MARID / SMTP D. Crocker
Internet-Draft Brandenburg InternetWorking
Expires: January 1, 2005 July 3, 2004
Internet Mail Architecture
draft-crocker-email-arch-01
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Over its thirty year history, Internet mail has undergone significant
changes in scale and complexity. The first standardized architecture
for email specified a simple split between the user world and the
transmission world, in the form of Mail User Agents (MUA) and Mail
Transfer Agents (MTA). Over time each of these has divided into
multiple, specialized modules. Public discussion and agreement about
the nature of the changes to Internet mail has not kept pace, and
abuses of the Internet mail service have brought these issues into
stark relief. This draft offers clarifications and enhancements, to
provide a more consistent base for community discussion of email
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service problems and proposed email service enhancements.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Document Changes . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Discussion venue . . . . . . . . . . . . . . . . . . . . . . . 4
2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 5
2.1 User . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Provider . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Email Identities . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3 Message Identifers . . . . . . . . . . . . . . . . . . . . . . 10
3.4 Identity Reference Convention . . . . . . . . . . . . . . . . 10
4. Email System Architecture . . . . . . . . . . . . . . . . . . 10
4.1 Architectural Components . . . . . . . . . . . . . . . . . . . 11
4.2 Operational Configuration . . . . . . . . . . . . . . . . . . 19
4.3 Layers of Identity References . . . . . . . . . . . . . . . . 20
5. Message Data . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1 Envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2 Message Headers . . . . . . . . . . . . . . . . . . . . . . . 21
5.3 Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Two Levels of Store-And-Forward . . . . . . . . . . . . . . . 21
6.1 MTA Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2 MUA Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 32
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 34
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
Intellectual Property and Copyright Statements . . . . . . . . 35
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1. Introduction
Over its thirty year history, Internet mail has undergone significant
changes in scale and complexity. The first standardized architecture
for email specified a simple split between the user world and the
transmission world, in the form of Mail User Agents (MUA) and Mail
Transfer Agents (MTA). Over time each of these has sub-divided into
more specialized modules.
The basic style and use of names, addresses and message structure
have remained remarkably constant. However each has benefited from
significant elaborations. Public discussion and agreement about the
nature of these changes has not kept pace, and abuses of the Internet
mail service have brought these issues into stark relief.
The current draft seeks to:
1. Document changes that have taken place in refining the email
model
2. Clarify functional roles for the architectural components
3. Clarify identity-related issues, across the email service
4. Provide a common venue for further defining and citing modern
Internet mail architecture
1.1 Service Overview
End-to-end Internet mail exchange is accomplished by using a
standardized infrastructure comprising:
1. An email object
2. Global addressing
3. A connected sequence of point-to-point transfer mechanisms
4. No prior arrangement between originator and recipient
5. No prior arrangement between point-to-point transfer services,
over the open Internet
The end-to-end portion of the service is the message. Broadly the
message, itself, is divided between handling control information and
user message payload.
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A precept to the design of Internet mail is to permit
interoperability with no prior, direct administrative arrangement
between the participants. That is, all participants rely on having
the core services be universally supported, either directly or
through gateways that translate between Internet mail standards and
other email conventions.
For localized environments (edge networks) prior, administrative
arrangement can include access control, routing and lookup service
configuration. In recent years one change to local environments is
an increased requirement for authentication or, at least,
accountability. In these cases, the server performs explicit
validation of the client's identity.
1.2 Document Changes
The major changes from the previous version of this document are:
Actors: Addition of the User/Relay/Provider construct of actors.
Labeling of these roles has also been added to the tables showing
architectural function. The distinction of Actors, versus
architectural system components, is not typical for discussions of
email. Therefore it is likely that the construct needs
refinement. In particular, please review the table assignments.
MDA/MS/MUA: The construct of the Message Store has been added. This
change is intended to reflect the consensus view from online
discussion, rather than being the editor's view, which has in any
event changed... However it is likely that it will need
significant revision or replacement. Please review it carefully!
Message Identifiers: Discussion of message identifiers has been added
to the section on Email Identities.
1.3 Discussion venue
NOTE: This document is the work of a single person, about a topic
with considerable diversity of views. It is certain to be
incomplete and inaccurate. Some errors simply need to be
reported; they will get fixed. Others need to be discussed by the
community, because the real requirement is to develop common
community views. To this end, please treat the draft as a
touchstone for public discussion.
Discussion about this document should be directed to the:
<mailto:ietf-smtp@imc.org>
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mailing list. The <http://www.imc.org/ietf-smtp/index.htm> is the
most active, long-standing venue for discussing email architecture.
Although this list is primarily for discussing only the SMTP
protocol, it is recommended that discussion of this draft take place
on that mailing list. This list tends to attend to end-to-end
infrastructure and architecture issues more than other email-related
mailing lists.
o The <mailto:ietf-822@imc.org> list also is pertinent
<http://www.imc.org/ietf-822/index.html>. However it's focus is
on the message, itself, so that transfer issues are typically
excluded. In addition, this list has not be very active recently.
o A currently active mailing list, likely to impact Internet mail
architecture, is <mailto:ietf-mxcomp@imc.org>. This list is
devoted to matters of spam control, so that underlying matters of
Internet mail architecture are probably best deferred to a more
general list, such as ietf-smtp.
o Also currently active is the <mailto:lemonade@ietf.org>, which is
considering enhancements for interaction between thin MUAs and
MSAs.
