the ADMD or host that assigns the
identifier, and the left side contains a string that is globally identifier, and the left side contains a string that is globally
opaque and serves to uniquely identify the message within the domain opaque and serves to uniquely identify the message within the domain
referenced on the right side. The duration of uniqueness for the referenced on the right side. The duration of uniqueness for the
message identifier is undefined. message identifier is undefined.
When a message is revised in any way, the decision whether to assign When a message is revised in any way, the decision whether to assign
a new Message-ID: requires a subjective assessment to determine a new Message-ID: requires a subjective assessment to determine
whether the editorial content has been changed enough to constitute a whether the editorial content has been changed enough to constitute a
new message. [RFC2822] states that "a message identifier pertains to new message. [RFC5322] states that "a message identifier pertains to
exactly one instantiation of a particular message; subsequent exactly one instantiation of a particular message; subsequent
revisions to the message each receive new message identifiers." Yet revisions to the message each receive new message identifiers." Yet
experience suggests that some flexibility is needed. An impossible experience suggests that some flexibility is needed. An impossible
test is whether the recipient will consider the new message to be test is whether the recipient will consider the new message to be
equivalent to the old one. For most components of Internet Mail, equivalent to the old one. For most components of Internet Mail,
there is no way to predict a specific recipient's preferences on this 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 matter. Both creating and failing to create a new Message-ID: have
their downsides. their downsides.
Here are some guidelines and examples: Here are some guidelines and examples:
* If a message is changed only in form, such as character- * If a message is changed only in form, such as character
encoding, it is still the same message. encoding, it is still the same message.
* If a message has minor additions to the content, such as a * If a message has minor additions to the content, such as a
mailing list tag at the beginning of the RFC2822.Subject header mailing list tag at the beginning of the RFC5322.Subject header
field, or some mailing list administrative information added to field, or some mailing list administrative information added to
the end of the primary body-part text, it is probably the same the end of the primary body-part text, it is probably the same
message. message.
* If a message has viruses deleted from it, it is probably the * If a message has viruses deleted from it, it is probably the
same message. same message.
* If a message has offensive words deleted from it, some * If a message has offensive words deleted from it, some
recipients will consider it the same message, but some will recipients will consider it the same message, but some will
not. not.
skipping to change at page 19, line 22 skipping to change at page 20, line 40
* If a message is translated into a different language, some * If a message is translated into a different language, some
recipients will consider it the same message, but some will recipients will consider it the same message, but some will
not. not.
* If a message is included in a digest of messages, the digest * If a message is included in a digest of messages, the digest
constitutes a new message. constitutes a new message.
* If a message is forwarded by a recipient, what is forwarded is * If a message is forwarded by a recipient, what is forwarded is
a new message. a new message.
* If a message is "redirected", such as using RFC2822 "Resent-*" * If a message is "redirected", such as using IMF "Resent-*"
header fields, some recipients will consider it the same header fields, some recipients will consider it the same
message, but some will not. message, but some will not.
The absence of both objective, precise criteria for re-generating a The absence of both objective, precise criteria for regenerating a
Message-ID: and strong protection associated with the string means Message-ID: and strong protection associated with the string means
that the presence of an ID can permit an assessment that is that the presence of an ID can permit an assessment that is
marginally better than a heuristic, but the ID certainly has no value marginally better than a heuristic, but the ID certainly has no value
on its own for strict formal reference or comparison. For that on its own for strict formal reference or comparison. For that
reason, the Message-ID: SHOULD NOT be used for any function that has reason, the Message-ID: SHOULD NOT be used for any function that has
security implications. security implications.
3.4.2. ENVID 3.4.2. ENVID
The ENVID (envelope identifier) can be used for message-tracking The ENVID (envelope identifier) can be used for message-tracking
purposes [RFC3885] concerning a single posting/delivery transfer. purposes ([RFC3885], [RFC3464]) concerning a single posting/delivery
The ENVID labels a single transit of the MHS by a specific message. transfer. The ENVID labels a single transit of the MHS by a specific
So, the ENVID is used for one message posting, until that message is message. So, the ENVID is used for one message posting, until that
delivered. A re-posting of the message, such as by a Mediator, does message is delivered. A re-posting of the message, such as by a
not re-use that ENVID, but can use a new one, even though the message Mediator, does not reuse that ENVID, but can use a new one, even
might legitimately retain its original Message-ID:. though the message might legitimately retain its original
Message-ID:.
The format of an ENVID is free form. Although its creator might The format of an ENVID is free form. Although its creator might
choose to impose structure on the string, none is imposed by Internet choose to impose structure on the string, none is imposed by Internet
standards. By implication, the scope of the string is defined by the standards. By implication, the scope of the string is defined by the
domain name of the Return Address. domain name of the Return Address.
4. Services and Standards 4. Services and Standards
The Internet Mail architecture comprises six basic types of The Internet Mail architecture comprises six basic types of
functionality, which are arranged to support a store-and-forward functionality, which are arranged to support a store-and-forward
skipping to change at page 21, line 23 skipping to change at page 23, line 23
. +-----+ | . +--------+ . . +-----+ | . +--------+ .
. | . ***************** ^ . . | . ***************** ^ .
. +-----V-.----*------------+ * . . . +-----V-.----*------------+ * . .
. MSA | +-------+ * +------+ | * . . . MSA | +-------+ * +------+ | * . .
. | | aMSA +-(S)->| hMSA | | * . . . | | aMSA +-(S)->| hMSA | | * . .
. | +-------+ * +--+---+ | * . . . | +-------+ * +--+---+ | * . .
V +------------*------+-----+ * . . V +------------*------+-----+ * . .
//==========\\ * V {smtp * . . //==========\\ * V {smtp * . .
|| MESSAGE || * +------+ * //===+===\\ . || MESSAGE || * +------+ * //===+===\\ .
||----------|| MHS * | MTA | * || dsn || . ||----------|| MHS * | MTA | * || dsn || .
|| Envelope || * +--+---+ * \\=======// . || ENVELOPE || * +--+---+ * \\=======// .
|| SMTP || * V {smtp * ^ ^ . || smtp || * V {smtp * ^ ^ .
|| Content || * +------+ * . . //==+==\\ || CONTENT || * +------+ * . . //==+==\\
|| RFC2822 || * | MTA +....*...... . || mdn || || imf || * | MTA +....*...... . || mdn ||
|| MIME || * +--+---+ * . \\=====// || mime || * +--+---+ * . \\=====//
\\==========// * smtp}| {local * . ^ \\==========// * smtp}| {local * . ^
. MDA * | {lmtp * . . . MDA * | {lmtp * . .
. +----------------+------V-----+ * . . . +----------------+------V-----+ * . .
. | +----------+ * +------+ | * . . . | +----------+ * +------+ | * . .
. | | | * | | +..*.......... . . | | | * | | +..*.......... .
. | | rMDA |<-(D)--+ hMDA | | * . . | | rMDA |<-(D)--+ hMDA | | * .
. | | | * | | |<.*........ . . | | | * | | |<.*........ .
. | +-+------+-+ * +------+ | * . . . | +-+------+-+ * +------+ | * . .
. +------+---------*------------+ * . . . +------+---------*------------+ * . .
. | ***************** . . . smtp,local}| ***************** . .
. V{smtp,imap,pop,local . . . V . .
. +-----+ //===+===\\ . . +-----+ //===+===\\ .
. | rMS | || sieve || . . | rMS | || sieve || .
. +--+--+ \\=======// . . +--+--+ \\=======// .
. |{imap,pop,local ^ . . |{imap,pop,local ^ .
. V . . . V . .
. ++==========++ . . . ++==========++ . .
. || || . . . || || . .
.......>|| rMUA ++........................... . .......>|| rMUA ++........................... .
|| ++................................... || ++...................................
++==========++ ++==========++
Legend: == lines indicate primary (possibly indirect) transfers or
Figure 5: Protocols and Services roles; == bpxes indicate data objects; ... lines indicate supporting
transfers or roles; *** lines indicate aggregated service
Figure 5: Protocols and Services
4.1. Message Data 4.1. Message Data
The purpose of the Mail Handling Service (MHS) is to exchange a The purpose of the Mail Handling Service (MHS) is to exchange an IMF
message object among participants [RFC2822], [RFC0822]. All of its message object among participants [RFC5322]. All of its underlying
underlying mechanisms serve to deliver that message from its Author mechanisms serve to deliver that message from its Author to its
to its Recipients. A message can be explicitly labeled as to its Recipients. A message can be explicitly labeled as to its nature
nature [RFC3458]. [RFC3458].
