< draft-crocker-email-arch-12.txt   draft-crocker-email-arch-13.txt >
SMTP D. Crocker SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track April 12, 2009 Intended status: Standards Track May 15, 2009
Expires: October 14, 2009 Expires: November 16, 2009
Internet Mail Architecture Internet Mail Architecture
draft-crocker-email-arch-12 draft-crocker-email-arch-13
Status of this Memo Status of this Memo
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Document Conventions . . . . . . . . . . . . . . . . . . . 7 1.2. The Role of This Architecture . . . . . . . . . . . . . . 7
2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 8 1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 8
2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 8 2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 9
2.2. Mail Handling Service (MHS) Actors . . . . . . . . . . . . 11 2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 14 2.2. Message Handling Service (MHS) Actors . . . . . . . . . . 12
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 15
3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 17 3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2. Scope of Email Address Use . . . . . . . . . . . . . . . . 18 3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 19 3.2. Scope of Email Address Use . . . . . . . . . . . . . . . . 19
3.4. Message Identifier . . . . . . . . . . . . . . . . . . . . 19 3.3. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 20
4. Services and Standards . . . . . . . . . . . . . . . . . . . . 21 3.4. Message Identifier . . . . . . . . . . . . . . . . . . . . 20
4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 24 4. Services and Standards . . . . . . . . . . . . . . . . . . . . 22
4.1.4. Identity References in a Message . . . . . . . . . . . 25 4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 25
4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 29 4.1.4. Identity References in a Message . . . . . . . . . . . 27
4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 31 4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 30
4.4. Transition Modes . . . . . . . . . . . . . . . . . . . . . 35 4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 32
4.5. Implementation and Operation . . . . . . . . . . . . . . . 35 4.4. Transition Modes . . . . . . . . . . . . . . . . . . . . . 36
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4.5. Implementation and Operation . . . . . . . . . . . . . . . 36
5.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . . . 37 5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.2. ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 38 5.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 40 5.2. ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 41
5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 42 5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 42
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 42 5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 43
6.1. Security Considerations . . . . . . . . . . . . . . . . . 43 6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 43
6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 44 6.1. Security Considerations . . . . . . . . . . . . . . . . . 44
6.3. Internationalization . . . . . . . . . . . . . . . . . . . 44 6.2. IANA Considerations . . . . . . . . . . . . . . . . . . . 45
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45 6.3. Internationalization . . . . . . . . . . . . . . . . . . . 45
7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 45 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
7.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 47 7.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 46
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 49 7.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 48
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 50
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 52 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 54
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 The underlying technical standards for Internet Mail comprise a rich
array of functional capabilities. The specifications form the core: array of functional capabilities. These specifications form the
core:
* Simple Mail Transfer Protocol (SMTP) [RFC0821], [RFC2821], * Simple Mail Transfer Protocol (SMTP) [RFC0821], [RFC2821],
[RFC5321] moves a message through the Internet. [RFC5321] moves a message through the Internet.
* Internet Mail Format (IMF) [RFC0733], [RFC0822], [RFC2822], * Internet Mail Format (IMF) [RFC0733], [RFC0822], [RFC2822],
[RFC5321] defines a message object. [RFC5322] defines a message object.
* Multipurpose Internet Mail Extensions (MIME) [RFC2045] defines * Multipurpose Internet Mail Extensions (MIME) [RFC2045] defines
an enhancement to the message object that permits using multi- an enhancement to the message object that permits using multi-
media attachments. 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
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* Clarifying functional roles for the architectural components * Clarifying functional roles for the architectural components
* 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 Message User
(MUA), and the transfer world, in the form of the Mail Handling Agents (MUA), and the transfer world, in the form of the Message
Service (MHS), which is composed of Mail Transfer Agents (MTA) Handling Service (MHS), which is composed of Message Transfer Agents
[RFC1506]. The MHS accepts a message from one User and delivers it (MTA) [RFC1506]. The MHS accepts a message from one User and
to one or more other users, creating a virtual MUA-to-MUA exchange delivers it to one or more other users, creating a virtual MUA-to-MUA
environment. 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, remain
remaining remarkably constant. The original distinction between the remarkably constant. The original distinction between the user level
user level and transfer level remains, but with elaborations in each. and transfer level remains, but with elaborations in each. The term
The term "Internet Mail" is used to refer to the entire collection of "Internet Mail" is used to refer to the entire collection of user and
user and transfer components and services. 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 2.1.4. 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 |
|| +--------+ || +--------+
|| ^ || ^
+--------+ || +--------+ . +--------+ || +--------+ .
| User +==++=========>| User | . | User +==++=========>| User | .