2. Email Actor Roles
Discussion of email architecture requires distinguishing different
actors within the service, and being clear about the job each
performs in the overall handling of mail. For this level of
discussion "the service" has the task of performing a single,
end-to-end transfer. Protracted, iterative exchanges, such as those
used for collaboration over time, are beyond the scope of (this
version of) this document. Actors often will be associated with
entirely independent organizations from other actors participating in
an end-to-end email transfer.
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The following depicts the relationships among participants in
Internet Mail. Although related to a technical architecture, its
focus is on participant responsibilities, rather than functional
modules. Hence the labels used are different than for classic email
architecture. This figure depicts the relationships among the
actors. It shows the Submitter as distinct from the Originator,
although it is common for them to be the same actor. The figure also
shows multiple Relays in the sequence. It is legal to have only one,
and for intra-organization mail services, this is common.
User (Originator, Author)
| -+
Submitter | Provider
| -+
| -+
Relay |
| | | Provider
| Relay |
| | -+
| | -+
| | -+ |
| User (Forwarder) | |
| [ Intermediate ] | |Provider
| [ Recipient ] | |
| [ Originator ] | Provider |
| [ Submitter ] | |
| | -+ |
| Relay |
| | -+
| | -+
Relay |
| | Provider
User (Recipient) |
-+
2.1 User
Users are customers of the email relaying service. They represent
the sources and sinks of that service.
Three types of users are distinguished:
2.1.1 Originator
Also called "Author", this is a user-level participant responsible
for creating original content and requesting its transmission. The
email service operates to send and deliver mail among Originators and
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Recipients.
2.1.2 Submitter
The Submitter is responsible for ensuring that a message is valid for
posting and then submitting it into the mail transfer service. It
primarily serves the Originator and often it is the same entity.
The Submitter has the responsibility for any additional
originator-related administrative tasks associated with message
transmission and delivery. Notably this pertains to error and
delivery notices.
It may be helpful to think of the Submitter as more like the editor
or publisher of a periodical, rather than simply the administrative
assistant for the Originator. Hence, the Submitter is best held
accountable for the message content, even when they did not create
any or most of it.
2.1.3 Recipient
The Recipient is a consumer of delivered content. The recipient is
specified as an addressee, in the envelope.
2.1.4 Forwarder
Email often transits intermediate, user-level points, called
Forwarders. The task of a Forwarder is to perform additional
processing, such as replacing one target address for one or more
others, and then submitting the message for further transmission.
Examples are recipient-controlled aliasing and, of course, mailing
list redistribution services. A Forwarder performs a natural
sequence of email steps:
o Service the mailbox specified in the envelope and accept arriving
messages.
o Reformulate message content and addressing, according to the
policies of the administrator of the Forwarder. Request (further)
message transmission. Note that an Intermediate Originator
operates with dual allegiance, notably its operating authority,
such as the mailing list administrator, as well as the "original"
originator.
o Perform the usual Submitter tasks.
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2.2 Relay
A mail relay performs email transfer-service routing and
store-and-forward. It (re-)transmits the message on towards it
recipient(s). A basic transfer operation is between a client and a
server Relay. A set of Relays composes a mail handling service
network. This is above any underlying packet-switching network that
they might be using.
2.3 Provider
Providers operate component services. As shown in the Figure, it is
possible for Providers to host services for other Providers. Common
examples are:
Enterprise Service Providers: Operating an organization's internal
data and/or mail operations.
Internet Service Providers: Operating underlying data communication
services that, in turn, are used by one or more Relays and Users.
It is not their job to perform email functions, but to provide an
environment in which those functions can be performed.
Mail Service Providers: Operate email services, such as for
end-users, or mailing lists.
Operational pragmatics often dictate that Providers be involved in
detailed administration and enforcement issues, to help insure the
health of the overall Internet Mail service.
3. Email Identities
Internet mail uses two forms of identity. The most common is the
mailbox address <addr-spec> [RFC2822]. The other form is the <domain
name> [RFC1034].
3.1 Mailbox Addresses
An addr-spec has two distinct parts, divided by an at-sign ("@").
The right-hand side contains a globally interpreted name for an
administrative domain. This domain name might refer to an entire
organization, or to a collection of machines integrated into a
homogeneous service, or to a single machine. Domain names are
defined and operated through the DNS [RFC1034], [RFC1035].
The left-hand side of the at-sign contains a string that is globally
opaque and is called the <local-part>. It is to be interpreted only
by the entity specified in the address's right-hand side. All other
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entities must treat the local-part as a uninterpreted, literal string
and must preserve all of its original details. As such, its
distribution is equivalent to sending a "cookie" that is only
interpreted upon being returned to its originator.
3.1.1 Global Standards for Local-Part
It is common for sites to have local conventions for sub-structure to
the left-hand side of an addr-spec. This permits sub-addressing,
such as for distinguishing different discussion groups by the same
participant. However it must be stressed that these conventions are
strictly private to the user's organization and must not be
interpreted by any domain except the one listed in the right-hand
side of the add-spec.
A small class of addresses have an elaboration on basic email
addressing, with a standardized, global schema for the local-part.
These are conventions between originating end-systems and recipient
gateways, and they are invisible to the public email transfer
infrastructure. When an originator is explicitly sending via a
gateway out of the Internet, there are coding conventions for the
local-part, so that the originator can formulate instructions for the
gateway. Standardized examples of this are the telephone numbering
formats for VPIM [RFC2421], such as "+16137637582@vpim.example.com",
and iFax [RFC2304], such as "FAX=+12027653000/
T33S=1387@ifax.example.com".
