Internet Mail Architecture
draft-crocker-email-arch-10

Status of this Memo

By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on August 28, 2008.

Abstract

Over its thirty-five year history Internet Mail has undergone significant changes in scale and complexity, as it has become a global infrastructure service. The first standardized architecture for networked email specified little more than a simple split between the user world and the transmission world. Core aspects of the service, such as the styles of mailbox address and basic message format, have remained remarkably constant. However today's Internet Mail is distinguished by many independent operators, many different components for providing service to users and many others for performing message transfer. Public discussion of the service often lacks common terminology and a common frame of reference for these components and their activities. Having a common reference model and terminology facilitates discussion about problems with the service, changes in policy, or enhancement to the service's functionality. This document offers an enhanced Internet Mail architecture that targets description of the existing service, in order to facilitate clearer and more efficient technical, operations and policy discussions about email.

Table of Contents

1.  Introduction
    1.1.  Background
    1.2.  Service Overview
    1.3.  Document Conventions
    1.4.  Changes to Previous Version
2.  Responsible Actor Roles
    2.1.  User Actors
    2.2.  Mail Handling Service (MHS) Actors
    2.3.  Administrative Actors
3.  Identities
    3.1.  Mailbox
    3.2.  Domain Names
    3.3.  Message Identifier
4.  Services and Standards
    4.1.  Message Data
    4.2.  User-Level Services
    4.3.  MHS-Level Services
5.  Mediators
    5.1.  Aliasing
    5.2.  Re-Sending
    5.3.  Mailing Lists
    5.4.  Gateways
    5.5.  Boundary Filter
6.  Considerations
    6.1.  Security Considerations
    6.2.  IANA Considerations
7.  References
    7.1.  Normative
    7.2.  Informative
Appendix A.  Acknowledgements
§  Index
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

Over its thirty-five year history Internet Mail has undergone significant changes in scale and complexity, as it has become a global infrastructure service. The changes have been evolutionary, rather than revolutionary, reflecting a strong desire to preserve its installed base of users and utility. Today, Internet Mail is distinguished by many independent operators, many different components for providing service to users and many other components for performing message transfer.

Public collaboration on email technical, operations and policy activities, including those responding to the challenges of email abuse, has brought in a much wider range of participants than email's technical community originally had. In order to do work on a large, complex system, they need to share the same view of how it is put together, as well as what terms to use to refer to the pieces and their activities. Otherwise, it is difficult to know exactly what another participant means. It is these differences in each person's perspective that motivates this document, to describe the realities of the current system. Internet mail is the subject of ongoing technical, operations and policy work, and the discussions often are hindered by different models of email service design and different meanings for the same terms. This architecture document seeks to facilitate clearer and more efficient technical, operations and policy exchanges about email.

This document offers an enhanced Internet Mail architecture to reflect the current service. In particular it:

• Documents refinements to the email model
• Clarifies functional roles for the architectural components
• Clarifies identity-related issues, across the email service
• Defines terminology for architectural components and their interactions

1.1.  Background

The first standardized architecture for networked email specified a simple split between the user world, in the form of Mail User Agents (MUA), and the transmission world, in the form of the Mail Handling Service (MHS) composed of Mail Transfer Agents (MTA). The MHS is responsible for accepting a message from one User and delivering it to one or more others, creating a virtual MUA-to-MUA exchange environment.

As shown in Figure 1 (Basic Internet Mail Service Model) this defines two logical "layers" of interoperability. One is directly between Users. The other is between the neighboring components, along the transfer path. In addition, there is interoperability between the layers, first when a message is posted from the User to the MHS and later when it is delivered from the MHS to the User.

The operational service has evolved sub-divisions for each of these layers into more specialized modules. Core aspects of the service, such as mailbox addressing and message format style, have remained remarkably constant. So the original distinction between user-level concerns and transfer-level concerns is retained, but with an elaboration to each level of the architecture. The term "Internet Mail" is used to refer to the entire collection of user and transfer components and services.