A message comprises a transit-handling envelope and the message A message comprises a transit-handling envelope and the message
content. The envelope contains information used by the MHS. The content. The envelope contains information used by the MHS. The
content is divided into a structured header and the body. The header content is divided into a structured header and the body. The header
comprises transit handling trace information and structured fields comprises transit handling trace information and structured fields
that are part of the Author's message content. The body can be that are part of the Author's message content. The body can be
unstructured lines of text or a tree of multi-media subordinate unstructured lines of text or a tree of multi-media subordinate
objects, called "body-parts" or attachments [RFC2045], [RFC2046], objects, called "body-parts" or attachments [RFC2045], [RFC2046],
[RFC2047], [RFC4288], [RFC4289], [RFC2049]. [RFC2047], [RFC4288], [RFC4289], [RFC2049].
In addition, Internet Mail has a few conventions for special control In addition, Internet Mail has a few conventions for special control
data, notably: data, notably:
Delivery Status Notification (DSN): Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) is a message that can be A Delivery Status Notification (DSN) is a message that can be
generated by the MHS (MSA, MTA, or MDA) and sent to the generated by the MHS (MSA, MTA, or MDA) and sent to the
RFC2821.MailFrom address. An MDA and MTA are shown as sources RFC5321.MailFrom address. An MDA and MTA are shown as sources
of DSNs in Figure 5, and the destination is shown as Returns. of DSNs in Figure 5, and the destination is shown as Returns.
DSNs provide information about message transit, such as DSNs provide information about message transit, such as
transfer errors or successful delivery. [RFC3461] transfer errors or successful delivery. [RFC3461]
Message Disposition Notification (MDN): Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) is a message that A Message Disposition Notification (MDN) is a message that
provides information about post-delivery processing, such as provides information about post-delivery processing, such as
indicating that the message has been displayed [RFC3798] or the indicating that the message has been displayed [RFC3798] or the
form of content that can be supported [RFC3297]. It can be form of content that can be supported [RFC3297]. It can be
generated by an rMUA and is sent to the Disposition- generated by an rMUA and is sent to the Disposition-
Notification-To addresses. The mailbox for this is shown as Notification-To addresses. The mailbox for this is shown as
Disp in Figure 5. Disp in Figure 5.
Message Filtering (SIEVE): Message Filtering (SIEVE):
Sieve is a scripting language used to specify conditions for Sieve is a scripting language used to specify conditions for
differential handling of mail, typically at the time of differential handling of mail, typically at the time of
delivery [RFC5228]. Scripts can be conveyed in a variety of delivery [RFC5228]. Scripts can be conveyed in a variety of
ways, as a MIME part. Figure 5 shows a Sieve script going ways, such as a MIME part in a message. Figure 5 shows a Sieve
from the rMUA to the MDA. However, filtering can be done at script going from the rMUA to the MDA. However, filtering can
many different points along the transit path, and any one or be done at many different points along the transit path, and
more of them might be subject to Sieve directives, especially any one or more of them might be subject to Sieve directives,
within a single ADMD. the Figure 5 shows only one relationship, especially within a single ADMD. the Figure 5 shows only one
for (relative) simplicity. relationship, for (relative) simplicity.
4.1.1. Envelope 4.1.1. Envelope
Internet Mail has a fragmented framework for transit-related handling Internet Mail has a fragmented framework for transit-related handling
information. Information that is used directly by the MHS is called information. Information that is used directly by the MHS is called
the "envelope." It directs handling activities by the transfer the "envelope." It directs handling activities by the transfer
service and is carried in transfer service commands. That is, the service and is carried in transfer service commands. That is, the
envelope exists in the transfer protocol SMTP. [RFC2821] envelope exists in the transfer protocol SMTP. [RFC5321]
Trace information, such as RFC2822.Received, is recorded in the Trace information, such as RFC5322.Received, is recorded in the
message header and is not subsequently altered. [RFC2822] message header and is not subsequently altered. [RFC5322]
4.1.2. Header Fields 4.1.2. Header Fields
Header fields are attribute name/value pairs that cover an extensible Header fields are attribute name/value pairs that cover an extensible
range of email service parameters, structured user content, and user range of email service parameters, structured user content, and user
transaction meta-information. The core set of header fields is transaction meta-information. The core set of header fields is
defined in [RFC2822], [RFC0822]. It is common practice to extend defined in [RFC5322]. It is common practice to extend this set for
this set for different applications. Procedures for registering different applications. Procedures for registering header fields are
header fields are defined in [RFC3864]. An extensive set of existing defined in [RFC3864]. An extensive set of existing header field
header field registrations is provided in [RFC4021]. registrations is provided in [RFC4021].
One danger of placing additional information in header fields is that One danger of placing additional information in header fields is that
Gateways often alter or delete them. Gateways often alter or delete them.
4.1.3. Body 4.1.3. Body
The body of a message might be lines of ASCII text or a The body of a message might be lines of ASCII text or a
hierarchically structured composition of multi-media body-part hierarchically structured composition of multi-media body-part
attachments, using MIME. [RFC2045], [RFC2046], [RFC2047], [RFC4288], attachments, using MIME. [RFC2045], [RFC2046], [RFC2047], [RFC4288],
[RFC2049] [RFC2049]
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| | Received: | Originator, Relay, | | | Received: | Originator, Relay, |
| | | Receiver | | | | Receiver |
| | Return-Path: | MDA, from MailFrom | | | Return-Path: | MDA, from MailFrom |
| | Resent-*: | Mediator | | | Resent-*: | Mediator |
| | List-Id: | Mediator | | | List-Id: | Mediator |
| | List-*: | Mediator | | | List-*: | Mediator |
| SMTP | HELO/EHLO | Latest Relay Client | | SMTP | HELO/EHLO | Latest Relay Client |
| | ENVID | Originator | | | ENVID | Originator |
| | MailFrom | Originator | | | MailFrom | Originator |
| | RcptTo | Author | | | RcptTo | Author |
| | ORCPT | Author | | | ORCPT | Originator |
| IP | Source Address | Latest Relay Client | | IP | Source Address | Latest Relay Client |
+----------------------+----------------+---------------------------+ +----------------------+----------------+---------------------------+
Legend: Layer - The part of the email architecture that uses the
identifier; Field - The protocol construct that contains the
identifier; Set By - The actor role responsible for specifying the
identifier value (and this can be different from the actor that
performs the fill-in function for the protocol construct)
Table 1: Layered Identities Table 1: Layered Identities
These are the most common address-related fields: These are the most common address-related fields:
RFC2822.From: Set by - Author RFC5322.From: Set by - Author
Names and addresses for authors of the message content are Names and addresses for authors of the message content are
listed in the From: field. listed in the From: field.
RFC2822.Reply-To: Set by - Author RFC5322.Reply-To: Set by - Author
If a Recipient sends a reply message that would otherwise use If a Recipient sends a reply message that would otherwise use
the RFC2822.From field addresses in the original message, the the RFC5322.From field addresses in the original message, the
addresses in the RFC2822.Reply-To field are used instead. In addresses in the RFC5322.Reply-To field are used instead. In
other words, this field overrides the From: field for responses other words, this field overrides the From: field for responses
from Recipients. from Recipients.
RFC2822.Sender: Set by - Originator RFC5322.Sender: Set by - Originator
This field specifies the address responsible for submitting the This field specifies the address responsible for submitting the
message to the transfer service. This field can be omitted if message to the transfer service. This field can be omitted if
it contains the same address as RFC2822.From. However, it contains the same address as RFC5322.From. However,
omitting this field does not mean that no Sender is specified; omitting this field does not mean that no Sender is specified;
it means that that header field is virtual and that the address it means that that header field is virtual and that the address
in the From: field MUST be used. in the From: field MUST be used.
Specification of the notifications Return addresses, which are Specification of the notifications Return addresses, which are
contained in RFC2821.MailFrom, is made by the RFC2822.Sender. contained in RFC5321.MailFrom, is made by the RFC5322.Sender.
Typically the Return address is the same as the Sender address. Typically the Return address is the same as the Sender address.
However, some usage scenarios require it to be different. However, some usage scenarios require it to be different.
RFC2822.To/.CC: Set by - Author RFC5322.To/.CC: Set by - Author
These fields specify MUA Recipient addresses. However, some or These fields specify MUA Recipient addresses. However, some or
all of the addresses in these fields might not be present in all of the addresses in these fields might not be present in
the RFC2821.RcptTo commands. the RFC5321.RcptTo commands.
The distinction between To and CC is subjective. Generally, a The distinction between To and CC is subjective. Generally, a
To addressee is considered primary and is expected to take To addressee is considered primary and is expected to take
action on the message. A CC addressee typically receives a action on the message. A CC addressee typically receives a
copy as a courtesy. copy as a courtesy.