3.1.2 Scope of Email Address Use
Email addresses are being used far beyond their original email
transfer and delivery role. In practical terms, email strings have
become a common form of user identity on the Internet. What is
essential, then, is to be clear about the nature and role of an
identity string in a particular context and to be clear about the
entity responsible for setting that string.
3.2 Domain Names
A domain name is a global reference to an Internet resource, such as
a host, a service or a network. A name usually maps to one or more
IP Addresses. A domain name can be administered to refer to
individual users, but this is not common practice. The name is
structure as a hierarchical sequence of sub-names, separated by dots
(".").
When not part of a mailbox address, a domain name is used in Internet
mail to refer to a node that took action upon the message, such as
providing the administrative scope for a message identifier, or
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performing transfer processing.
3.3 Message Identifers
Message identifiers have two distinct parts, divided by an at-sign
("@"). The right-hand side contains a globally interpreted name for
the administrative domain assigning the identifier. The left-hand
side of the at-sign contains a string that is globally opaque and
serves to uniquely identify the message within the domain referenced
on the right-hand side. The duration of uniqueness for the message
identifier is undefined.
The identifier may be assigned by the user or by any component of the
system along the path. Although Internet mail standards provide for
a single identifier, more than one is sometimes assigned.
3.4 Identity Reference Convention
In this document, fields references to identities are labeled in a
two-part, dotted notation. The first part cites the document
defining the identity and the second defines the name of the
identity. Hence, <RFC2822.From> is the From field in an email
content header, and <RFC2821.MailFrom> is the address in the SMTP
"Mail From" command.
4. Email System Architecture
NOTE: A discussion about any interesting system architecture is often
complicated by confusion between architecture versus
implementation. An architecture defines the conceptual functions
of a service, divided into discrete conceptual modules. An
implementation of that architecture may combine or separate
architectural components, as needed for a particular operational
environment. It is important not to confuse the engineering
decisions that are made to implement a product, with the
architectural abstractions used to define conceptual functions.
Modern Internet email architecture distinguishes four types of
components, arranged to support a store-and-forward service
architecture:
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+-----------<-oMUA <-------------------------------+
| | < smtp, |
(envelope) V submission |
RFC2822 MSA <-------------------------+ |
MIME | < smtp | |
| MTA dsn |
| | < smtp | |
| MTA ->----------------------->+ |
| | < smtp |
| . |
| . |
| . |
| V |
| MTA |
| | < local, smtp, lmtp |
| MDA <-------------------------+ |
| | | | |
| MS-1 | < local, | |
| | | | < pop, sieve |
| | | | < imap, | |
| | MS-2 < smtp, or | mdn
| | | < web | |
V | V | |
+---------> rMUA ->--------------------------+-----+
Software implementations of these architectural components often
compress them, such as having the same software do MSA, MTA and MDA
functions. However the requirements for each of these components of
the architecture are becoming more extensive. So, their separation
is increasingly common.
4.1 Architectural Components
4.1.1 Mail User Agent (MUA)
An <MUA> works on behalf of end-users and end-user applications. It
is their "representative" within the email service.
At the origination side of the service, the <oMUA> is used to create
a message and perform initial "submission" into the transfer
infrastructure, via an <MSA>. It may also perform any creation- and
posting-time archival. An MUA outbox is part of the origination-side
MUA.
The recipient-side <rMUA> works on behalf of the end-user to process
received mail. This includes generating user-level return control
messages, display and disposition of the received message, and
closing or expanding the user communication loop, by initiating
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replies and forwarding new messages.
An MUA may, itself, have a distributed architecture, such as
implementing a "thin" user interface module on a limited end-user
device, with the bulk of the MUA functionality operated remotely on a
more capable server. An example of such an architecture might use
IMAP [RFC3501] for most of the interactions between an MUA client and
an MUA server.
A special class of MUA functions perform message forwarding, as
discussed in the [2] section.