For Internet Mail the term "end-to-end" usually refers to a single posting and the set of deliveries directly resulting from its single transiting of the MHS. A common exception is with group dialogue that is mediated via a mailing list, so that two postings occur before intended recipients receive an Author's message, as discussed in Section 2.1.4 (Mediator). In fact some uses of email consider the entire email service -- including Author and Recipient -- as a subordinate component. For these services "end-to-end" refers to points outside of the email service. Examples are voicemail over email [RFC3801] (Vaudreuil, G. and G. Parsons, “,” June 2004.), EDI over email [RFC1767] (Crocker, D., “MIME Encapsulation of EDI Objects,” March 1995.) and facsimile over email [RFC4142] (Crocker, D. and G. Klyne, “Full-mode Fax Profile for Internet Mail: FFPIM,” December 2005.).

                              +--------+
            +---------------->|  User  |
            |                 +--------+
            |                      ^
+--------+  |          +--------+  .
|  User  +--+--------->|  User  |  .
+--------+  |          +--------+  .
    .       |               ^      .
    .       |   +--------+  .      .
    .       +-->|  User  |  .      .
    .           +--------+  .      .
    .                ^      .      .
    .                .      .      .
    V                .      .      .
+---+----------------+------+------+---+
|   .                .      .      .   |
|   +...............>+      .      .   |
|   .                       .      .   |
|   +......................>+      .   |
|   .                              .   |
|   +.............................>+   |
|                                      |
|     Mail Handling Service (MHS)      |
+--------------------------------------+

1.2.  Service Overview

End-to-end Internet Mail exchange is accomplished by using a standardized infrastructure comprising:

• An email object
• Global addressing
• An asynchronous sequence of point-to-point transfer mechanisms
• No prior arrangement between Author and Recipient
• No prior arrangement between point-to-point transfer services, over the open Internet
• No requirement for Author and Recipient to be online at the same time.

The end-to-end portion of the service is the email object, called a message. Broadly the message, itself, distinguishes between control information for handling, versus the author's message content.

A precept to the design of mail over the open Internet is permitting user-to-user and MTA-to-MTA interoperability to take place with no prior, direct arrangement between the independent administrative authorities responsible for handling a message. That is, all participants rely on the core services being universally supported and accessible, either directly or through gateways that translate between Internet Mail and email environments that conform to other standards. Given the importance of spontaneity and serendipity in the world of human communications, this lack of prearrangement between participants is a core benefit of Internet Mail and remains a core requirement for it.

Within localized networks at the edge of the public Internet, prior administrative arrangement often is required and can include access control, routing constraints and information query service configuration. In recent years one change to local environments is an increased requirement for authentication or, at least, accountability. In these cases a server performs explicit validation of the client's identity.

1.3.  Document Conventions

In this document, references to structured fields of a message use a two-part dotted notation. The first part cites the document that contains the specification for the field and the second is the name of the field. Hence <RFC2822.From> is the From: field in an email content header and <RFC2821.MailFrom> is the address in the SMTP "Mail From" command.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

Discussion venue:

Please direct discussion about this document to the IETF-SMTP mailing list http://www.imc.org/ietf-smtp.

1.4.  Changes to Previous Version

INSTRUCTIONS TO THE RFC EDITOR:

Remove this sub-section prior to publication.

Many small editing changes, for wordsmithing improvements to make details more consistent. This section documents the nature and basis for changes with significant impact.

Originator->Author:

The term "Originator" is used by RFC 2822 more broadly than just the From: field, which specifically defines who the author of the content is. I believe this distinguishes two constructs, one for the content author and one for the first agency that handles the message, in terms of the transfer service. So the change from "Originator" to "Author" seems pretty straightforward. The challenge is in using the term Originator, as defined in RFC 2822 and applying it to the system's architecture.

Source->Originator:

This change is more of a challenge. We need the "Originator" term and construct, but the architecture is already complex enough. Hence, adding a new construct seems like a very poor resolution. The document has used "Source" as an MHS term for the MSA set of functions. While one could argue against re-labeling it as Originator, I believe this is a reasonable choice and likely to be comfortable for community use, since "Source" does not have an established history.