RFC2822.BCC: Set by - Author RFC5322.BCC: Set by - Author
A copy of the message might be sent to an addressee whose A copy of the message might be sent to an addressee whose
participation is not to be disclosed to the RFC2822.To or participation is not to be disclosed to the RFC5322.To or
RFC2822.CC Recipients and, usually, not to the other BCC RFC5322.CC Recipients and, usually, not to the other BCC
Recipients. The BCC: header field indicates a message copy to Recipients. The BCC: header field indicates a message copy to
such a Recipient. Use of this field is discussed in [RFC2822]. such a Recipient. Use of this field is discussed in [RFC5322].
RFC2821.HELO/.EHLO: Set by - Originator, MSA, MTA RFC5321.HELO/.EHLO: Set by - Originator, MSA, MTA
Any SMTP client -- including Originator, MSA, or MTA -- can Any SMTP client -- including Originator, MSA, or MTA -- can
specify its hosting domain identity for the SMTP HELO or EHLO specify its hosting domain identity for the SMTP HELO or EHLO
command operation. command operation.
RFC3461.ENVID: Set by - Originator RFC3461.ENVID: Set by - Originator
The MSA can specify an opaque string, to be included in a DSN, The MSA can specify an opaque string, to be included in a DSN,
as a means of assisting the Return address recipient in as a means of assisting the Return address recipient in
identifying the message that produced a DSN or message identifying the message that produced a DSN or message
tracking. tracking.
RFC2821.MailFrom: Set by - Originator RFC5321.MailFrom: Set by - Originator
This field is an end-to-end string that specifies an email This field is an end-to-end string that specifies an email
address for receiving return control information, such as address for receiving return control information, such as
returned messages. The name of this field is misleading, returned messages. The name of this field is misleading,
because it is not required to specify either the Author or the because it is not required to specify either the Author or the
actor responsible for submitting the message. Rather, the actor responsible for submitting the message. Rather, the
actor responsible for submission specifies the RFC2821.MailFrom actor responsible for submission specifies the RFC5321.MailFrom
address. Ultimately, the simple basis for deciding which address. Ultimately, the simple basis for deciding which
address needs to be in the RFC2821.MailFrom field is to address needs to be in the RFC5321.MailFrom field is to
determine which address must be informed about transfer-level determine which address must be informed about transfer-level
problems (and possibly successes.) problems (and possibly successes.)
RFC2821.RcptTo: Set by - Author, Final MTA, MDA. RFC5321.RcptTo: Set by - Author, Final MTA, MDA.
This field specifies the MUA mailbox address of a Recipient. This field specifies the MUA mailbox address of a Recipient.
The string might not be visible in the message content header. The string might not be visible in the message content header.
For example, the message destination address header fields, For example, the IMF destination address header fields, such as
such as RFC2822.To, might specify a mailing list mailbox, while RFC5322.To, might specify a mailing list mailbox, while the
the RFC2821.RcptTo address specifies a member of that list. RFC5321.RcptTo address specifies a member of that list.
RFC2821.ORCPT: Set by - Author. RFC5321.ORCPT: Set by - Author.
This is an optional parameter to the RCPT command, indicating This is an optional parameter to the RCPT command, indicating
the original address to which the current RCPT TO address the original address to which the current RCPT TO address
corresponds, after a mapping was performed during transit. An corresponds, after a mapping was performed during transit. An
ORCPT is the only reliable way to correlate a DSN from a multi- ORCPT is the only reliable way to correlate a DSN from a multi-
recipient message transfer with the intended recipient. recipient message transfer with the intended recipient.
RFC2821.Received: Set by - Originator, Relay, Mediator, Dest RFC5321.Received: Set by - Originator, Relay, Mediator, Dest
This field contains trace information, including originating This field contains trace information, including originating
host, Relays, Mediators, and MSA host domain names and/or IP host, Relays, Mediators, and MSA host domain names and/or IP
Addresses. Addresses.
RFC2821.Return-Path: Set by - Originator RFC5321.Return-Path: Set by - Originator
The MDA records the RFC2821.MailFrom address into the The MDA records the RFC5321.MailFrom address into the
RFC2822.Return-Path field. RFC5322.Return-Path field.
RFC2919.List-Id: Set by - Mediator Author RFC2919.List-Id: Set by - Mediator Author
This field provides a globally unique mailing list naming This field provides a globally unique mailing list naming
framework that is independent of particular hosts. [RFC2919] framework that is independent of particular hosts. [RFC2919]
The identifier is in the form of a domain name; however, the The identifier is in the form of a domain name; however, the
string usually is constructed by combining the two parts of an string usually is constructed by combining the two parts of an
email address. The result is rarely a true domain name, listed email address. The result is rarely a true domain name, listed
in the domain name service, although it can be. in the domain name service, although it can be.
RFC2369.List-*: Set by - Mediator Author RFC2369.List-*: Set by - Mediator Author
[RFC2369] defines a collection of message header fields for use [RFC2369] defines a collection of message header fields for use
by mailing lists. In effect, they supply list-specific by mailing lists. In effect, they supply list-specific
parameters for common mailing list user operations. The parameters for common mailing list user operations. The
skipping to change at page 27, line 48 skipping to change at page 30, line 4
4.2.1. Mail User Agent (MUA) 4.2.1. Mail User Agent (MUA)
A Mail User Agent (MUA) works on behalf of User actors and User A Mail User Agent (MUA) works on behalf of User actors and User
applications. It is their representative within the email service. applications. It is their representative within the email service.
The Author MUA (aMUA) creates a message and performs initial The Author MUA (aMUA) creates a message and performs initial
submission into the transfer infrastructure via a Mail Submission submission into the transfer infrastructure via a Mail Submission
Agent (MSA). It can also perform any creation- and posting-time Agent (MSA). It can also perform any creation- and posting-time
archival in its Message Store (aMS). An MUA aMS can organize archival in its Message Store (aMS). An MUA aMS can organize
messages in many different ways. A common model uses aggregations, messages in many different ways. A common model uses aggregations,
called "folders". This model allows a folder for messages under called "folders"; in IMAP they are called "mailboxes". This model
development (Drafts), a folder for messages waiting to be sent allows a folder for messages under development (Drafts), a folder for
(Queued or Unsent), and a folder for messages that have been messages waiting to be sent (Queued or Unsent), and a folder for
successfully posted for transfer (Sent). But none of these folders messages that have been successfully posted for transfer (Sent). But
is required. For example, IMAP allows drafts to be stored in any none of these folders is required. For example, IMAP allows drafts
folder; so no Drafts folder is present. to be stored in any folder; so no Drafts folder needs to be present.
The Recipient MUA (rMUA) works on behalf of the Recipient to process The Recipient MUA (rMUA) works on behalf of the Recipient to process
received mail. This processing includes generating user-level received mail. This processing includes generating user-level
disposition control messages, displaying and disposing of the disposition control messages, displaying and disposing of the
received message, and closing or expanding the user communication received message, and closing or expanding the user communication
loop by initiating replies and forwarding new messages. loop by initiating replies and forwarding new messages.
NOTE: Although not shown in Figure 5, an MUA itself can have a NOTE: Although not shown in Figure 5, an MUA itself can have a
distributed implementation, such as a "thin" user interface distributed implementation, such as a "thin" user interface
module on a constrained device such as a smartphone, with most module on a constrained device such as a smartphone, with most
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An MUA can be automated, on behalf of a user who is not present at 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 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 that has a timed-initiation feature. These services are not to be
confused with a mailing list Mediator, since there is no incoming confused with a mailing list Mediator, since there is no incoming
message triggering the activity of the automated service. message triggering the activity of the automated service.
A popular and problematic MUA is an automatic responder, such as one A popular and problematic MUA is an automatic responder, such as one
that sends out-of-office notices. This behavior might be confused that sends out-of-office notices. This behavior might be confused
with that of a Mediator, but this MUA is generating a new message. with that of a Mediator, but this MUA is generating a new message.
Automatic responders can annoy users of mailing lists unless they Automatic responders can annoy users of mailing lists unless they
follow [RFC3834]. ****** The recommendations in RFC 3834 are an follow [RFC3834].
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: The identity fields are relevant to a typical MUA:
RFC2822.From RFC5322.From
RFC5322.Reply-To
RFC2822.Reply-To RFC5322.Sender
RFC2822.Sender RFC5322.To, RFC5322.CC
RFC2822.To, RFC2822.CC
RFC2822.BCC RFC5322.BCC
4.2.2. Message Store (MS) 4.2.2. Message Store (MS)
An MUA can employ a long-term Message Store (MS). Figure 5 depicts 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 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. 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 An MS acquires messages from an MDA either proactively by a local
using POP or IMAP. The MUA accesses the MS either by a local mechanism or even with a standardized mechanism such as SMTP(!) or
mechanism or by using POP or IMAP. Using POP for message access, reactively by using POP or IMAP. The MUA accesses the MS either by a
rather than bulk transfer, is rare, awkward, and largely non- local mechanism or by using POP or IMAP. Using POP for individual
standard. message accesses, rather than for bulk transfer, is relatively rare
and inefficient.