+---+----+ || +--------+ . +---+----+ || +--------+ .
. || ^ . . || ^ .
. || +--------+ . . . || +--------+ . .
. ++==>| User | . . . ++==>| User | . .
. +--------+ . . . +--------+ . .
. ^ . . . ^ . .
. . . . . . . .
V . . . V . . .
+---+-----------------+------+------+---+ +---+-----------------+------+------+---+
| . . . . | | . . . . |
| .................>. . . | | .................>. . . |
| . . . | | . . . |
| ........................>. . | | ........................>. . |
| . . | | . . |
| ...............................>. | | ...............................>. |
| | | |
| Mail Handling Service (MHS) | | Message Handling Service (MHS) |
+---------------------------------------+ +---------------------------------------+
Legend: == lines indicate primary (possibly indirect) transfers; ...
lines indicate supporting transfers Legend: == lines indicate primary (possibly indirect) transfers or roles
... lines indicate supporting transfers or roles
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
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Within localized networks at the edge of the public Internet, prior Within localized networks at the edge of the public Internet, prior
administrative arrangement often is required and can include access administrative arrangement often is required and can include access
control, routing constraints, and configuration of the information control, routing constraints, and configuration of the information
query service. Although recipient authentication has usually been query service. Although recipient authentication has usually been
required for message access since the beginning of Internet Mail, in required for message access since the beginning of Internet Mail, in
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. The Role of This Architecture
An Internet service is an integration of related capabilities among
two or more participating nodes. The capabilities are accomplished
across the Internet by one or more protocols. What connects a
protocol to a service is an architecture. An architecture specifies
how the protocols implement the service by defining the logical
components of a service and the relationships among them. From that
logical view, a service defines what is being done, an architecture
defines where the pieces are (in relation to each other) and a
protocol defines how particular capabilities are performed.
As such, an architecture will more formally describe a service at a
relatively high level. A protocol which implements some portion of a
service will conform to the architecture to a greater or lesser
extent, depending on the pragmatic tradeoffs they make when trying to
implement the architecture in the context of real-world constraints.
Failure to precisely follow an architecture is not a failure of the
protocol, nor is failure to precisely cast a protocol a failure of
the architecture. Where a protocol varies from the architecture, it
should of course explain the reason for the variance. However, such
variance is not a mark against a protocol: Happily, the IETF prefers
running code to architectural purity.
In this particular case, this architecture attempts to define the
logical components of Internet email and does so post hoc, trying to
capture the architectural principles that the current email protocols
embody. To different extents, email protocols will conform to this
architecture more or less well. Insofar as this architecture differs
from those protocols, the reasons are generally well understood and
are required for interoperation. The differences are not a sign that
protocols need to be fixed. However, this architecture is a best
attempt at a logical model of Internet email, and insofar as new
protocol development varies from this architecture, it is necessary
for designers to understand those differences and explain them
carefully.
1.3. 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 <RFC5322.From> is the IMF From: header field in an email Hence <RFC5322.From> is the IMF From: header field in an email
content header and <RFC5321.MailFrom> is the address in the SMTP content header and <RFC5321.MailFrom> is the address in the SMTP
"Mail From" command. "Mail From" command.
When occurring without the IMF (rfc5322) qualifier, header field When occurring without the IMF (RFC 5322) qualifier, header field
names are 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].
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to the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>. to the IETF-SMTP mailing list <http://www.imc.org/ietf-smtp>.
2. Responsible Actor Roles 2. Responsible Actor Roles
Internet Mail is a highly distributed service, with a variety of Internet Mail is a highly distributed service, with a variety of
actors playing different roles. These actors fall into three basic actors playing different roles. These actors fall into three basic
types: types:
* User * User
* Mail Handling Service (MHS) * Message Handling Service (MHS)
* ADministrative Management Domain (ADMD) * ADministrative Management Domain (ADMD)
Although related to a technical architecture, the focus on actors Although related to a technical architecture, the focus on actors
concerns participant responsibilities, rather than functionality of concerns participant responsibilities, rather than functionality of
modules. For that reason, the labels used are different from those modules. For that reason, the labels used are different from those
used in classic email architecture diagrams. used in classic email architecture diagrams.
2.1. User Actors 2.1. User Actors
skipping to change at page 9, line 5 skipping to change at page 10, line 5
* Recipients * Recipients
* Return Handlers * Return Handlers
* Mediators * Mediators
Figure 2 shows the primary and secondary flows of messages among Figure 2 shows the primary and secondary flows of messages among
them. them.