Bounce->Return:

'bounce address' is not accurate, because the address is used for more than that, but it *is* as established term within portions of the broader email community. I also believe the extensive discussion on this point, last year, justifies the change.

The problem with saying "Bounce" is that is not merely linguistically impure, it is plain wrong and has already caused serious problems. Witness SPF. Frankly, we need to fix RFC2821, but that's a separate battle to fight and not one for this forum.

Although not a verbatim use of "Reverse Path", the related term that seems to work publicly is "Return Address". It is already established in the bricks-and-mortar postal world and seems to have some acceptance within parts of the email community. (I've done a draft white paper on authentication for the Messaging Anti-Abuse Working Group and the membership had some debate about this vocabulary choice and converged on agreeing to it.)

Is the envelope part of the message?

I don't remember whether we resolved this. For a variety of reasons, I believe the message includes its envelope, and am encouraged to find RFC2822upd says:

In the context of electronic mail, messages are viewed as having an envelope and contents. The envelope contains whatever information is needed to accomplish transmission and delivery. (See [I‑D.klensin‑rfc2821bis] (Klensin, J., “Simple Mail Transfer Protocol,” November 2007.) for a discussion of the envelope.) The contents comprise the object to be delivered to the recipient. This specification applies only to the format and some of the semantics of message contents.

rfc2821bis says:

SMTP transports a mail object. A mail object contains an envelope and content.

I think these justify having the term 'message' as including the object.

Examples of 'new' messages:

Section Section 3.3.1 (Message-ID)contains a list of examples, discussing scenarios that might or might not be viewed as creating a "new" message, rather than retaining an existing one. The list has been expanded.

2.  Responsible Actor Roles

Internet Mail is a highly distributed service, with a variety of actors serving different roles. These divide into 3 basic types:

• User
• Mail Handling Service (MHS)
• ADministrative Management Domain (ADMD)

Although related to a technical architecture, the focus on Actors concerns participant responsibilities, rather than on functionality of modules. Hence the labels used are different than for classic email architecture diagrams.

2.1.  User Actors

Users are the sources and sinks of messages. They can be humans, organizations or processes. They can have an exchange that iterates and they can expand or contract the set of users participating in a set of exchanges. In Internet Mail there are three types of user-level Actors:

• Authors
• Recipients
• Mediators

From the User-level perspective all mail transfer activities are performed by a monolithic Mail Handling Service (MHS), even though the actual service can be provided by many independent organizations. Users are customers of this unified service.

The following figure depicts the flow of messages among Actors:

+------------+
|            |<---------------------------+
|   Author   |<----------------+          |
|            |<----+           |          |
+-+---+----+-+     |           |          |
  |   |    |       |           |          |
  |   |    V       |           |          |
  |   |  +---------+-+         |          |
  |   |  | Recipient |         |          |
  |   |  +-----------+         |          |
  |   |                        |          |
  |   |       +--------+       |          |
  |   |       |        |       |          |
  |   V       V        |       |          |
  | +-----------+    +-+-------+-+        |
  | | Mediator  +--->| Recipient |        |
  | +-----------+    +-----------+        |
  |                                       |
  |       +-----------------------------+ |
  |       |                +----------+ | |
  |       |                |          | | |
  V       V                V          | | |
+-----------+    +-----------+    +---+-+-+---+
| Mediator  +--->| Mediator  +--->| Recipient |
+-----------+    +-----------+    +-----------+

2.1.1.  Author

This is the user-level participant responsible for creating the message, its contents and its list of recipient addresses. The MHS operates to send and deliver mail among Authors and Recipients. As described below, the MHS has a "Source" role that correlates with the user-level Author role.

2.1.2.  Recipient

The Recipient is a consumer of delivered message content. As described below, the MHS has a "Dest[ination]" role that correlates with the user-level Recipient role.