Identity fields set by the MUA include:
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2822.From | Originator | Names and addresses |
| | | for author(s) of |
| | | the message content |
| | | are listed in the |
| | | From header |
| RFC2822.Reply-To | Originator | If a message |
| | | recipient sends a |
| | | message that would |
| | | otherwise use the |
| | | RFC2822.From field |
| | | information in the |
| | | original message, |
| | | they are to use the |
| | | contents of the |
| | | RFC2822.Reply-To |
| | | field instead. In |
| | | other words, this |
| | | field is a direct |
| | | override of the |
| | | From field, for |
| | | responses from |
| | | recipients. |
| RFC2822.Sender | Submitter | This specifies the |
| | | address responsible |
| | | for submission into |
| | | the transfer |
| | | service. For |
| | | efficiency, this |
| | | field should be |
| | | omitted if it |
| | | contains the same |
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| | | address as |
| | | RFC2822.From. |
| | | However this does |
| | | not mean there is |
| | | no Sender |
| | | specified. Rather, |
| | | it means that that |
| | | header is virtual |
| | | and that the |
| | | address in the From |
| | | field must be used. |
| | | Specification of |
| | | the error return |
| | | addresses (the |
| | | "bounces" address, |
| | | contained in |
| | | RFC2821.MailFrom) |
| | | is made by the |
| | | Sender. Typically |
| | | the bounce address |
| | | is the same as the |
| | | Sender address. |
| | | However some usage |
| | | scenarios require |
| | | it to be different. |
| RFC2822.To, | Recipient | These specify MUA |
| RFC2822.CC | | recipient |
| | | addresses. The |
| | | distinction between |
| | | To and CC is |
| | | subjective. |
| | | Generally, a To |
| | | addressee is |
| | | considered primary |
| | | and is expected to |
| | | take action on the |
| | | message. A CC |
| | | addressee typically |
| | | receives a copy |
| | | only for their |
| | | information. |
| RFC2822.BCC | Recipient | A message might be |
| | | copied to an |
| | | addressee who is |
| | | not to be disclosed |
| | | to the RFC2822.TO |
| | | or RFC2822.CC |
| | | recipients. The BCC |
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| | | header indicates a |
| | | message copy to |
| | | such a recipient. |
| | | Typically, the |
| | | field lists no |
| | | addresses or only |
| | | lists the address |
| | | of the single |
| | | recipient receiving |
| | | the copy. This |
| | | ensures that even |
| | | other BCC |
| | | recipients do not |
| | | know of each other. |
| | | An MUA will |
| | | typically make |
| | | separate postings |
| | | for TO and CC |
| | | recipients, versus |
| | | BCC recipients. The |
| | | former will see no |
| | | indication that any |
| | | BCCs were sent, |
| | | whereas the latter |
| | | have a BCC field |
| | | present. It might |
| | | be empty, contain a |
| | | comment, or contain |
| | | one or more BCC |
| | | addresses, |
| | | depending upon the |
| | | preferences or the |
| | | Originator. |
+----------------------+----------------------+---------------------+
Table 1: Message Identities
4.1.2 Mail Submission Agent (MSA)
An <MSA> accepts the message submission from the oMUA and conditions
it for insertion into the global email transfer network, according to
the policies of the hosting network and the requirements of Internet
standards. It implements a server function to MUAs and a client
function to MTAs (or MDAs).
Examples of MSA-styled functions, in the world of paper mail, might
range across the very different capabilities of administrative
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assistants, postal drop boxes, and post office front-counter
employees.
The MUA/MSA interface can be implemented within single host and use
private conventions for their interactions. Historically,
standards-based MUA/MSA interactions have used SMTP [RFC2821].
However a recent alternative is SUBMISSION [RFC2476]. Although
SUBMISSION derives from SMTP, it operates on a separate TCP port, and
will typically impose distinct requirements, such as access
authorization.
Identities set by the MSA include:
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2821.HELO or | Submitter | The MSA may specify |
| RFC2821.EHLO | | its hosting domain |
| | | identity for the |
| | | SMTP HELO or EHLO |
| | | command operation. |
| RFC2821.MailFrom | Submitter | This is an |
| | | end-to-end string |
| | | that specifies an |
| | | email address for |
| | | receiving return |
| | | control |
| | | information, such |
| | | as "bounces". The |
| | | name of this field |
| | | is misleading, |
| | | because it is not |
| | | required to specify |
| | | either the author |
| | | or the agent |
| | | responsible for |
| | | submitting the |
| | | message. Rather, |
| | | the agent |
| | | responsible for |
| | | submission |
| | | specifies the |
| | | RFC2821.MailFrom |
| | | address. Ultimately |
| | | the simple basis |
| | | for deciding what |
| | | address needs to be |
| | | in the |
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| | | RFC2821.MailFrom is |
| | | to determine what |
| | | address needs to be |
| | | informed about |
| | | transmission-level |
| | | problems (and, |
| | | possibly, |
| | | successes.) |
| RFC2821.Rcpt-To | Recipient | This specifies the |
| | | MUA inbox address |
| | | of a recipient. The |
| | | string might not be |
| | | visible in the |
| | | message content |
| | | headers. For |
| | | example, the |
| | | message destination |
| | | address headers, |
| | | such as RFC2822.To, |
| | | might specify a |
| | | mailing list |
| | | address, while the |
| | | RFC2821.Rcpt-To |
| | | address specifies a |
| | | member of that |
| | | list. |
| RFC2821.Received | Submitter | An MSA may record a |
| | | Received header, to |
| | | indicate initial |
| | | submission trace |
| | | information, |
| | | including |
| | | originating host |
| | | and MSA host domain |
| | | names and/or IP |
| | | Addresses. |
+----------------------+----------------------+---------------------+
Table 2: MSA Identities
4.1.3 Mail Transfer Agent (MTA)
An <MTA> relays a message to another other MTA or to an <MDA>, in a
point-to-point exchange. Relaying is performed by a sequence of
MTAs, until the message reaches its destination MDA. Hence an MTA
implements both client and server MTA functionality.
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The basic functionality of an MTA is similar to that of a packet
switch or IP router. That is, it does email store-and-forward email,
with a routing decision determining where the next-hop destination
shall be. The primary "routing" mechanism for Internet mail is the
DNS MX record [RFC1035]. As with most "link layer" mechanisms
Internet mail's SMTP supports a basic level of reliability, by virtue
of providing for retransmission after al transfer failure. However
the degree of persistence by an MTA can be highly variable.
However email objects are typically much larger than the payload of a
packet or datagram, and the end-to-end latencies are typically much
higher. Contrary to typical packet switches (and Instant Messaging
services) Internet mail MTAs typically store messages in a manner
that allows recovery across services interruptions, such as host
system shutdown.
Internet mail primarily uses SMTP [RFC2821], [RFC0821] to effect
point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and ODMR [RFC2645]
An important characteristic of MTA-MTA communications, over the open
Internet, is that they do not require prior arrangement between the
independent administrations operating the different MTAs. Given the
importance of spontaneity and serendipity in the world of human
communications, this lack of prearrangement, between the
participants, is a core benefit of Internet mail and remains a core
requirement for it.