++==========++ ++==========++
|| Author ||<..................................<.. || Author ||<..................................<..
++=++=++=++=++ . ++=++=++=++=++ .
|| || || ++===========++ . || || || ++===========++ .
|| || ++====>|| Recipient || . || || ++====>|| Recipient || .
|| || ++=====+=====++ . || || ++=====+=====++ .
|| || . . || || . .
|| || ..........................>.+ || || ..........................>.+
|| || . || || .
|| || ................... . || || ................... .
|| || . . . || || . . .
|| || V . . || || V . .
|| || +-----------+ ++=====+=====++ . || || +-----------+ ++=====+=====++ .
|| ++========>| Mediator +===>|| Recipient || . || ++========>| Mediator +===>|| Recipient || .
|| +-----+-----+ ++=====+=====++ . || +-----+-----+ ++=====+=====++ .
|| . . . || . . .
|| ..................+.......>.+ || ..................+.......>.+
|| . || .
|| ..............+.................. . || ..............+.................. .
|| . . . . || . . . .
\/ V V ' . \/ V V ' .
+-----------+ +-----------+ ++=====+=====++ . +-----------+ +-----------+ ++=====+=====++ .
| Mediator +===>| Mediator +===>|| Recipient || . | Mediator +===>| Mediator +===>|| Recipient || .
+-----+-----+ +-----+-----+ ++=====+=====++ . +-----+-----+ +-----+-----+ ++=====+=====++ .
. . . . . . . .
.................+.................+.......>.. .................+.................+.......>..
Legend: == lines indicate primary (possibly indirect) transfers; ...
lines indicate supporting transfers Legend: == lines indicate primary (possibly indirect) transfers or roles
... lines indicate supporting transfers or roles
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 message transfer activities are
by a monolithic Mail Handling Service (MHS), even though the actual performed by a monolithic Message Handling Service (MHS), even though
service can be provided by many independent organizations. Users are the actual service can be provided by many independent organizations.
customers of this unified service. Users are 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
User. User.
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
skipping to change at page 11, line 17 skipping to change at page 12, line 17
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. Message Handling Service (MHS) Actors
The Mail Handling Service (MHS) performs a single end-to-end transfer The Message Handling Service (MHS) performs a single end-to-end
on behalf of the Author to reach the Recipient addresses specified in transfer on behalf of the Author to reach the Recipient addresses
the original RFC5321.RcptTo commands. Exchanges that are either specified in the original RFC5321.RcptTo commands. Exchanges that
mediated or iterative and protracted, such as those used for are either mediated or iterative and protracted, such as those used
collaboration over time are handled by the User actors, not by the for collaboration over time are handled by the User actors, not by
MHS actors. the 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 Legend: == lines indicate primary (possibly indirect) transfers or roles
roles; ... lines indicate supporting 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
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2.2.2. Relay 2.2.2. Relay
The Relay performs MHS-level transfer-service routing and store-and- The Relay performs MHS-level transfer-service routing and store-and-
forward, by transmitting or retransmitting the message to its forward, by transmitting or retransmitting the message to its
Recipients. The Relay adds trace information [RFC2505] but does not Recipients. The Relay adds trace information [RFC2505] but does not
modify the envelope information or the message content semantics. It modify the envelope information or the message content semantics. It
can modify message content representation, such as changing the form can modify message content representation, such as changing the form
of transfer encoding from binary to text, but only as required to of transfer encoding from binary to text, but only as required to
meet the capabilities of the next hop in the MHS. meet the capabilities of the next hop in the MHS.
A Mail Handling Service (MHS) network consists of a set of Relays. A Message Handling System (MHS) network consists of a set of Relays.
This network is above any underlying packet-switching network that This network is above any underlying packet-switching network that
might be used and below any Gateways or other Mediators. might be used and below any Gateways or other Mediators.
In other words, email scenarios can involve three distinct In other words, email scenarios can involve three distinct
architectural layers, each providing its own type of data of store- architectural layers, each providing its own type of data of store-
and-forward service: and-forward service:
* User Mediators * User Mediators
* MHS Relays * MHS Relays
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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 | | | | | | Recipient | | Author | | | | | | Recipient |
| . | | | | | | ^ | | . | | | | | | ^ |
| V | | | | | | . | | V | | | | | | . |
| Edge..+....>|.Transit.+....>|-Edge..+....>|..Consumer | | Edge..+....>|.Transit.+....>|-Edge..+....>|..Consumer |
| | | | | | | | | | | | | | | |
+--------+ +---------+ +-------+ +-----------+ +--------+ +---------+ +-------+ +-----------+
Legend: == lines indicate primary (possibly indirect) transfers or
roles; ... lines indicate supporting transfers or roles 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
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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, The forms of identity used by Internet Mail are: mailbox, domain
message-ID and ENVID. Each must be globally unique. name, 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 receives mail. It is a conceptual entity that does not
which does not necessarily pertain to file storage." [RFC5322] 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 [RFC5321]. 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
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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.