A Recipient can close the user-level communication loop by creating and submitting a new message that replies to an Author. An example of an automated form of reply is the Message Disposition Notification (MDN), which informs the Author about the Recipient's handling of the message. (See Section 4.1 (Message Data).)

2.1.3.  Return Handler

The Return Handler -- also called "Bounce Handler" -- receives and services notifications generated by the MHS, as a result of efforts to transfer or deliver the message. Notices can be about failures or completions and are sent to an address that is specified by the Source. This Return handling address (also known as a Return address) might have no visible characteristics in common with the address of the Author or Source.

2.1.4.  Mediator

A Mediator receives, aggregates, reformulates and redistributes messages as part of a potentially-protracted, higher-level exchange among Users. It is easy to confuse this user-level activity with the underlying MHS transfer exchanges. However they serve very different purposes and operate in very different ways. Mediators are considered extensively in Section 5 (Mediators).

When mail is delivered to a receiving mediator specified in the RFC2821.RcptTo command, the MHS handles it the same way as for any other Recipient. That is, the MHS only sees posting and delivery sources and sinks and does not see (later) re-posting as a continuation of a process. Hence when submitting messages, the Mediator is an Author.

The distinctive aspects of a Mediator are, therefore, above the MHS. A Mediator preserves the Author information of the message it reformulates, but may make meaningful changes to the content. Hence the MHS sees a new message, but Users receive a message that is interpreted as primarily being from -- or, at least, initiated by -- the author of the original message. The role of a Mediator permits distinct, active creativity, rather than being limited to the more constrained job of merely connecting together other participants. Hence it is really the Mediator that is responsible for the new message.

A Mediator's task can be complex and contingent, such as modifying and adding content or regulating which users are allowed to participate and when. The popular example of this role is a group mailing list. A sequence of Mediators may even perform a series of formal steps, such as reviewing, modifying and approving a purchase request.

Because a Mediator originates messages, it can also receive replies. So a Mediator really is a full-fledged User.

Gateway:

A Gateway is a particularly interesting form of Mediator. It is a hybrid of User and Relay that interconnects heterogeneous mail services. Its goal is to emulate a Relay, and a detailed discussion is in Section 2.2.3 (Gateway).

2.2.  Mail Handling Service (MHS) Actors

The Mail Handling Service (MHS) has the task of performing a single, end-to-end transfer on behalf of the Author and reaching the Recipient address(es) specified in the original RFC2821.RcptTo commands. Mediated or protracted, iterative exchanges, such as those used for collaboration over time, are part of the User-level service, and are not part of this transfer-level Handling Service.

The following figure depicts the relationships among transfer participants in Internet Mail. It shows the Source as distinct from the Author, and Dest[ination] as distinct from Recipient, although it is common for each pair to be the same actor. Transfers typically entail one or more Relays. However direct delivery from the Source to Destination is possible. For intra-organization mail services, it is common to have only one Relay.

+------------+                           +-----------+
|   Author   |         +--------+        | Recipient |
+-----+------+   +....>| Return |        +-----------+
      |          .     +--------+              ^
      |          .         ^                   |
//===================================================\\
||    |          .         |  Mail Handling    |     ||
||    |          .         |  Service (MHS)    |     ||
      V          .         |                   |
  +---------+    .         ^              +----+---+
  |         |    .         |              |        |
  | Origin  +....+         +-<------------+  Dest  |
  |         |              |              |        |
  +----+----+              |              +--------+
       |                   |                   ^
       |  +-------------->-+-<-------------+   |
       V  |                |               |   |
  +-------+-+         +----+----+        +-+---+---+
  |  Relay  +-->...-->|  Relay  +------->|  Relay  |
  +---------+         +----+----+        +---------+
                           |
                           V
                      +---------+
                      | Gateway +-->...
                      +---------+

2.2.1.  Originator

The Originator role is responsible for ensuring that a message is valid for posting and then submitting it to a Relay. Validity includes conformance with Internet Mail standards, as well as with local operational policies. The Originator can simply review the message for conformance and reject it if there are errors, or it can create some or all of the necessary information.