4.3. MHS-Level Services 4.3. MHS-Level Services
4.3.1. Mail Submission Agent (MSA) 4.3.1. Mail Submission Agent (MSA)
A Mail Submission Agent (MSA) accepts the message submitted by the A Mail Submission Agent (MSA) accepts the message submitted by the
aMUA and enforces the policies of the hosting ADMD and the aMUA and enforces the policies of the hosting ADMD and the
requirements of Internet standards. An MSA represents an unusual requirements of Internet standards. An MSA represents an unusual
functional dichotomy. It represents the interests of the Author functional dichotomy. It represents the interests of the Author
(aMUA) during message posting, to facilitate posting success; it also (aMUA) during message posting, to facilitate posting success; it also
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transition, within the MSA. transition, within the MSA.
The hMSA takes transit responsibility for a message that conforms to The hMSA takes transit responsibility for a message that conforms to
the relevant Internet standards and to local site policies. It the relevant Internet standards and to local site policies. It
rejects messages that are not in conformance. The MSA performs final rejects messages that are not in conformance. The MSA performs final
message preparation for submission and effects the transfer of message preparation for submission and effects the transfer of
responsibility to the MHS, via the hMSA. The amount of preparation responsibility to the MHS, via the hMSA. The amount of preparation
depends upon the local implementations. Examples of oMSA tasks depends upon the local implementations. Examples of oMSA tasks
include adding header fields, such as Date: and Message-ID:, and include adding header fields, such as Date: and Message-ID:, and
modifying portions of the message from local notations to Internet modifying portions of the message from local notations to Internet
standards, such as expanding an address to its formal RFC2822 standards, such as expanding an address to its formal IMF
representation. representation.
Historically, standards-based MUA/MSA message postings have used Historically, standards-based MUA/MSA message postings have used
SMTP. [RFC2821] The standard currently preferred is SUBMISSION. SMTP. [RFC5321] The standard currently preferred is SUBMISSION.
[RFC4409] Although SUBMISSION derives from SMTP, it uses a separate [RFC4409] Although SUBMISSION derives from SMTP, it uses a separate
TCP port and imposes distinct requirements, such as access TCP port and imposes distinct requirements, such as access
authorization. authorization.
These identities are relevant to the MSA: These identities are relevant to the MSA:
RFC2821.HELO/.EHLO RFC5321.HELO/.EHLO
RFC3461.ENVID RFC3461.ENVID
RFC2821.MailFrom RFC5321.MailFrom
RFC2821.RcptTo RFC5321.RcptTo
RFC2821.Received RFC5321.Received
RFC0791.SourceAddr RFC0791.SourceAddr
4.3.2. Mail Transfer Agent (MTA) 4.3.2. Mail Transfer Agent (MTA)
A Mail Transfer Agent (MTA) relays mail for one application-level A Mail Transfer Agent (MTA) relays mail for one application-level
"hop." It is like a packet-switch or IP router in that its job is to "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 make routing assessments and to move the message closer to the
Recipients. Of course, email objects are typically much larger than Recipients. Of course, email objects are typically much larger than
the payload of a packet or datagram, and the end-to-end latencies are the payload of a packet or datagram, and the end-to-end latencies are
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content. A change in data form, such as to MIME Content-Transfer- content. A change in data form, such as to MIME Content-Transfer-
Encoding, is within the purview of an MTA, but removal or replacement Encoding, is within the purview of an MTA, but removal or replacement
of body content is not. An MTA also adds trace information. of body content is not. An MTA also adds trace information.
[RFC2505] [RFC2505]
NOTE: Within a destination ADMD, email relaying modules can NOTE: Within a destination ADMD, email relaying modules can
make a variety of changes to the message, prior to delivery. make a variety of changes to the message, prior to delivery.
In such cases, these modules are acting as Gateways, rather In such cases, these modules are acting as Gateways, rather
than MTAs. than MTAs.
Internet Mail uses SMTP [RFC2821], [RFC0821] primarily to effect Internet Mail uses SMTP [RFC5321], [RFC0821] primarily to effect
point-to-point transfers between peer MTAs. Other transfer point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]. As with mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]. As with
most network layer mechanisms, the Internet Mail SMTP supports a most network layer mechanisms, the Internet Mail SMTP supports a
basic level of reliability, by virtue of providing for retransmission basic level of reliability, by virtue of providing for retransmission
after a temporary transfer failure. Unlike typical packet switches after a temporary transfer failure. Unlike typical packet switches
(and Instant Messaging services), Internet Mail MTAs are expected to (and Instant Messaging services), Internet Mail MTAs are expected to
store messages in a manner that allows recovery across service store messages in a manner that allows recovery across service
interruptions, such as host system shutdown. The degree of such interruptions, such as host system shutdown. The degree of such
robustness and persistence by an MTA can vary. The base SMTP robustness and persistence by an MTA can vary. The base SMTP
specification provides a framework for protocol response codes. An specification provides a framework for protocol response codes. An
extensible enhancement to this framework is defined in [RFC5248] extensible enhancement to this framework is defined in [RFC5248]
The primary routing mechanism for Internet Mail is the DNS MX record Although quite basic, the primary routing mechanism for Internet Mail
[RFC1035], which specifies an MTA through which the queried domain is the DNS MX record [RFC1035], which specifies an MTA through which
can be reached. This mechanism presumes a public, or at least a the queried domain can be reached. This mechanism presumes a public,
common, backbone that permits any attached MTA to connect to any or at least a common, backbone that permits any attached MTA to
other. connect to any other.
MTAs can perform any of these well-established roles: MTAs can perform any of these well-established roles:
Boundary MTA: An MTA that is part of an ADMD and interacts with 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 MTAs in other ADMDs. This is also called a Border MTA. There
can be different Boundary MTAs, according to the direction of can be different Boundary MTAs, according to the direction of
mail-flow. mail-flow.
Outbound MTA: An MTA that relays messages to other ADMDs. Outbound MTA: An MTA that relays messages to other ADMDs.
Inbound MTA: An MTA that receives inbound SMTP messages from Inbound MTA: An MTA that receives inbound SMTP messages from
MTA Relays in other ADMDs, for example, an MTA running on MTA Relays in other ADMDs, for example, an MTA running on
the host listed as the target of an MX record. the host listed as the target of an MX record.
Final MTA: The MTA that transfers a message to the MDA. Final MTA: The MTA that transfers a message to the MDA.
These identities are relevant to the MTA: These identities are relevant to the MTA:
RFC2821.HELO/.EHLO RFC5321.HELO/.EHLO
RFC3461.ENVID RFC3461.ENVID
RFC2821.MailFrom RFC5321.MailFrom
RFC2821.RcptTo
RFC2822.Received: Set by - Relay Server RFC5321.RcptTo
RFC5322.Received: Set by - Relay Server
RFC0791.SourceAddr RFC0791.SourceAddr
4.3.3. Mail Delivery Agent (MDA) 4.3.3. Mail Delivery Agent (MDA)
A transfer of responsibility from the MHS to a Recipient's A transfer of responsibility from the MHS to a Recipient's
environment (mailbox) is called "delivery." In the architecture, as environment (mailbox) is called "delivery." In the architecture, as
depicted in Figure 5, delivery takes place within a Mail Delivery depicted in Figure 5, delivery takes place within a Mail Delivery
Agent (MDA) and is shown as the (D) transition from the MHS-oriented Agent (MDA) and is shown as the (D) transition from the MHS-oriented
MDA component (hMDA) to the Recipient-oriented MDA component (rMDA). MDA component (hMDA) to the Recipient-oriented MDA component (rMDA).
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mechanism. Transfer from an MDA to an MS uses an access protocol, mechanism. Transfer from an MDA to an MS uses an access protocol,
such as POP or IMAP. such as POP or IMAP.
NOTE: The term "delivery" can refer to the formal, MHS function NOTE: The term "delivery" can refer to the formal, MHS function
specified here or to the first time a message is displayed to a specified here or to the first time a message is displayed to a
Recipient. A simple, practical test for whether the MHS-based Recipient. A simple, practical test for whether the MHS-based
definition applies is whether a DSN can be generated. definition applies is whether a DSN can be generated.
These identities are relevant to the MDA: These identities are relevant to the MDA:
RFC2821.Return-Path: Set by - Author Originator or Mediator RFC5321.Return-Path: Set by - Author Originator or Mediator
Originator Originator
The MDA records the RFC5321.MailFrom address into the
RFC5322.Return-Path field.
The MDA records the RFC2821.MailFrom address into the RFC5322.Received: Set by - MDA server
RFC2822.Return-Path field.
RFC2822.Received: Set by - MDA server
An MDA can record a Received: header field to indicate trace An MDA can record a Received: header field to indicate trace
information, including source host and receiving host domain information, including source host and receiving host domain
names and/or IP Addresses. names and/or IP Addresses.