Basic email addressing defines the <local-part> as being globally Basic email addressing defines the <local-part> as being globally
opaque. However there are some uses of email that add a opaque. However there are some uses of email that add a
standardized, global schema to the value, such as between an author standardized, global schema to the value, such as between an author
and a Gateway. The <local-part> details remain invisible to the and a Gateway. The <local-part> details remain invisible to the
public email transfer infrastructure, but provide addressing and public email transfer infrastructure, but provide addressing and
handling instructions for further processing by the Gateway. handling instructions for further processing by the Gateway.
Standardized examples of such conventions are the telephone numbering Standardized examples of these conventions are the telephone
formats for VPIM [RFC3801] such as: numbering formats for VPIM [RFC3801], 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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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 an individual user, but this is not common practice. The refer to an individual user, but this is not common practice. The
name is structured as a hierarchical sequence of names, separated by name is structured as a hierarchical sequence of labels, separated by
dots (.), 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. There can be many names in the sequence -- that is, the sequence. There can be many names in the sequence -- that is, the
depth of the hierarchy can be substantial. Domain names are defined depth of the hierarchy can be substantial. Domain names are defined
and operated through the Domain Name System (DNS) [RFC1034], and operated through the Domain Name System (DNS) [RFC1034],
[RFC1035], [RFC2181]. [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:. IMF provides for, at most, a single Message-ID:. The Message-ID: for
The Message-ID: for a single message, which is a user-level IMF tag, a single message, which is a user-level IMF tag, has a variety of
has a variety of uses including threading, aiding identification of uses including threading, aiding identification of duplicates, and
duplicates, and DSN tracking. [RFC5322]. The Originator assigns the DSN tracking. The Originator assigns the Message-ID:. The
Message-ID:. The Recipient's ADMD is the intended consumer of the Recipient's ADMD is the intended consumer of the Message-ID:,
Message-ID:, although any actor along the transfer path can use it. 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
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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
service. As shown in Figure 5, each type can have multiple service. As shown in Figure 5, each type can have multiple
instances, some of which represent specialized roles. This section instances, some of which represent specialized roles. This section
considers the activities and relationships among these components, considers the activities and relationships among these components,
and the Internet Mail standards that apply to them. and the Internet Mail standards that apply to them.
Message Message
Mail User Agent (MUA) Message User Agent (MUA)
Author MUA (aMUA) Author MUA (aMUA)
Recipient MUA (rMUA) Recipient MUA (rMUA)
Message Submission Agent (MSA) Message Submission Agent (MSA)
Author-focused MSA functions (aMSA) Author-focused MSA functions (aMSA)
MHS-focused MSA functions (hMSA) MHS-focused MSA functions (hMSA)
skipping to change at page 23, line 5 skipping to change at page 24, line 5
Message Store (MS) Message Store (MS)
Author MS (aMS) Author MS (aMS)
Recipient MS (rMS) Recipient MS (rMS)
This figure shows function modules and the standardized protocols This figure shows function modules and the standardized protocols
used between them. used between them.
++========++ ++========++
|| || +-------+ || || +-------+
...........++ aMUA ||<............................+ Disp | ...........++ aMUA ||<............................+ Disp |
. || || +-------+ . || || +-------+
. ++=+==+===++ ^ . ++=+==+===++ ^
. local,imap}| |{smtp,submission . . local,imap}| |{smtp,submission .
. +-----+ | | +--------+ . . +-----+ | | +--------+ .
. | aMS |<---+ | ........................>| Return | . . | aMS |<---+ | ........................>| Return | .
. +-----+ | . +--------+ . . +-----+ | . +--------+ .
. | . ***************** ^ . . | . ***************** ^ .
. +-----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 || * +------+ * . . //==+==\\
|| imf || * | 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}| ***************** . . . smtp,local}| ***************** . .
. V . . . V . .
. +-----+ //===+===\\ . . +-----+ //===+===\\ .
. | rMS | || sieve || . . | rMS | || sieve || .
. +--+--+ \\=======// . . +--+--+ \\=======// .
. |{imap,pop,local ^ . . |{imap,pop,local ^ .
. V . . . V . .