The Originator operates with dual "allegiance". It serves the Author and often it is the same entity. However its role in assuring validity means that it MUST also represent the local operator of the MHS, that is, the local ADministrative Management Domain (ADMD).

The Originator also has the responsibility for any post-submission, Author-related administrative tasks associated with message transmission and delivery. Notably this pertains to error and delivery notices. Hence Source is best held accountable for the message content, even when they did not create any or most of it.

2.2.2.  Relay

A mail Relay performs email transfer-service routing and store-and-forward by (re-)transmitting the message on towards its Recipient(s). A Relay can add trace information. However it does not modify existing envelope information or the message content semantics. It can modify message content syntax, such as a change from binary to text transfer-encoding form, only as required to meet the capabilities of the next hop in the MHS.

A set of Relays composes a Mail Handling Service (MHS) network. This is above any underlying packet-switching network that they might be using and below any gateways or other user-level Mediators.

In other words, interesting email scenarios can involve three distinct architectural layers of store-and-forward service:

• User Mediators
• MHS Relays
• Packet Switches

with the bottom-most usually being the Internet's IP service. The most basic email scenarios involve Relays and Switches.

Aborting a message transfer results in having the Relay become an Author and sending an error message to the Return address. The potential for looping is avoided by having this message, itself, contain no Return address.

2.2.3.  Gateway

A Gateway is a hybrid form of User and Relay that interconnects heterogeneous mail services. Its purpose is simply to emulate a Relay and the closer it comes to this, the better. However it operates at the User level, because it MUST be able to modify message content.

Differences between mail services can be as small as minor syntax variations, but usually encompass significant, semantic distinctions. One difference could have the concept of an email address being a hierarchical, machine-specific address, versus having it be a flat, global name space. Another difference could be between text-only content, versus multi-media. Hence the Relay function in a Gateway offers significant design challenges, to make the result be as seamless as possible. The most significant challenge is in ensuring the user-to-user functionality that matches syntax and semantics of independent email standards suites.

The basic test of a Gateway's adequacy is, of course, whether an Author on one side of a Gateway can send a useful message to a Recipient on the other side, without requiring changes to any of the components in the Author's or Recipient's mail services, other than adding the Gateway. To each of these otherwise independent services, the Gateway will appear to be a "native" participant. However the ultimate test of a Gateway's adequacy is whether the Author and Recipient can sustain a dialogue. In particular can a Recipient's MUA automatically formulate a valid Reply that will reach the initial Author?

2.3.  Administrative Actors

Actors often are associated with different organizations, each with its own administrative authority. This operational independence, coupled with the need for interaction between groups, provides the motivation for distinguishing among ADministrative Management Domains (ADMD). Each ADMD can have vastly different operating policies and trust-based decision-making. An obvious example is the distinction between mail that is exchanged within a single organization, versus mail that is exchanged between independent organizations. The rules for handling these two types of traffic tend to be quite different. That difference requires defining the boundaries of each, and this requires the ADMD construct.

Operation of Internet Mail services is apportioned to different providers (or operators). Each can be an independent ADMD. This independence of administrative decision-making defines boundaries that distinguish different portions of the Internet Mail service. Examples include an end-user operating their desktop client, a department operating a local Relay, an IT department operating an enterprise Relay and an ISP operating a public shared email service. These can be configured into many combinations of administrative and operational relationships, with each ADMD potentially having a complex arrangement of functional components. Figure 4 (ADMD Example) depicts relationships among ADMDs. The benefit of having the ADMD construct is to facilitate discussion about designs and operations that need to distinguish between "internal" issues and "external" ones.

The architectural impact of needing to have boundaries between ADMD's is discussed in [Tussle] (Clark, D., Wroclawski, J., Sollins, K., and R. Braden, “Tussle in Cyberspace: Defining Tomorrow’s Internet,” 2002.). Most significant is that the entities communicating across ADMD boundaries will typically have an added burden to enforce organizational policies concerning "external" communications. At a more mundane level, routing mail between ADMDs can be an issue, such as needing to route mail for partners over specially-trusted paths.