4.4. Transition Modes 4.4. Transition Modes
From the origination site to the point of delivery, Internet Mail From the origination site to the point of delivery, Internet Mail
usually follows a "push" model. That is, the actor that holds the usually follows a "push" model. That is, the actor that holds the
message initiates transfer to the next venue, typically with SMTP message initiates transfer to the next venue, typically with SMTP
[RFC2821] or LMTP [RFC2033]. With a "pull" model, the actor that [RFC5321] or LMTP [RFC2033]. With a "pull" model, the actor that
holds the message waits for the actor in the next venue to initiate a holds the message waits for the actor in the next venue to initiate a
request for transfer. Standardized mechanisms for pull-based MHS request for transfer. Standardized mechanisms for pull-based MHS
transfer are ETRN [RFC1985] and ODMR [RFC2645]. transfer are ETRN [RFC1985] and ODMR [RFC2645].
After delivery, the Recipient's MUA (or MS) can gain access by having 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 the message pushed to it or by having the receiver of access pull the
message, such as by using POP [RFC1939] and IMAP [RFC3501]. message, such as by using POP [RFC1939] and IMAP [RFC3501].
4.5. Implementation and Operation 4.5. Implementation and Operation
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in a sequence of independent transmissions through some number of in a sequence of independent transmissions through some number of
MTAs. A very different task is a sequence of postings and deliveries MTAs. A very different task is a sequence of postings and deliveries
through Mediators. A Mediator forwards a message, through a re- through Mediators. A Mediator forwards a message, through a re-
posting process. The Mediator shares some functionality with basic posting process. The Mediator shares some functionality with basic
MTA relaying, but has greater flexibility in both addressing and MTA relaying, but has greater flexibility in both addressing and
content than is available to MTAs. content than is available to MTAs.
This is the core set of message information that is commonly set by This is the core set of message information that is commonly set by
all types of Mediators: all types of Mediators:
RFC2821.HELO/.EHLO: Set by - Mediator Originator RFC5321.HELO/.EHLO: Set by - Mediator Originator
RFC3461.ENVID: Set by - Mediator Originator RFC3461.ENVID: Set by - Mediator Originator
RFC2821.RcptTo: Set by - Mediator Author RFC5321.RcptTo: Set by - Mediator Author
RFC2821.Received: Set by - Mediator Dest RFC5321.Received: Set by - Mediator Dest
The Mediator can record received information, to indicate the The Mediator can record received information, to indicate the
delivery to the original address and submission to the alias delivery to the original address and submission to the alias
address. The trace of Received: header fields can include address. The trace of Received: header fields can include
everything from original posting, through relaying, to final everything from original posting, through relaying, to final
delivery. delivery.
The aspect of a Mediator that distinguishes it from any other MUA The aspect of a Mediator that distinguishes it from any other MUA
creating a message is that a Mediator preserves the integrity and creating a message is that a Mediator preserves the integrity and
tone of the original message, including the essential aspects of its tone of the original message, including the essential aspects of its
skipping to change at page 35, line 41 skipping to change at page 37, line 52
5.1. Alias 5.1. Alias
One function of an MDA is to determine the internal location of a 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- mailbox in order to perform delivery. An Alias is a simple re-
addressing facility that provides one or more new Internet Mail addressing facility that provides one or more new Internet Mail
addresses, rather than a single, internal one; the message continues addresses, rather than a single, internal one; the message continues
through the transfer service, for delivery to one or more alternate through the transfer service, for delivery to one or more alternate
addresses. Although typically implemented as part of an MDA, this addresses. Although typically implemented as part of an MDA, this
facility is a Recipient function. It resubmits the message, although facility is a Recipient function. It resubmits the message, although
all handling information except the envelope recipient all handling information except the envelope recipient
(rfc2821.RcptTo) address is retained. In particular, the Return (rfc5321.RcptTo) address is retained. In particular, the Return
address (rfc2821.MailFrom) is unchanged. address (rfc5321.MailFrom) is unchanged.
What is distinctive about this forwarding mechanism is how closely it What is distinctive about this forwarding mechanism is how closely it
resembles normal MTA store-and-forward relaying. Its only resembles normal MTA store-and-forward relaying. Its only
significant difference is that it changes the RFC2821.RcptTo value. significant difference is that it changes the RFC5321.RcptTo value.
Because this change is so small, aliasing can be viewed as a part of Because this change is so small, aliasing can be viewed as a part of
the lower-level mail relaying activity. However, this small change the lower-level mail relaying activity. However, this small change
has a large semantic impact: The designated recipient has chosen a has a large semantic impact: The designated recipient has chosen a
new recipient. new recipient.
NOTE: When the replacement list includes more than one address, NOTE: When the replacement list includes more than one address,
the alias is increasingly likely to have delivery problems. the alias is increasingly likely to have delivery problems.
Any problem reports go to the original Author, not the Any problem reports go to the original Author, not the
administrator of the alias entry. This makes it more difficult administrator of the alias entry. This makes it more difficult
to resolve the problem, because the original Author has no to resolve the problem, because the original Author has no
knowledge of the Alias mechanism. knowledge of the Alias mechanism.
Alias typically changes only envelope information: Including the core set of message information listed at the beginning
of this section, Alias typically changes:
RFC2822.To/.CC/.BCC: Set by - Author RFC5322.To/.CC/.BCC: Set by - Author
These fields retain their original addresses. These fields retain their original addresses.
RFC2821.MailFrom: Set by - Author RFC5321.MailFrom: Set by - Author
The benefit of retaining the original MailFrom value is to The benefit of retaining the original MailFrom value is to
ensure that an actor related to the originating ADMD knows ensure that an actor related to the originating ADMD knows
there has been a delivery problem. On the other hand, the there has been a delivery problem. On the other hand, the
responsibility for handling problems, when transiting from the responsibility for handling problems, when transiting from the
original recipient mailbox to the alias mailbox usually lies original recipient mailbox to the alias mailbox usually lies
with that original Recipient, because the Alias mechanism is with that original Recipient, because the Alias mechanism is
strictly under that Recipient's control. Retaining the strictly under that Recipient's control. Retaining the
original MailFrom address prevents this. original MailFrom address prevents this.
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Also called the ReDirector, the ReSender's actions differ from Also called the ReDirector, the ReSender's actions differ from
forwarding because the Mediator "splices" a message's addressing forwarding because the Mediator "splices" a message's addressing
information to connect the Author of the original message with the information to connect the Author of the original message with the
Recipient of the new message. This connection permits them to have Recipient of the new message. This connection permits them to have
direct exchange, using their normal MUA Reply functions, while also direct exchange, using their normal MUA Reply functions, while also
recording full reference information about the Recipient who served recording full reference information about the Recipient who served
as a Mediator. Hence, the new Recipient sees the message as being as a Mediator. Hence, the new Recipient sees the message as being
from the original Author, even if the Mediator adds commentary. from the original Author, even if the Mediator adds commentary.
These identities are relevant to a resent message: Including the core set of message information listed at the beginning
of this section, these identities are relevant to a resent message:
RFC5322.From: Set by - original Author
RFC2822.From: Set by - original Author
Names and addresses for the original Author of the message Names and addresses for the original Author of the message
content are retained. The free-form (display-name) portion of content are retained. The free-form (display-name) portion of
the address might be modified to provide informal reference to the address might be modified to provide informal reference to
the ReSender. the ReSender.
RFC2822.Reply-To: Set by - original Author RFC5322.Reply-To: Set by - original Author
If this field is present in the original message, it is If this field is present in the original message, it is
retained in the resent message. retained in the resent message.
RFC2822.Sender: Set by - Author's Originator or Mediator RFC5322.Sender: Set by - Author's Originator or Mediator
Originator. Originator.
RFC2822.To/.CC/.BCC: Set by - original Author RFC5322.To/.CC/.BCC: Set by - original Author
These fields specify the original message Recipients. These fields specify the original message Recipients.
RFC2822.Resent-From: Set by - Mediator Author RFC5322.Resent-From: Set by - Mediator Author
This address is of the original Recipient who is redirecting This address is of the original Recipient who is redirecting
the message. Otherwise, the same rules apply to the Resent- the message. Otherwise, the same rules apply to the Resent-
From: field as to an original RFC2822.From field. From: field as to an original RFC5322.From field.
RFC2822.Resent-Sender: Set by - Mediator Originator RFC5322.Resent-Sender: Set by - Mediator Originator
The address of the actor responsible for resubmitting the The address of the actor responsible for resubmitting the
message. As with RFC2822.Sender, this field can be omitted message. As with RFC5322.Sender, this field can be omitted
when it contains the same address as RFC2822.Resent-From. when it contains the same address as RFC5322.Resent-From.