. ++==========++ . . . ++==========++ . .
. || || . . . || || . .
.......>|| rMUA ++........................... . .......>|| rMUA ++........................... .
|| ++................................... || ++...................................
++==========++ ++==========++
Legend: == lines indicate primary (possibly indirect) transfers or
roles; == bpxes indicate data objects; ... lines indicate supporting Legend: == lines indicate primary (possibly indirect) transfers or roles
transfers or roles; *** lines indicate aggregated service == boxes indicate data objects
... lines indicate supporting transfers or roles
*** lines indicate aggregated service
Figure 5: Protocols and Services Figure 5: Protocols and Services
4.1. Message Data 4.1. Message Data
The purpose of the Mail Handling Service (MHS) is to exchange an IMF The purpose of the Message Handling System (MHS) is to exchange an
message object among participants [RFC5322]. All of its underlying IMF message object among participants [RFC5322]. All of its
mechanisms serve to deliver that message from its Author to its underlying mechanisms serve to deliver that message from its Author
Recipients. A message can be explicitly labeled as to its nature to its Recipients. A message can be explicitly labeled as to its
[RFC3458]. nature [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].
skipping to change at page 28, line 26 skipping to change at page 29, line 33
problems (and possibly successes.) problems (and possibly successes.)
RFC5321.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 IMF destination address header fields, such as For example, the IMF destination address header fields, such as
RFC5322.To, might specify a mailing list mailbox, while the RFC5322.To, might specify a mailing list mailbox, while the
RFC5321.RcptTo address specifies a member of that list. RFC5321.RcptTo address specifies a member of that list.
RFC5321.ORCPT: Set by - Author. RFC5321.ORCPT: Set by - Originator.
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.
RFC5321.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.
RFC5321.Return-Path: Set by - Originator RFC5321.Return-Path: Set by - Originator
The MDA records the RFC5321.MailFrom address into the The MDA records the RFC5321.MailFrom address into the
RFC5322.Return-Path field. RFC5321.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
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from those that occur at lower layers of the Internet Mail MHS from those that occur at lower layers of the Internet Mail MHS
architecture that is, in turn, above the Internet Transport layer. architecture that is, in turn, above the Internet Transport layer.
Because the motivation for email, and much of its use, is for Because the motivation for email, and much of its use, is for
interaction among people, the nature and details of these protocol interaction among people, the nature and details of these protocol
exchanges often are determined by the needs of interpersonal and exchanges often are determined by the needs of interpersonal and
group communication. To accommodate the idiosyncratic behavior group communication. To accommodate the idiosyncratic behavior
inherent in such communication, only subjective guidelines, rather inherent in such communication, only subjective guidelines, rather
than strict rules, can be offered for some aspects of system than strict rules, can be offered for some aspects of system
behavior. Mailing Lists provide particularly salient examples. behavior. Mailing Lists provide particularly salient examples.
4.2.1. Mail User Agent (MUA) 4.2.1. Message User Agent (MUA)
A Mail User Agent (MUA) works on behalf of User actors and User A Message 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"; in IMAP they are called "mailboxes". This model called "folders"; in IMAP they are called "mailboxes". This model
allows a folder for messages under development (Drafts), a folder for allows a folder for messages under development (Drafts), a folder for
messages waiting to be sent (Queued or Unsent), and a folder for messages waiting to be sent (Queued or Unsent), and a folder for
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RFC3461.ENVID RFC3461.ENVID
RFC5321.MailFrom RFC5321.MailFrom
RFC5321.RcptTo RFC5321.RcptTo
RFC5321.Received RFC5321.Received
RFC0791.SourceAddr RFC0791.SourceAddr
4.3.2. Mail Transfer Agent (MTA) 4.3.2. Message Transfer Agent (MTA)
A Mail Transfer Agent (MTA) relays mail for one application-level A Message 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
typically much higher. Relaying is performed by a sequence of MTAs, typically much higher. Relaying is performed by a sequence of MTAs,
until the message reaches a destination MDA. Hence, an MTA until the message reaches a destination MDA. Hence, an MTA
implements both client and server MTA functionality; it does not implements both client and server MTA functionality; it does not
change addresses in the envelope or reformulate the editorial change addresses in the envelope or reformulate the editorial
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 [RFC5321], [RFC0821] primarily to effect Internet Mail uses SMTP [RFC5321], [RFC2821], [RFC0821] primarily to
point-to-point transfers between peer MTAs. Other transfer effect 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]
Although quite basic, the primary routing mechanism for Internet Mail Although quite basic, the dominant routing mechanism for Internet
is the DNS MX record [RFC1035], which specifies an MTA through which Mail is the DNS MX record [RFC1035], which specifies an MTA through
the queried domain can be reached. This mechanism presumes a public, which the queried domain can be reached. This mechanism presumes a
or at least a common, backbone that permits any attached MTA to public, or at least a common, backbone that permits any attached MTA
connect to any other. to 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.
skipping to change at page 35, line 4 skipping to change at page 36, line 7
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:
RFC5321.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 The MDA records the RFC5321.MailFrom address into the
RFC5322.Return-Path field. RFC5321.Return-Path field.