Basic types of ADMDs include --

Edge:

Independent transfer services, in networks at the edge of the open Internet Mail service.

User:

End-user services. This might be subsumed under the Edge service, such as is common for web-based email access.

Transit:

These are Mail Service Providers (MSP) offering value-added capabilities for Edge ADMDs, such as aggregation and filtering.

Note that Transit services are quite different from packet-level switching operation. Whereas end-to-end packet transfers usually go through intermediate routers, email exchange across the open Internet is often directly between the Boundary MTAs of Edge ADMDs, at the email level. This further highlights the differences discussed in Section 2.2.2 (Relay)

+-------+                           +-------+    +-------+
| ADMD1 |                           | ADMD3 |    | ADMD4 |
| ----- |                           | ----- |    | ----- |
|       |   +---------------------->|       |    |       |
| User  |   |                       |-Edge--+--->|-User  |
|  |    |   |    +---------+   +--->|       |    |       |
|  V    |   |    |  ADMD2  |   |    +-------+    +-------+
| Edge--+---+    |  -----  |   |
|       |   |    |         |   |
+-------+   +----|-Transit-+---+
                 |         |
                 +---------+

Edge networks can use proprietary email standards internally. However the distinction between Transit network and Edge network transfer services is primarily significant because it highlights the need for concern over interaction and protection between independent administrations. In particular this distinction calls for additional care in assessing transitions of responsibility, as well as the accountability and authorization relationships among participants in email transfer.

The interactions between functional components within an ADMD are subject to the policies of that domain. Policies can cover such things as:

• Reliability
• Access control
• Accountability
• Content evaluation and modification

They can be implemented in different functional components, according to the needs of the ADMD. For example see [RFC5068] (Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and E. Allman, “Email Submission Operations: Access and Accountability Requirements,” Nov 2007.).

User, Edge and Transit services can be offered by providers that operate component services or sets of services. Further it is possible for one ADMD to host services for other ADMDs.

Common ADMD examples are --

Enterprise Service Providers:

Operating an organization's internal data and/or mail services.

Internet Service Providers:

Operating underlying data communication services that, in turn, are used by one or more Relays and Users. It is not necessarily their job to perform email functions, but they can, instead, provide an environment in which those functions can be performed.

Mail Service Providers:

Operating 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 ensure the health of the overall Internet Mail Service. This can include operators of lower-level packet services.

3.  Identities

Internet Mail uses three forms of identity: mailbox, domain name and message-id. Each is required to be globally unique.

3.1.  Mailbox

"A mailbox sends and receives mail. It is a conceptual entity which does not necessarily pertain to file storage." [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.)

A mailbox is specified as an Internet Mail address <addr-spec>. It has two distinct parts, divided by an at-sign ("@"). The right-hand side is a globally interpreted domain name that is associated with an ADMD. Domain Names are discussed in Section 3.2 (Domain Names). Formal Internet Mail addressing syntax can support source routes, to indicate the path through which a message should be sent. Although legal, the use of source routes is not part of the modern Internet Mail service and it is not discussed further.

The portion to the left of the at-sign contains a string that is globally opaque and is called the <local-part>. It is to be interpreted only by the entity specified by the address's right-hand side domain name. All other entities MUST treat the local-part as a uninterpreted literal string and MUST preserve all of its original details. As such its public distribution is equivalent to sending a Web browser "cookie" that is only interpreted upon being returned to its Author.

3.1.1.  Global Standards for Local-Part

It is common for sites to have local structuring conventions for the left-hand side <local-part> of an <addr-spec>. This permits sub-addressing, such as for distinguishing different discussion groups used by the same participant. However it is worth stressing that these conventions are strictly private to the user's organization and SHOULD NOT be interpreted by any domain except the one listed in the right-hand side of the addr-spec. The exceptions are those specialized services conforming to standardized conventions, as noted below.