Identities set by the MTA include:
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2821.HELO | Relay | The MTA may specify |
| | | its hosting domain |
| | | identity for the |
| | | SMTP HELO or EHLO |
| | | command operation. |
| RFC2821.Return-Path | Originator | The MDA records the |
| | | RFC2821.MailFrom |
| | | address into an |
| | | RFC2822 header |
| | | named Return-Path. |
| RFC2822.Received | Relay | An MTA must record |
| | | a Received header, |
| | | to indicate trace |
| | | information, |
| | | including source |
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| | | host and receiving |
| | | host domain names |
| | | and/or IP |
| | | Addresses. |
+----------------------+----------------------+---------------------+
Table 3: MTA Identities
4.1.4 Mail Delivery Agent (MDA)
The <MDA> delivers email to the recipient's inbox.
An MDA can provide distinctive, address-based functionality, made
possible by its detailed knowledge of the properties of the
destination address. This knowledge might also be present earlier in
an MTA relaying sequence that ends with the MDA, such as at an
organizational gateway. However it is required for the MDA, if only
because the MDA must know where to store the message. This knowledge
is used to achieve differential handling of messages.
Using Internet protocols, delivery is effected with POP [RFC1939],
IMAP [RFC3501]. SMTP permits "push" delivery to the recipient
system, at the imitative of the upstream email service. POP is used
for "pull" delivery at the initiative of the recipient system.
Notably, SMTP and POP effect a transfer of message control from the
email service to the recipient host. In contrast, IMAP provides
on-going, interactive access to a message store, and does not effect
a transfer of message control to the end-user host. Instead, control
stays with the message store host that is being access by the user.
Identities set by the MDA include:
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2821.HELO or | Relay | The MDA may specify |
| RFC2821.EHLO | | its hosting domain |
| | | identity for the |
| | | SMTP HELO or EHLO |
| | | command operation. |
| RFC2822.Received | h | An MTA must record |
| | | a Received header, |
| | | to indicate trace |
| | | information, |
| | | including source |
| | | host and receiving |
| | | host domain names |
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| | | and/or IP |
| | | Addresses. |
+----------------------+----------------------+---------------------+
Table 4: MDA Identities
4.1.5 Message Store
An MUA's uses a long-term Message Store (MS). A rich set of choices
for the use of that store derives from permitting more than one to be
associated with a single user, demonstrated as MS-1 and MS-2 in the
Figure. MS-1 is shown as being remote from the MUA and MS-2 as being
local. Further the relationship between two message store may vary.
Between the MDA and the MUA, these choices are supported by a wide
variety of protocol options.
The operational relationship among two MSs can be:
Online: Only a remote MS is used, with messages being accessible only
when the MUA is attached to the MS, and the MUA repeatedly fetches
all or part of a message, from one session to the next.
Offline: The MS is local to the user, and messages are moved from any
remote store, rather than (also) being retained there.
Disconnected: A remote MS and a local MS synchronize all or parts of
their contents, while connected. The user may make changes while
disconnected, and the two stores are re-synchronized upon
reconnection.
4.2 Operational Configuration
Mail service components can be arranged into numerous organizational
structures, each with independent software and administration. One
common arrangement is to distinguish:
1. an open, core, global email transfer infrastructure
2. independent transfer services in networks at the edge of the core
3. end-user services
Edge networks may use proprietary email standards. However the
distinction between "public" network and edge network transfer
services is primarily significant because it highlights the need for
concern over interaction and protection between independent
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administrations. In particular, this distinctions calls for
additional care in assessing transitions of responsibility, as well
as the accountability and authorization relationships among
participants in email transfer.
On the other hand, real-world operations of Internet mail
environments do impose boundaries such as access control at
organizational firewalls to the Internet. It should be noted that
the current Internet Mail architecture offers no special constructs
for these configuration choices. The current design of Internet mail
is for a seamless, end-to-end store-and-forward sequence. It is
possible that the architectural enhancement will not require new
protocols, but rather will require clarification of best practises,
as exemplified by a recent effort [ID-spamops]
4.3 Layers of Identity References
For a message in transit, the core identity fields combine into
+-----------------+--------------+-----------------------------+
| Layer | Field | Set By |
+-----------------+--------------+-----------------------------+
| Message Content | MIME Headers | Originator |
| Message Headers | From | Originator |
| | Sender | Submitter |
| | Reply-To | Originator |
| | To, CC, BCC | Originator |
| | Received | Submitter, Relay, Recipient |
| | Return-Path | MDA from MailFrom |
| SMTP | HELO | Latest Relay Client |
| | MailFrom | Submitter |
| | RCPT-TO | Submitter |
| IP | IP Address | Latest Relay Client |
+-----------------+--------------+-----------------------------+
5. Message Data
5.1 Envelope
Information that is directly used or produced by the email transfer
service is called the "envelope". It controls and records handling
activities by the transfer service. Internet mail has a fragmented
framework for handling this "handling" information. The envelope
exists partly in the transfer protocol SMTP [RFC2821] and partly in
the message object [RFC2822].
Direct envelope addressing information, as well as optional transfer
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directives, are carried in-band by MTAs. All other envelope
information, such as trace records, is carried within the content
headers. Upon delivery, SMTP-level envelope information is typically
encoded within additional content headers, such as Return-Path and
Received (From and For).
5.2 Message Headers
Headers are attribute/value pairs covering an extensible range of
email service, user content and user transaction meta-information.