RFC5322.Received: Set by - MDA server RFC5322.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
skipping to change at page 44, line 15 skipping to change at page 45, line 15
[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
The core Internet email standards are based on the use of US-ASCII. The core Internet email standards are based on the use of US-ASCII.
That is SMTP [RFC5321] and IMF [RFC5322], as well as their That is SMTP [RFC5321] and IMF [RFC5322], as well as their
predecessors, describe the transport and composition of messages predecessors. They describe the transport and composition of
composed strictly of US-ASCII 7-bit encoded characters. The messages as composed strictly of US-ASCII 7-bit encoded characters.
standards have been incrementally enhanced to allow for characters The standards have been incrementally enhanced to allow for
outside of this limited set, while retaining mechanisms for characters outside of this limited set, while retaining mechanisms
backwards-compatibility. Specifically: for backwards-compatibility. Specifically:
o The MIME specifications [RFC2045], [RFC2046], [RFC2047] and o The MIME specifications [RFC2045], [RFC2046], [RFC2047] and
[RFC2298] allow for the use of coded character sets and character [RFC2049] allow for the use of coded character sets and character
encoding schemes ("charsets" in MIME terminology) other than US- encoding schemes ("charsets" in MIME terminology) other than US-
ASCII. MIME's [RFC2046] allows the textual content of a message ASCII. MIME's [RFC2046] allows the textual content of a message
to have a label affixed that specifies the charset used in that to have a label affixed that specifies the charset used in that
content. Equally MIME's [RFC2047] allows the textual content of content. Equally MIME's [RFC2047] allows the textual content of
certain header fields in a message to be similarly labeled. certain header fields in a message to be similarly labeled.
However, since messages might be transported over SMTP However, since messages might be transported over SMTP
implementations only capable of transporting 7-bit encoded implementations only capable of transporting 7-bit encoded
characters, MIME's [RFC2045] also provides for "content transfer characters, MIME's [RFC2045] also provides for "content transfer
encoding" so that characters of other charsets can be re-encoded encoding" so that characters of other charsets can be re-encoded
as an overlay to US-ASCII. as an overlay to US-ASCII.
skipping to change at page 44, line 50 skipping to change at page 45, line 50
encoding is still required for those. encoding is still required for those.
o A series of experimental protocols on Email Address o A series of experimental protocols on Email Address
Internationalization (EAI) have been released that extend SMTP and Internationalization (EAI) have been released that extend SMTP and
IMF to allow for 8-bit encoded characters to appear in addresses IMF to allow for 8-bit encoded characters to appear in addresses
and other information throughout the header fields of messages. and other information throughout the header fields of messages.
[RFC5335] specifies the format of such message header fields [RFC5335] specifies the format of such message header fields
(which encode the characters in UTF-8), and [RFC5336] specifies an (which encode the characters in UTF-8), and [RFC5336] specifies an
SMTP option for the transport of these messages. SMTP option for the transport of these messages.
o MIME's [RFC2045] and [RFC2046] allow for the transport of true
multimedia material, which has obvious applicability to
internationalization.
o The formats for delivery status notifications (DSNs--[RFC3462],
[RFC3463], [RFC3464]) and message disposition notifications
(MDNs--[RFC3798]) include both a structured and unstructured
representation of the notification. In the event that the
unstructured representation is in the wrong language or is
otherwise unsuitable for use, this allows an MUA to construct its
own appropriately localized representation of notification for
display to the user.
o POP and IMAP do not introduce internationalization issues.
Hence, the use of UTF-8 is fully established in existing Internet Hence, the use of UTF-8 is fully established in existing Internet
mail. However support for long-standing encoding forms is retained mail. However support for long-standing encoding forms is retained
and is still used. 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.
skipping to change at page 45, line 43 skipping to change at page 47, line 11
[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] Gellens, R., "On-Demand Mail Relay (ODMR) SMTP with
Addresses", RFC 2645, August 1999. Dynamic IP 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.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, [RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001. April 2001.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field [RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists", and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001. RFC 2919, March 2001.