RFC2822.Resent-To/-CC/-BCC: Set by: Mediator Author RFC5322.Resent-To/-CC/-BCC: Set by: Mediator Author
The addresses of the new Recipients who are now able to reply The addresses of the new Recipients who are now able to reply
to the original author. to the original author.
RFC2821.MailFrom: Set by - Mediator Originator RFC5321.MailFrom: Set by - Mediator Originator
The actor responsible for resubmission (RFC5322.Resent-Sender)
The actor responsible for resubmission (RFC2822.Resent-Sender)
is also responsible for specifying the new MailFrom address. is also responsible for specifying the new MailFrom address.
5.3. Mailing Lists 5.3. Mailing Lists
A Mailing List receives messages as an explicit addressee and then A Mailing List receives messages as an explicit addressee and then
re-posts them to a list of subscribed members. The Mailing List 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. 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 In addition to sending the new message to a potentially large number
of new Recipients, the Mailing List can modify content, for example, of new Recipients, the Mailing List can modify content, for example,
by deleting attachments, converting the format, and adding list- by deleting attachments, converting the format, and adding list-
specific comments. Mailing Lists also archive messages posted by specific comments. Mailing Lists also archive messages posted by
Authors. Still the message retains characteristics of being from the Authors. Still the message retains characteristics of being from the
original Author. original Author.
These identities are relevant to a mailing list processor, when Including the core set of message information listed at the beginning
submitting a message: of this section, these identities are relevant to a mailing list
processor, when submitting a message:
RFC2919.List-Id: Set by - Mediator Author RFC2919.List-Id: Set by - Mediator Author
RFC2369.List-*: Set by - Mediator Author RFC2369.List-*: Set by - Mediator Author
RFC2822.From: Set by - original Author RFC5322.From: Set by - original Author
Names and email addresses for the original Author of the Names and email addresses for the original Author of the
message content are retained. message content are retained.
RFC2822.Reply-To: Set by - Mediator or original Author RFC5322.Reply-To: Set by - Mediator or original Author
Although problematic, it is common for a Mailing List to assign Although problematic, it is common for a Mailing List to assign
its own addresses to the Reply-To: header field of messages its own addresses to the Reply-To: header field of messages
that it posts. This assignment is intended to ensure that that it posts. This assignment is intended to ensure that
replies go to all list members, rather than to only the replies go to all list members, rather than to only the
original Author. As a User actor, a Mailing List is the Author original Author. As a User actor, a Mailing List is the Author
of the new message and can legitimately set the Reply-To: of the new message and can legitimately set the Reply-To:
value. As a Mediator attempting to represent the message on value. As a Mediator attempting to represent the message on
behalf of its original Author, creating or modifying a behalf of its original Author, creating or modifying a
Reply-To: field can be viewed as violating that Author's Reply-To: field can be viewed as violating that Author's
intent. Modifying the field to include the list address can intent. When the Reply-To is modified in this way, a reply
send to the entire list replies that are meant only for the that is meant only for the original Author will instead go to
original Author. When the Mailing List does not set the field, the entire list. When the Mailing List does not set the field,
a reply meant for the entire list can instead go only to the a reply meant for the entire list can instead go only to the
original Author. At best, either choice is a matter of group original Author. At best, either choice is a matter of group
culture for the particular list. culture for the particular list.
RFC2822.Sender: Set by - Author Originator or Mediator RFC5322.Sender: Set by - Author Originator or Mediator
Originator Originator
This field usually specifies the address of the actor This field usually specifies the address of the actor
responsible for Mailing List operations. Mailing Lists that responsible for Mailing List operations. Mailing Lists that
operate in a manner similar to a simple MTA Relay preserve as operate in a manner similar to a simple MTA Relay preserve as
much of the original handling information as possible, much of the original handling information as possible,
including the original RFC2822.Sender field. (Note that this including the original RFC5322.Sender field. (Note that this
mode of operation causes the Mailing List to behave much like mode of operation causes the Mailing List to behave much like
an Alias, with a possible difference in number of new an Alias, with a possible difference in number of new
addressees.) addressees.)
RFC2822.To/.CC: Set by - original Author RFC5322.To/.CC: Set by - original Author
These fields usually contain the original list of Recipient These fields usually contain the original list of Recipient
addresses. addresses.
RFC2821.MailFrom: Set by - Mediator Originator RFC5321.MailFrom: Set by - Mediator Originator
Because a Mailing List can modify the content of a message in 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 any way, it is responsible for that content; that is, it is an
Author. As such, the Return Address is specified by the Author. As such, the Return Address is specified by the
Mailing List. Although it is plausible for the Mailing List to Mailing List. Although it is plausible for the Mailing List to
re-use the Return Address employed by the original Originator, re-use the Return Address employed by the original Originator,
notifications sent to that address after a message has been notifications sent to that address after a message has been
processed by a Mailing List could be problematic. processed by a Mailing List could be problematic.
5.4. Gateways 5.4. Gateways
skipping to change at page 39, line 39 skipping to change at page 41, line 51
standards, but significantly different administrative policies, it is standards, but significantly different administrative policies, it is
easy to view a Gateway as merely an MTA. easy to view a Gateway as merely an MTA.
The critical distinction between an MTA and a Gateway is that a The critical distinction between an MTA and a Gateway is that a
Gateway can make substantive changes to a message to map between the Gateway can make substantive changes to a message to map between the
standards. In virtually all cases, this mapping results in some standards. In virtually all cases, this mapping results in some
degree of semantic loss. The challenge of Gateway design is to degree of semantic loss. The challenge of Gateway design is to
minimize this loss. Standardized gateways to Internet Mail are minimize this loss. Standardized gateways to Internet Mail are
facsimile [RFC4143], voicemail [RFC3801], and MMS [RFC4356] facsimile [RFC4143], voicemail [RFC3801], and MMS [RFC4356]
A Gateway can set any identity field available to an MUA. These A Gateway can set any identity field available to an MUA. Including
identities are typically relevant to Gateways: the core set of message information listed at the beginning of this
section, these identities are typically relevant to Gateways:
RFC2822.From: Set by - original Author RFC5322.From: Set by - original Author
Names and addresses for the original Author of the message Names and addresses for the original Author of the message
content are retained. As for all original addressing content are retained. As for all original addressing
information in the message, the Gateway can translate addresses information in the message, the Gateway can translate addresses
as required to continue to be useful in the target environment. as required to continue to be useful in the target environment.
RFC2822.Reply-To: Set by - original Author RFC5322.Reply-To: Set by - original Author
The Gateway SHOULD retain this information, if it is present. The Gateway SHOULD retain this information, if it is present.
The ability to perform a successful reply by a Recipient is a The ability to perform a successful reply by a Recipient is a
typical test of Gateway functionality. typical test of Gateway functionality.
RFC2822.Sender: Set by - Author Originator or Mediator RFC5322.Sender: Set by - Author Originator or Mediator
Originator Originator
This field can retain the original value or can be set to a new This field can retain the original value or can be set to a new
address. address.
RFC2822.To/.CC/.BCC: Set by - original Recipient RFC5322.To/.CC/.BCC: Set by - original Recipient
These fields usually retain their original addresses. These fields usually retain their original addresses.
RFC2821.MailFrom: Set by - Author Originator or Mediator RFC5321.MailFrom: Set by - Author Originator or Mediator
Originator Originator
The actor responsible for handling the message can specify a The actor responsible for handling the message can specify a
new address to receive handling notices. new address to receive handling notices.
5.5. Boundary Filter 5.5. Boundary Filter
To enforce security boundaries, organizations can subject messages to To enforce security boundaries, organizations can subject messages to
analysis, for conformance with its safety policies. An example is analysis, for conformance with its safety policies. An example is
detection of content classed as spam or a virus. A filter might detection of content classed as spam or a virus. A filter might
skipping to change at page 40, line 37 skipping to change at page 43, line 4
5.5. Boundary Filter 5.5. Boundary Filter
To enforce security boundaries, organizations can subject messages to To enforce security boundaries, organizations can subject messages to
analysis, for conformance with its safety policies. An example is analysis, for conformance with its safety policies. An example is
detection of content classed as spam or a virus. A filter might detection of content classed as spam or a virus. A filter might
alter the content, to render it safe, such as by removing content alter the content, to render it safe, such as by removing content
deemed unacceptable. Typically, these actions add content to the deemed unacceptable. Typically, these actions add content to the
message that records the actions. message that records the actions.
6. Considerations 6. Considerations
6.1. Security Considerations 6.1. Security Considerations
This document describes the existing Internet Mail architecture. It This document describes the existing Internet Mail architecture. It
introduces no new capabilities. The security considerations of this introduces no new capabilities. The security considerations of this
deployed architecture are documented extensively in the technical deployed architecture are documented extensively in the technical
specifications referenced by this document. These specifications specifications referenced by this document. These specifications
cover classic security topics, such as authentication and privacy. cover classic security topics, such as authentication and privacy.