There are a few types of addresses that have an elaboration on basic email addressing, with a standardized, global schema for the local-part. These are conventions between authoring systems and Recipient Gateways, and they are invisible to the public email transfer infrastructure. When an Author is explicitly sending via a Gateway out of the Internet, there are coding conventions for the local-part, so that the Author can formulate instructions for the Gateway. Standardized examples of this are the telephone numbering formats for VPIM [RFC3801] (Vaudreuil, G. and G. Parsons, “,” June 2004.), such as "+16137637582@vpim.example.com", and iFax [RFC3192] (Allocchio, C., “Minimal FAX address format in Internet Mail,” October 2001.), 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, an email address string has become the common identifier for representing online identity. 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. For example, see: Section 4.1.4 (Identity References in a Message), Section 4.3.3 (Mail Delivery Agent (MDA)), Section 5 (Mediators).

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 domain name usually maps to one or more IP Addresses. Conceptually the name might encompass an entire organization, a collection of machines integrated into a homogeneous service, or only a single machine. A domain name can be administered to refer to individual users, but this is not common practice. The name is structured as a hierarchical sequence of sub-names, separated by dots ("."), with the top of the hierarchy being on the right-end of the sequence. Domain names are defined and operated through the Domain Name System (DNS) [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.), [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.), [RFC2181] (Elz, R. and R. Bush, “Clarifications to the DNS Specification,” July 1997.).

When not part of a mailbox address, a domain name is used in Internet Mail to refer to the ADMD or the host that took action upon the message, such as providing the administrative scope for a message identifier, or performing transfer processing.

3.3.  Message Identifier

There are two standardized tags for identifying messages: Message-ID and ENVID.

3.3.1.  Message-ID

The Message-ID is a user-level tag, primarily used for threading and for eliminating duplicates [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.). Any actor within the Originating ADMD can assign the Message-ID. The recipient's ADMD is the intended consumer of the Message-ID, although any actor along the transfer path might use it. Internet Mail standards provide for a single Message-ID; however more than one is sometimes assigned.

Like a mailbox address, a Message-ID has two distinct parts, divided by an at-sign ("@"). The right-hand side is globally interpreted and specifies the ADMD or host assigning the identifier. The left-hand side 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.

When a message is revised in any way, the question of whether to assign a new Message-ID requires a subjective assessment, deciding whether the editorial content has been changed enough to constitute a new message. [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.) says "a message identifier pertains to exactly one instantiation of a particular message; subsequent revisions to the message each receive new message identifiers." However real-world experience dictates some flexibility. An impossible test is whether the recipient will consider the new message to be equivalent to the old. For most components of Internet Mail, there is no way to predict a specific recipient's preferences on this matter. Both creating and failing to create a new Message-ID have their downsides.

The best that can be offered, here, are some guidelines and examples:

• If a message is changed only in terms of form, such as character-encoding, it clearly is still the same message.
• If a message has minor additions to the content, such as a mailing list tag at the beginning of the RFC2822.Subject header field, or some mailing list administrative information added to the end of the primary body-part's text, then it probably is still the same message.
• If a message has viruses deleted from it, it probably is still the same message.
• If a message has offensive words deleted from it, then some recipients will consider it the same message, but some will not.
• If a message is translated into a different language, then some recipients will consider it the same message, but some will not.
• If a message is included in a digest of messages, it the digest constitutes a new message.
• If a message is forwarded by a recipient, what is forwarded is considered to be a new message.
• If a message is "redirected", such as using RFC2822 "Redirect-*" headers, some recipients will consider it the same message, but some will not.

The absence of objective, precise criteria for Message-ID re-generation, along with the absence of strong protection associated with the string, means that the presence of an ID can permit an assessment that is marginally better than a heuristic, but the ID certainly has no value on its own for strict formal reference or comparison. Hence Message-ID SHOULD NOT be used for any function that has security implications.