[RFC3192] Allocchio, C., "Minimal FAX address format in Internet [RFC3192] Allocchio, C., "Minimal FAX address format in Internet
Mail", RFC 2304, October 2001. Mail", RFC 3192, October 2001.
[RFC3297] Klyne, G., Iwazaki, R., and D. Crocker, "Content [RFC3297] Klyne, G., Iwazaki, R., and D. Crocker, "Content
Negotiation for Messaging Services based on Email", Negotiation for Messaging Services based on Email",
RFC 3297, July 2002. RFC 3297, July 2002.
[RFC3458] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message [RFC3458] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003. Context for Internet Mail", RFC 3458, January 2003.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service [RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003. RFC 3461, January 2003.
[RFC3462] Vaudreuil, G., "The Multipart/Report Content Type for the
Reporting of Mail System Administrative Messages",
RFC 3462, January 2003.
[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[RFC3501] Crispin, M., "Internet Message Access Protocol - Version [RFC3501] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003. 4rev1", RFC 3501, March 2003.
[RFC3798] Hansen, T. and G. Vaudreuil, "Message Disposition [RFC3798] Hansen, T. and G. Vaudreuil, "Message Disposition
Notification", RFC 3798, May 2004. Notification", RFC 3798, May 2004.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to [RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004. Electronic Mail", RFC 3834, August 2004.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
skipping to change at page 47, line 25 skipping to change at page 48, line 43
[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, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008. October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008. October 2008.
[RFC5335] TWNIC, "Internationalized Email Headers", RFC 5335, [RFC5335] Abel, Y., Ed., "Internationalized Email Headers",
September 2008. RFC 5335, September 2008.
7.2. Informative 7.2. Informative
[MAIL-I18N]
Internet Mail Consortium, "Using International Characters
in Internet Mail", IMC IMCR-010, August 1998.
[RFC0733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson, [RFC0733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
"Standard for the Format of ARPA Network Text Messages", "Standard for the Format of ARPA Network Text Messages",
RFC 733, November 1977. 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 [RFC1506] Houttuin, J., "A Tutorial on Gatewaying between X.400 and
Internet Mail", RFC 1506, August 1993. Internet Mail", RFC 1506, August 1993.
[RFC1652] MCI, Innosoft, Dover Beach Consulting, Inc., Network [RFC1652] Klensin, J., Freed, N., Ed., Rose, M., Stefferud, E., and
Management Associates, Inc., and Silicon Graphics, Inc., D. Crocker, "SMTP Service Extension for 8bit-
"SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, MIMEtransport", RFC 1652, July 1994.
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,
October 1996. October 1996.
[RFC2142] Crocker, D., "Mailbox Names for Common services, Roles and [RFC2142] Crocker, D., "Mailbox Names for Common services, Roles and
Functions", RFC 2142, May 1997. Functions", RFC 2142, May 1997.
[RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998. [RFC2442] Freed, N., Newman, D., Belissen, J., and M. Hoy, "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.
[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.
skipping to change at page 49, line 30 skipping to change at page 50, line 44
September 2008. 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 began in 2004 and has evolved through numerous rounds of This work began in 2004 and has evolved through numerous rounds of
community review; it derives from a section in an early version of community review; it derives from a section in an early version of
[RFC5068]. Over its 4 years of development, the draft has gone [RFC5068]. Over its 5 years of development, the draft has gone
through 12 incremental versions, with vigorous community review that through 13 incremental versions, with vigorous community review that
produced many substantive changes. Review was performed in the IETF produced many substantive changes. Review was performed in the IETF
and other email technical venues. Although not a formal activity of and other email technical venues. Although not a formal activity of
the IETF, issues with the document's contents were resolved using the the IETF, issues with the document's contents were resolved using the
classic style of IETF community open, group decision-making. The classic style of IETF community open, group decision-making. The
document is already cited in other work, such as for IMAP and Sieve document is already cited in other work, such as for IMAP and Sieve
specifications and for academic classwork. The step of standardizing specifications and for academic classwork. The step of standardizing
is useful to provide a solid and stable reference to the Internet's is useful to provide a solid and stable reference to the Internet's
now-complex email service. now-complex email service.
Details of the Originator actor role was greatly clarified during Details of the Originator actor role was greatly clarified during
skipping to change at page 50, line 7 skipping to change at page 51, line 22
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, and Greg Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, Greg
Vaudreuil. Vaudreuil, Patrick Cain, Paul Hoffman, Vijay Gurbani, and Hans
Lachman.