For example, email transfer protocols can use standardized mechanisms For example, email transfer protocols can use standardized mechanisms
for operation over authenticated and/or encrypted links, and message for operation over authenticated and/or encrypted links, and message
content has similar protection standards available. Examples of such content has similar protection standards available. Examples of such
mechanisms include SMTP-TLS [RFC3207], SMTP-Auth [RFC2554], OpenPGP mechanisms include SMTP-TLS [RFC3207], SMTP-Auth [RFC4954], OpenPGP
[RFC4880], and S/MIME [RFC3851]. [RFC4880], and S/MIME [RFC3851].
The core of the Internet Mail architecture does not impose any The core of the Internet Mail architecture does not impose any
security requirements or functions on the end-to-end or hop-by-hop security requirements or functions on the end-to-end or hop-by-hop
components. For example, it does not require participant components. For example, it does not require participant
authentication and does not attempt to prevent data disclosure. authentication and does not attempt to prevent data disclosure.
Particular message attributes might expose specific security Particular message attributes might expose specific security
considerations. For example, the blind carbon copy feature of the considerations. For example, the blind carbon copy feature of the
architecture invites disclosure concerns, as discussed in section 7.2 architecture invites disclosure concerns, as discussed in section 7.2
of [RFC2821] and section 5 of [RFC2822]. Transport of text or non- of [RFC5321] and section 5 of [RFC5322]. Transport of text or non-
text content in this architecture has security considerations that text content in this architecture has security considerations that
are discussed in [RFC2822], [RFC2045], [RFC2046], and [RFC4288] as are discussed in [RFC5322], [RFC2045], [RFC2046], and [RFC4288] as
well as the security considerations present in the IANA media types well as the security considerations present in the IANA media types
registry for the respective types. registry for the respective types.
Agents that automatically respond to email raise significant security Agents that automatically respond to email raise significant security
considerations, as discussed in [RFC3834]. Gateway behaviors affect considerations, as discussed in [RFC3834]. Gateway behaviors affect
end-to-end security services, as discussed in [RFC2480]. Security end-to-end security services, as discussed in [RFC2480]. Security
considerations for boundary filters are discussed in [RFC5228]. considerations for boundary filters are discussed in [RFC5228].
See section 7.1 of [RFC2821] for a discussion of the topic of See section 7.1 of [RFC5321] for a discussion of the topic of
origination validation. As mentioned in Section 4.1.4, it is common origination validation. As mentioned in Section 4.1.4, it is common
practice for components of this architecture to use the practice for components of this architecture to use the
[RFC0791].SourceAddr to make policy decisions [RFC2505], although the [RFC0791].SourceAddr to make policy decisions [RFC2505], although the
address can be "spoofed". It is possible to use it without address can be "spoofed". It is possible to use it without
authorization. SMTP and Submission authentication [RFC2554], authorization. SMTP and Submission authentication [RFC4954],
[RFC4409] provide more secure alternatives. [RFC4409] provide more secure alternatives.
The discussion of trust boundaries, ADMDs, actors, roles, and The discussion of trust boundaries, ADMDs, actors, roles, and
responsibilities in this document highlights the relevance and responsibilities in this document highlights the relevance and
potential complexity of security factors for operation of an Internet potential complexity of security factors for operation of an Internet
mail service. The core design of Internet Mail to encourage open and mail service. The core design of Internet Mail to encourage open and
casual exchange of messages has met with scaling challenges, as the casual exchange of messages has met with scaling challenges, as the
population of email participants has grown to include those with population of email participants has grown to include those with
problematic practices. For example, spam, as defined in [RFC2505], problematic practices. For example, spam, as defined in [RFC2505],
is a by-product of this architecture. A number of standards track or is a by-product of this architecture. A number of standards track or
BCP documents on the subject have been issued. [RFC2505], [RFC5068], BCP documents on the subject have been issued. [RFC2505], [RFC5068],
[RFC3685] [RFC3685]
6.2. IANA Considerations 6.2. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
6.3. Internationalization 6.3. Internationalization
Because its origins date back to the use of ASCII, Internet Mail has The core Internet email standards are based on the use of US-ASCII.
had an ongoing challenge to support the wide range of necessary That is SMTP [RFC5321] and IMF [RFC5322], as well as their
international data representations. For a discussion of this topic, predecessors, describe the transport and composition of messages
see [MAIL-I18N]. composed strictly of US-ASCII 7-bit encoded characters. The
standards have been incrementally enhanced to allow for characters
outside of this limited set, while retaining mechanisms for
backwards-compatibility. Specifically:
o The MIME specifications [RFC2045], [RFC2046], [RFC2047] and
[RFC2298] allow for the use of coded character sets and character
encoding schemes ("charsets" in MIME terminology) other than US-
ASCII. MIME's [RFC2046] allows the textual content of a message
to have a label affixed that specifies the charset used in that
content. Equally MIME's [RFC2047] allows the textual content of
certain header fields in a message to be similarly labeled.
However, since messages might be transported over SMTP
implementations only capable of transporting 7-bit encoded
characters, MIME's [RFC2045] also provides for "content transfer
encoding" so that characters of other charsets can be re-encoded
as an overlay to US-ASCII.
o MIME's [RFC2045] allows for the textual content of a message to be
in an 8-bit character encoding scheme. In order to transport
these without re-encoding them, the SMTP specification supports an
option [RFC1652] that permits the transport of such textual
content. However, the [RFC1652] option does not address the use
of 8-bit content in message header fields, and therefore [RFC2047]
encoding is still required for those.
o A series of experimental protocols on Email Address
Internationalization (EAI) have been released that extend SMTP and
IMF to allow for 8-bit encoded characters to appear in addresses
and other information throughout the header fields of messages.
[RFC5335] specifies the format of such message header fields
(which encode the characters in UTF-8), and [RFC5336] specifies an
SMTP option for the transport of these messages.
Hence, the use of UTF-8 is fully established in existing Internet
mail. However support for long-standing encoding forms is retained
and is still used.
7. References 7. References
7.1. Normative 7.1. Normative
[RFC0791] Postel, J., "Internet Protocol", RFC 791, 1981 September. [RFC0791] Postel, J., "Internet Protocol", RFC 791, 1981 September.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
skipping to change at page 42, line 42 skipping to change at page 45, line 43
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996. Examples", RFC 2049, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997. Specification", RFC 2181, July 1997.
[RFC2298] Fajman, R., "An Extensible Message Format for Message
Disposition Notifications", RFC 2298, March 1998.
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax [RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998. Message Header Fields", RFC 2369, July 1998.
[RFC2645] "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP [RFC2645] "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
Addresses", RFC 2645, August 1999. Addresses", RFC 2645, August 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001. April 2001.
skipping to change at page 44, line 13 skipping to change at page 47, line 19
[RFC4550] Maes, S., , S., and Isode Ltd., "Internet Email to Support [RFC4550] Maes, S., , S., and Isode Ltd., "Internet Email to Support
Diverse Service Environments (Lemonade) Profile", Diverse Service Environments (Lemonade) Profile",
June 2006. June 2006.
[RFC5228] Showalter, T., "Sieve: A Mail Filtering Language", [RFC5228] Showalter, T., "Sieve: A Mail Filtering Language",
RFC 5228. RFC 5228.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", RFC 5248, June 2008. Mail System Status Codes", RFC 5248, June 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5335] TWNIC, "Internationalized Email Headers", RFC 5335,
September 2008.
7.2. Informative 7.2. Informative
[MAIL-I18N] [MAIL-I18N]
Internet Mail Consortium, "Using International Characters Internet Mail Consortium, "Using International Characters
in Internet Mail", IMC IMCR-010, August 1998. in Internet Mail", IMC IMCR-010, August 1998.
[RFC0733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
"Standard for the Format of ARPA Network Text Messages",
RFC 733, November 1977.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, [RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982. RFC 821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet [RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982. text messages", STD 11, RFC 822, August 1982.
[RFC1506] Houttuin, J., "A Tutorial on Gatewaying between X.400 and
Internet Mail", RFC 1506, August 1993.
[RFC1652] MCI, Innosoft, Dover Beach Consulting, Inc., Network
Management Associates, Inc., and Silicon Graphics, Inc.,
"SMTP Service Extension for 8bit-MIMEtransport", RFC 1652,
July 1994.
[RFC1733] Crispin, M., "Distributed Electronic Models in IMAP4", [RFC1733] Crispin, M., "Distributed Electronic Models in IMAP4",
December 1994. December 1994.
[RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects", [RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects",
RFC 1767, March 1995. RFC 1767, March 1995.