The core set of headers is defined in [RFC2822], [RFC0822]. It is
common to extend this set, for different applications. A complete
set of registered headers is being developed through [ID-hdr-reg].
One danger with placing additional information in headers is that
gateways often alter or delete them.
5.3 Body
The body of a message might simply be lines of ASCII text or it might
be structured into a composition of multi-media, body-part
attachments, using MIME [RFC2045], [RFC2046], [RFC2047], [RFC2048],
and [RFC2049]. It should be noted that MIME structures each
body-part into a recursive set of MIME Header meta-data and MIME
Content sections.
6. Two Levels of Store-And-Forward
Basic email transfer is accomplished with an asynchronous
store-and-forward communication infrastructure. This means that
moving a message from an originator to a recipient involves a
sequence of independent transmissions through some number of
intermediaries, called MTAs. A very different task is the user-level
process of re-posting a message through a new submission process,
after final delivery for an earlier transfer sequence. Such
MUA-based re-posting shares some functionality with basic MTA
relaying, but it enjoys a degree of freedom with both addressing and
content that is not available to MTAs.
The primary "routing" mechanism for Internet mail is the DNS MX
record [RFC1035]. It is an advertisement, by a recipient domain, of
hosts that are able to relay mail to it, within the portion of the
Internet served by this instance of the DNS.
6.1 MTA Relaying
MTAs relay mail. They are like packet-switches and IP routers.
Their job is to make routing assessments and to move the message
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payload data closer to the recipient. It is not their job to
reformulate the payload or to change addresses in the envelope or the
content.
6.2 MUA Forwarding
As discussed in <Forwarder> section, forwarding is performed by MUAs
that take a received message and submit it back to the transfer
service, for delivery to one or more different addresses. A
forwarded message may appear identical to a relayed message, such as
for Alias forwarders, or it may have minimal similarity, as with a
Reply.
6.2.1 MUA Basic Forwarding
The simplest type of forwarding involves creating an entirely new
message, with new content, that includes the original message between
Originator-1 and Recipient-1. However this forwarded communication
is between Recipient-1 (who could also be called Originator-2) and a
new recipient, Recipient-2. The forwarded message is therefore
independent of the original message exchange and creates a new
message dialogue.
6.2.2 MUA Re-Sending
A recipient may wish to declare that an alternate addressee should
take on responsibility for a message, or otherwise become involved in
the original communication. They do this through a user-level
forwarding function, called re-sending. The act of re-sending, or
re-directing, splices a communication between Originator-1 and
Recipient-1, to become a communication between Originator-1 and new
Recipient-2. In this case, the content of the new message is the old
message, including preservation of the essential aspects of the
original message's origination information.
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Identities specified in a resent message include
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2822.From | Originator | Names and email |
| | | addresses for the |
| | | original author(s) |
| | | of the message |
| | | content are |
| | | retained. The |
| | | free-form |
| | | (display-name) |
| | | portion of the |
| | | address might be |
| | | modified to provide |
| | | informal reference |
| | | to the agent |
| | | responsible for the |
| | | redirection. |
| RFC2822.Reply-To | Originator | If this field is |
| | | present in the |
| | | original message, |
| | | it should be |
| | | retained in the |
| | | Re-sent message. |
| RFC2822.Sender | Submitter | This field is |
| | | expected to contain |
| | | the original Sender |
| | | value. |
| RFC2822.TO, | Recipient | These specify the |
| RFC2822.CC, | | original message |
| RFC2822.BCC | | recipients. |
| RFC2822.Resent-From | Intermediate | The address of the |
| | Originator | original recipient |
| | | who is redirecting |
| | | the message. |
| | | Otherwise, the same |
| | | rules apply for the |
| | | Resent-From field |
| | | as for an original |
| | | RFC2822.From field |
| RFC2822.Resent-Sende | Intermediate | The address of the |
| r | Submitter | agent responsible |
| | | for re-submitting |
| | | the message. For |
| | | efficiency, this |
| | | field should be |
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| | | omitted if it |
| | | contains the same |
| | | address as |
| | | RFC2822.Resent-From |
| | | . However this does |
| | | not mean there is |
| | | no Resend-Sender |
| | | specified. Rather, |
| | | it means that that |
| | | header is virtual |
| | | and that the |
| | | address in the |
| | | Resent-From field |
| | | must be used. |
| | | Specification of |
| | | the error return |
| | | addresses (the |
| | | "bounces" address, |
| | | contained in |
| | | RFC2821.MailFrom) |
| | | is made by the |
| | | Resent-Sender. |
| | | Typically the |
| | | bounce address is |
| | | the same as the |
| | | Resent-Sender |
| | | address. However |
| | | some usage |
| | | scenarios require |
| | | it to be different. |
| RFC2822.Resent-To, | Recipient | The addresses of |
| RFC2822.Resent-cc, | | the new recipients |
| RFC2822.Resent-bcc | | who will now be |
| | | able to reply to |
| | | the original |
| | | author. |
| RFC2821.MailFrom | Intermediate | The agent |
| | Submitter | responsible for |
| | | re-submission |
| | | (RFC2822.Resent-Sen |
| | | der) is also |
| | | responsible for |
| | | specifying the new |
| | | RFC2821.MailFrom |
| | | address. |
| RFC2821.Rcpt-to | Recipient | This will contain |
| | | the address of a |
| | | new recipient |
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| RFC2822.Received | Intermediate | When re-sending a |
| | Submitter | message, the |
| | | submission agent |
| | | may record a |
| | | Received header, to |
| | | indicate the |
| | | transition from |
| | | original posting to |
| | | resubmission. |
+----------------------+----------------------+---------------------+
Table 6: ReSent Identities
6.2.3 MUA Reply
When a recipient formulates a response to a message, the new message
is not typically viewed as being a "forwarding" of the original.