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 The final stages of development for this document were guided by a
design team comprising: Alexey Melnikov, Pete Resnick, Carl S. design team comprising: Alexey Melnikov, Pete Resnick, Carl S.
Gutekunst, Jeff Macdonald, Randall Gellens, Tony Hansen and Tony Gutekunst, Jeff Macdonald, Randall Gellens, Tony Hansen and Tony
Finch. Pete Resnick developed the final version of the section on Finch. Pete Resnick developed the final version of the section on
internationalization. internationalization.
Index Index
12 13
7
7-bit 46
A A
accountability 13 accountability 14
accountable 13-14 accountable 14-15
Actor Actor
Administrative 14 Administrative 15
Author 9 Author 10
Consumer 15 Consumer 16
Edge 15 Edge 16
Gateway 14 Gateway 15
Originator 12 Originator 13
Recipient 10 Recipient 11
Return Handler 10 Return Handler 11
Transit 15 Transit 16
Actors Actors
MHS 11 MHS 12
ADMD 13-15, 19, 25, 31, 38 ADMD 14-16, 20, 26, 32, 39
Administrative Actors 14 Administrative Actors 15
Administrative Management Domain 13 Administrative Management Domain 14
aMSA 31 aMSA 32
Author 9, 12 Author 10, 13
author 35 author 36
B B
body-parts 24 body-parts 25
bounce handler 10 bounce handler 11
boundary 15 boundary 16
C C
Consumer Actor 15 charset 46
content 11, 13-14, 20, 24, 32 Consumer Actor 16
content 12, 14-15, 21, 25, 33
D D
delivery 5, 10, 12-14, 18, 24-25, 35, 38 delivery 5, 11, 13-15, 19, 25-26, 36, 39
Discussion of document 8 Discussion of document 8
DSN 46
E E
Edge Actor 15 EAI 46
Edge Actor 16
encoding 46
end-to-end 5 end-to-end 5
envelope 10, 13, 21, 24-25, 32, 38 envelope 11, 14, 22, 25-26, 33, 39
ETRN 35 ETRN 36
G G
Gateway 11, 14 Gateway 12, 15
H H
header 24 header 25
hMSA 31 hMSA 32
I I
IMAP 25, 32, 35-36, 46
IMF 20, 25, 46
Internet Mail 5 Internet Mail 5
L L
LMTP 35 LMTP 25, 36
local-part 18 local-part 19
M M
Mail 5 Mail 5
Mail From 38 Mail From 39
Mail Handling Service 5, 11 Mail Submission Agent 13
Mail Submission Agent 12 mailbox 39
Mail Transfer Agent 5 MDA 25, 39
Mail User Agent 5 MDN 11, 25, 46
mailbox 38 message 7, 25
MDA 38 Message Disposition Notification 11
MDN 10 Message Handling Service 5
message 7, 24 Message Handling System 12
Message Disposition Notification 10 Message Transfer Agent 5
MHS 5, 10-11, 13, 21-22, 24-25 Message User Agent 5
Actors 11 MHS 5, 11-12, 14, 22-23, 25-26
MSA 12, 31 Actors 12
MTA 5, 15 MIME 25, 46
boundary 15 MS 25
MUA 5, 14, 30-31 MSA 13, 25, 32
MTA 5, 16
boundary 16
MUA 5, 15, 25, 31-32
O O
ODMR 35 ODMR 36
Originator 10, 12 Originator 11, 13
P P
posting 5, 10, 12, 21, 30-31, 35, 38 POP 25, 32, 35-36, 46
pull 35 posting 5, 11, 13, 22, 31-32, 36, 39
push 35 pull 36
push 36
R R
RcptTo 11 RcptTo 12
Receiver 12 Receiver 13
Recipient 10, 12, 38 Recipient 11, 13, 39
recipient 35 recipient 36
relay 12 relay 13
responsibility 31 responsibility 32
responsible 13-14 responsible 14-15
Return address 38 Return address 39
Return Handler 10 Return Handler 11
role 10, 18 role 11, 19
Author 9 Author 10
Originator 12 Originator 13
Recipient 10 Recipient 11
S S
SIEVE 24 SIEVE 25-26
SMTP 35 SMTP 25, 36, 46
T T
transfer 12-14 transfer 13-15
Transit Actor 15 Transit Actor 16
transition 31 transition 32
U U
UA 5 UA 5
User Agent 5 User Agent 5
Author's Address Author's Address
Dave Crocker Dave Crocker
Brandenburg InternetWorking Brandenburg InternetWorking
675 Spruce Drive 675 Spruce Drive
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