[RFC1985] De Winter, J., "SMTP Service Extension for Remote [RFC1985] De Winter, J., "SMTP Service Extension for Remote
Message Queue Starting", August 1996. Message Queue Starting", August 1996.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033, [RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
skipping to change at page 44, line 48 skipping to change at page 48, line 28
Functions", RFC 2142, May 1997. Functions", RFC 2142, May 1997.
[RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998. [RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998.
[RFC2480] Freed, N., "Gateways and MIME Security Multiparts", [RFC2480] Freed, N., "Gateways and MIME Security Multiparts",
RFC 2480, January 1999. RFC 2480, January 1999.
[RFC2505] Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs", [RFC2505] Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs",
RFC 2505, February 1999. 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 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002. Transport Layer Security", RFC 3207, February 2002.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
January 2003.
[RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest [RFC3685] Daboo, C., "SIEVE Email Filtering: Spamtest and VirusTest
Extensions", RFC 3685, February 2004. Extensions", RFC 3685, February 2004.
[RFC3801] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet [RFC3801] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
Mail - version 2 (VPIMv2)", RFC 3801, June 2004. Mail - version 2 (VPIMv2)", RFC 3801, June 2004.
[RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification", Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004. RFC 3851, July 2004.
skipping to change at page 45, line 31 skipping to change at page 49, line 11
[RFC4143] Toyoda, K. and D. Crocker, "Facsimile Using Internet Mail [RFC4143] Toyoda, K. and D. Crocker, "Facsimile Using Internet Mail
(IFAX) Service of ENUM", RFC 4143, November 2005. (IFAX) Service of ENUM", RFC 4143, November 2005.
[RFC4356] Gellens, R., "Mapping Between the Multimedia Messaging [RFC4356] Gellens, R., "Mapping Between the Multimedia Messaging
Service (MMS) and Internet Mail", RFC 4356, January 2006. Service (MMS) and Internet Mail", RFC 4356, January 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007. Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954, July 2007.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and [RFC5068] Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and
E. Allman, "Email Submission Operations: Access and E. Allman, "Email Submission Operations: Access and
Accountability Requirements", RFC 5068, BCP 134, Nov 2007. Accountability Requirements", RFC 5068, BCP 134, Nov 2007.
[RFC5336] Yao, J., Ed. and W. Mao, Ed., "SMTP Extension for
Internationalized Email Addresses", RFC 5336,
September 2008.
[Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden, [Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
"Tussle in Cyberspace: Defining Tomorrow's Internet", "Tussle in Cyberspace: Defining Tomorrow's Internet",
ACM SIGCOMM, 2002. ACM SIGCOMM, 2002.
Appendix A. Acknowledgements Appendix A. Acknowledgements
This work derives from a section in an early version of [RFC5068]. This work began in 2004 and has evolved through numerous rounds of
Discussion of the Originator actor role was greatly clarified during community review; it derives from a section in an early version of
[RFC5068]. Over its 4 years of development, the draft has gone
through 12 incremental versions, with vigorous community review that
produced many substantive changes. Review was performed in the IETF
and other email technical venues. Although not a formal activity of
the IETF, issues with the document's contents were resolved using the
classic style of IETF community open, group decision-making. The
document is already cited in other work, such as for IMAP and Sieve
specifications and for academic classwork. The step of standardizing
is useful to provide a solid and stable reference to the Internet's
now-complex email service.
Details of the Originator actor role was greatly clarified during
discussions in the IETF's Marid working group. discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
insight on the framework and details of the original drafts, as did insight on the framework and details of the original drafts, as did
Chris Newman for the final versions, while also serving as cognizant Chris Newman for the final versions, while also serving as cognizant
Area Director for the document. Tony Hansen served as document Area Director for the document. Tony Hansen served as document
shepherd, through the IETF process. shepherd, through the IETF process.
Later reviews and suggestions were provided by Eric Allman, Nathaniel Later reviews and suggestions were provided by Eric Allman, Nathaniel
Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch,
Ned Freed, Eric Hall, Willemien Hoogendoorn, Brad Knowles, John Ned Freed, Eric Hall, Willemien Hoogendoorn, Brad Knowles, John
Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg, Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg,
Alexey Melnikov, der Mouse, S. Moonesamy, Daryl Odnert, Rahmat M. Alexey Melnikov, der Mouse, S. Moonesamy, Daryl Odnert, Rahmat M.
Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, Greg Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, and Greg
Vaudreuil. Vaudreuil.
Diligent early proof-reading was performed by Bruce Lilly. Diligent Diligent early proof-reading was performed by Bruce Lilly. Diligent
technical editing was provided by Susan Hunziker technical editing was provided by Susan Hunziker.
The final stages of development for this document were guided by a
design team comprising: Alexey Melnikov, Pete Resnick, Carl S.
Gutekunst, Jeff Macdonald, Randall Gellens, Tony Hansen and Tony
Finch. Pete Resnick developed the final version of the section on
internationalization.
Index Index
10 12
A A
accountability 11 accountability 13
accountable 12-13 accountable 13-14
Actor Actor
Administrative 13 Administrative 14
Author 8 Author 9
Consumer 14 Consumer 15
Edge 14 Edge 15
Gateway 12 Gateway 14
Originator 11 Originator 12
Recipient 9 Recipient 10
Return Handler 9 Return Handler 10
Transit 14 Transit 15
Actors Actors
MHS 10 MHS 11
ADMD 11, 13-14, 18, 23, 29, 36 ADMD 13-15, 19, 25, 31, 38
Administrative Actors 13 Administrative Actors 14
Administrative Management Domain 11 Administrative Management Domain 13
aMSA 29 aMSA 31
Author 8, 10 Author 9, 12
author 33 author 35
B B
body-parts 22 body-parts 24
bounce handler 9 bounce handler 10
boundary 14 boundary 15
C C
Consumer Actor 14 Consumer Actor 15
content 10, 12-13, 18, 22, 30 content 11, 13-14, 20, 24, 32
D D
delivery 4, 9-10, 12-13, 17, 22-23, 33, 35-36 delivery 5, 10, 12-14, 18, 24-25, 35, 38
Discussion of document 7 Discussion of document 8
E E
Edge Actor 14 Edge Actor 15
end-to-end 4 end-to-end 5
envelope 9, 12, 19, 22-23, 30, 35-36 envelope 10, 13, 21, 24-25, 32, 38
ETRN 33 ETRN 35
G G
Gateway 10, 12 Gateway 11, 14
H H
header 22 header 24
hMSA 29 hMSA 31
I I
Internet Mail 4 Internet Mail 5
L L
LMTP 33 LMTP 35
local-part 16 local-part 18
M M
Mail 4 Mail 5
Mail User Agent 4 Mail From 38
Mail From 35 Mail Handling Service 5, 11
Mail Handling Service 4, 10 Mail Submission Agent 12
Mail Submission Agent 11 Mail Transfer Agent 5
Mail Transfer Agent 4 Mail User Agent 5
mailbox 35 mailbox 38
MDA 35 MDA 38
MDN 9 MDN 10
message 6, 22 message 7, 24
Message Disposition Notification 9 Message Disposition Notification 10
MHS 4, 9-12, 19-20, 22-23 MHS 5, 10-11, 13, 21-22, 24-25
Actors 10 Actors 11
MSA 11, 29 MSA 12, 31
MTA 4, 14 MTA 5, 15
boundary 14 boundary 15
MUA 4, 13, 28-29 MUA 5, 14, 30-31
O O
ODMR 33 ODMR 35
Originator 10-11 Originator 10, 12
P P
posting 4, 9, 11, 19, 28-29, 33, 36 posting 5, 10, 12, 21, 30-31, 35, 38
pull 33 pull 35
push 33 push 35
R R
RcptTo 10 RcptTo 11
Receiver 10 Receiver 12
Recipient 9-10, 35 Recipient 10, 12, 38
recipient 33 recipient 35
relay 10 relay 12
responsibility 29 responsibility 31
responsible 12-13 responsible 13-14
Return address 35 Return address 38
Return Handler 9 Return Handler 10
role 9, 17 role 10, 18
Author 8 Author 9
Originator 11 Originator 12
Recipient 9 Recipient 10
S S
SIEVE 22 SIEVE 24
SMTP 33 SMTP 35
T T
transfer 10, 12-13 transfer 12-14
Transit Actor 14 Transit Actor 15
transition 29 transition 31
U U
UA 4 UA 5
User Agent 4 User Agent 5
Author's Address Author's Address
Dave Crocker Dave Crocker
Brandenburg InternetWorking Brandenburg InternetWorking
675 Spruce Drive 675 Spruce Drive
Sunnyvale, CA 94086 Sunnyvale, CA 94086
USA USA
Phone: +1.408.246.8253 Phone: +1.408.246.8253
Email: dcrocker@bbiw.net Email: dcrocker@bbiw.net
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