6.2.4 MUA Gateways
Gateways perform the basic routing and transfer work of message
relaying, but they also make any message or address modifications
that are needed to send the message into the next messaging
environment. When a gateway connects two differing messaging
services, its role is easy to identify and understand. When it
connects environments that have technical similarity, but may have
significant administrative differences, it is easy to think that a
gateway is merely an MTA. The critical distinguish between an MTA
and a gateway is that the latter modifies addresses and/or message
content.
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A gateway may set any identity field available to a regular MUA.
Identities typically set by gateways include
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2822.From | Originator | Names and email |
| | | addresses for the |
| | | original author(s) |
| | | of the message |
| | | content are |
| | | retained. As for |
| | | all original |
| | | addressing |
| | | information in the |
| | | message, the |
| | | gateway may |
| | | translate addresses |
| | | in whatever way |
| | | will allow them |
| | | continue to be |
| | | useful in the |
| | | target environment. |
| RFC2822.Reply-To | Originator | The gateway should |
| | | retain this |
| | | information, if it |
| | | is originally |
| | | present. The |
| | | ability to perform |
| | | a successful reply |
| | | by a gatewayed |
| | | recipient is a |
| | | typical test of |
| | | gateway |
| | | functionality. |
| RFC2822.Sender | Submitter | This may retain the |
| | | original value or |
| | | may be set to a new |
| | | address |
| RFC2822.TO, | Recipient | These usually |
| RFC2822.CC, | | retain their |
| RFC2822.BCC | | original addresses. |
| RFC2821.MailFrom | Submitter | The agent |
| | | responsible for |
| | | gatewaying the |
| | | message may choose |
| | | to specify a new |
| | | address to receive |
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| | | handling notices. |
| RFC2822.Received | Forwarder | The gateway may |
| | | record a Received |
| | | header, to indicate |
| | | the transition from |
| | | original posting to |
| | | the new messaging |
| | | environment. |
+----------------------+----------------------+---------------------+
Table 7: Gateway Identities
6.2.5 MUA Alias Handling
A simple re-addressing facility that is available in most MDA
implementations is called Aliasing. It is performed just before
placing a message into the specified recipient's inbox. Instead, the
message is submitted back to the transfer service, for delivery to
one or more alternate addresses. Although implemented as part of the
message delivery service, this facility is strictly a recipient user
function. In effect it resubmits the message to a new address, on
behalf of the listed recipient.
What is most distinctive about this forwarding mechanism is how
closely it compares to normal MTA store-and-forward. In reality its
only interesting difference is that it changes the RFC2821.RCPT-TO
value. Notably it does not typically change the RFC2821.Mailfrom
An MDA that is re-posting a message to an alias typically changes
only envelope information:
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2822.TO, | Recipient | These retain their |
| RFC2822.CC, | | original addresses. |
| RFC2822.BCC | | |
| RFC2821.Rcpt-To | Recipient | This field contains |
| | | an alias address. |
| RFC2821.MailFrom | Intermediate | The agent |
| | Submitter | responsible for |
| | | submission to an |
| | | alias address will |
| | | usually retain the |
| | | original address to |
| | | receive handling |
| | | notifications. The |
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| | | benefit of |
| | | retaining the |
| | | original MailFrom |
| | | value is to ensure |
| | | that the |
| | | origination-side |
| | | agent knows of that |
| | | there has been a |
| | | delivery problem. |
| | | On the other hand, |
| | | the responsibility |
| | | for the problem |
| | | usually lies with |
| | | the recipient, |
| | | since the Alias |
| | | mechanism is |
| | | strictly under the |
| | | recipient's |
| | | control. |
| RFC2821.Received | Intermediate | The agent should |
| | Recipient | record Received |
| | | information, to |
| | | indicate the |
| | | delivery to the |
| | | original address |
| | | and submission to |
| | | the alias address. |
| | | The trace of |
| | | Received headers |
| | | should include |
| | | everything from |
| | | original posting |
| | | through final |
| | | delivery to the |
| | | alias. |
+----------------------+----------------------+---------------------+
Table 8: Alias Identities
6.2.6 MUA Mailing Lists
Mailing lists have explicit email addresses and forward messages to a
list of subscribed members. Mailing list processing is a user-level
activity, outside of the core email transfer service. The mailing
list address is, therefore, associated with a distinct user-level
entity that can perform arbitrary actions upon the original message,
before submitting it to the mailing list membership. Hence, mailing
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lists are similar to gateways.
Identities set by a mailing list processor, when submitting a
message, include:
+----------------------+----------------------+---------------------+
| Identity | Actor | Description |
+----------------------+----------------------+---------------------+
| RFC2919.List-id | -- | This provides a |
| | | global mailing list |
| | | naming framework |
| | | that is independent |
| | | of particular |
| | | hosts. Although |
| | | [RFC2919] is a |
| | | standards-track |
| | | specification, it |
| | | has not gained |
| | | significant |
| | | adoption. |
| RFC2369.List-* | Recipient | [RFC2369] defines a |
| | | collection of |
| | | message headers for |
| | | use by mailing |
| | | lists. In effect, |
| | | they supply |
| | | list-specific |
| | | parameters for |
| | | common mailing list |
| | | user operations. |
| | | The identifiers for |
| | | these operations |
| | | are for the list, |
| | | itself, and the |
| | | user-as-subscriber. |
| | | |
| RFC2822.From | Originator | Names and email |
| | | addresses for the |
| | | original author(s) |
| | | of the message |
| | | content are |
| | | specified. |
| RFC2822.Reply-To | Originator | Mailing lists have |
| | | introduced an |
| | | ambiguity for the |
| | | Reply-To field. |
| | | Some List |
| | | operations choose |
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| | | to force all |
| | | replies to go to |
| | | all list members. |
| | | They achieve this |
| | | by placing the list |
| | | address into the |
| | | RFC2822.Reply-To |
| | | field. Hence, |
| | | direct, "private" |
| | | replies only to the |
| | | original author |
| | | cannot be achieved |
| | | by using the MUA's |
| | | typical "reply to |
| | | author" function. |
| | | If the author |
| | | created a Reply-To |
| | | field, its |
| | | information is |
| | | lost. |
| RFC2822.Sender | Submitter | This will usually |
| | | specify the address |
| | | of the agent |
| | | responsible for |
| | | mailing list |
| | | operations. |
| | | However, some |
| | | mailing lists |
| | | operate in a manner |
| | | very similar to a |
| | | simple MTA relay, |
| | | so that they |
| | | preserve as much of |
| | | the original |
| | | handling |
| | | information as |
| | | possible, including |
| | | the original |
| | | RFC2822.Sender |
| | | field. |
| RFC2822.TO, | Intermediate | These will usually |
| RFC2822.CC | Recipient | contain the |
| | | original list of |
| | | recipient |
| | | addresses. |
| RFC2821.MailFrom | Intermediate | This may contain |
| | Submitter | the original |
| | | address to be |
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| | | notified of |
| | | transmission |
| | | issues, or the |
| | | mailing list agent |
| | | may set it to |
| | | contain a new |
| | | notification |
| | | address. Typically, |
| | | the value is set to |
| | | a new address, so |
| | | that mailing list |
| | | members and posters |
| | | are not burdened |
| | | with |
| | | transmission-relate |
| | | d notifications. |
| RFC2821.Rcpt-To | Recipient | This contain the |
| | | address of a |
| | | mailing list |
| | | member. |
| RFC2821.Received | Intermediate | An Mailing List |
| | Recipient | Agent should record |
| | | a Received header, |
| | | to indicate the |
| | | transition from |
| | | original posting to |
| | | mailing list |
| | | forwarding. The |
| | | Agent may choose to |
| | | have the message |
| | | retain the original |
| | | set of Received |
| | | headers or may |
| | | choose to remove |
| | | them. In the latter |
| | | case, it should |
| | | ensure that the |
| | | original Received |
| | | headers are |
| | | otherwise |
| | | available, to |
| | | ensure later |
| | | accountability and |
| | | diagnostic access |
| | | to it. |
+----------------------+----------------------+---------------------+
Table 9: Mailing List Identities
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7. Security Considerations
This document does not specify any new Internet mail functionality.
Consequently it should introduce no new security considerations.
However its discussion of the roles and responsibilities for
different mail service modules, and the information they create,
highlights the considerable security considerations that must be
present when implementing any component of the Internet mail service.
8 References
[ID-hdr-reg]
"Registration of mail and MIME header fields",
draft-klyne-hdrreg-mail-04.txt (work in progress), Apr
2004.
[ID-marid-core]
Lyon, J. and M. Wong, "MTA Authentication Records in DNS",
draft-ietf-marid-core-01.txt (work in progress), June
2004.
[ID-spamops]
Hutzler, C., Crocker, D., Resnick, P., Sanderson, R. and
E. Allman, "Email Submission Between Independent
Networks", draft-spamops-00 (work in progress), March
2004.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
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Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2298] Fajman, R., "An Extensible Message Format for Message
Disposition Notifications", RFC 2298, March 1998.
[RFC2304] Allocchio, C., "Minimal FAX address format in Internet
Mail", RFC 2304, March 1998.
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.
[RFC2421] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
Mail - version 2", RFC 2421, September 1998.
[RFC2423] Vaudreuil, G. and G. Parsons, "VPIM Voice Message MIME
Sub-type Registration", RFC 2423, September 1998.
[RFC2442] "The Batch SMTP Media Type", RFC 2442, November 1998.
[RFC2476] Gellens, R. and J. Klensin, "Message Submission", RFC
2476, December 1998.
[RFC2645] "On-Demand Mail Relay (ODMR) SMTP with Dynamic IP
Addresses", RFC 2465, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
Crocker Expires January 1, 2005 [Page 33]
Internet-Draft ID-Mail-Arch July 2004
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC3028] Showalter, T., "Sieve: A Mail Filtering Language", RFC
3028, January 2001.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC
3461, January 2003.
[RFC3501] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[2] <Forwarder>
Author's Address
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
EMail: dcrocker@brandenburg.com
Appendix A. Acknowledgements
The Email Architecture section derives from draft-hutzler-spamops
[ID-spamops]. The text has been further elaborated.
Discussion of the Submitter actor role was greatly clarified by
[ID-marid-core]. Reference to this role has been written to align
with that document's label and discussion.
Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful
insight on the framework and details of early drafts. Additional
review and suggestions have been provided by Nathaniel Borenstein,
Chris Newman, Eric Hall, Tony Finch, Ed Bradford, Cyrus Daboo, Ned
Freed.
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