3.3.2.  ENVID

The ENVID (envelope identifier) is a tag that is primarily for use within Delivery Status Notifications (DSN), so that the Return Address (RFC2821.MailFrom) recipient can correlate the DSN with a particular message [RFC3461] (Moore, K., “Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs),” January 2003.). The ENVID is therefore used from one message posting, until the directly-resulting message deliveries. It does not survive re-postings.

The ENVID may also be used for message tracking purposes [RFC3885] (Allman, E. and T. Hansen, “SMTP Service Extension for Message Tracking,” September 2004.).

The format of an ENVID is free-form. Although its creator might choose to impose structure on the string, none is imposed by Internet standards. By implication, the scope of the string is defined by the domain name of the Return Address.

4.  Services and Standards

Internet Mail's architecture distinguishes among six basic types of functionality, arranged to support a store-and-forward service architecture. As shown in Figure 5 (Protocols and Services) these types can have multiple instances, some of which represent specialized sub-roles. This section considers the activities and relationships among these components, and the Internet Mail standards that apply to them.

1. Message
2. Mail User Agent (MUA)

   Originating MUA (oMUA)

   Receiving MUA (rMUA)

3. Message Submission Agent (MSA)

   Author-focussed MSA functions (oMSA)

   MHS-focussed MSA functions (hMSA)

4. Message Transfer Agent (MTA)
5. Message Delivery Agent (MDA)

   Recipient-focused MDA functions (rMDA)

   MHS-focussed MDA functions (hMDA)

6. Message Store (MS)
   1. Author MS (oMS)

      oMS on a remote server (soMS)

      oMS co-located with the oMUA (uoMS)

   2. Recipient MS (rms)

      rMS on a remote server (srMS)

      rMS co-located with the rMUA (urMS)

This section describes each functional component for Internet Mail, and the standards-based protocols associated with their operation.

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 service are becoming more extensive. So their separation is increasingly common.

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 can combine or separate architectural components, as needed for a particular operational environment.

A software system that primarily performs message relaying -- and therefore is an MTA -- might also include MDA functionality. That same MTA system might be able to interface with non-Internet email services and therefore qualify as a Gateway.

It is important not to confuse the engineering decisions made to implement a product, with the architectural abstractions used to define conceptual functions.

The following figure shows function modules and the standardized protocols used between them. Additional protocols and configurations are possible. Boxes defined by asterisks (*) represent functions that often are distributed among two or more systems.

                  +------+                              +-------+
      ............+ oMUA |..............................| Disp  |
      .           +--+-+-+                              +-------+
      .   local,imap}| |{smtp,submission                     ^
      .              | |                        +---------+  |
      . *******      | | .......................| Returns |  |
      . * oMS *<-----+ | .                      +---------+  |
      . *******        | .   *****************       ^       |
      .         +------V-.---*------------+  *       |       |
      .     MSA | +-------+  *   +------+ |  *       |       |
      .         | | oMSA  +--O-->| hMSA | |  *       |       |
      .         | +-------+  *   +--+---+ |  *       |       |
      .         +------------*------+-----+  *       |       |
//==========\\               *      V {smtp  *       |       |
|| MESSAGE  ||               *   +------+    *  //===+===\\  |
||----------||           MHS *   | MTA  |    *  ||  dsn  ||  |
|| Envelope ||               *   +--+---+    *  \\=======//  |
||  SMTP    ||               *      V {smtp  *     ^   ^     |
|| Content  ||               *   +------+    *     |   | //==+==\\
||  RFC2822 ||               *   | MTA  +----*-----+   | || mdn ||
||  MIME    ||               *   +--+---+    *         | \\=====//
\\==========//               * smtp}| {local *         |     |
      .          MDA         *      | {lmtp  *         |     |
      .         +------------+------V-----+  *         |     |
      .         | +------+   *   +------+ |  *         |     |
      .         | |      |   *   |      | +--*---------+     |
      .         | | rMDA |<--O---+ hMDA | |  *               |
      .         | |      |   *   |      | |<-*-------+       |
      .         | +-+----+   *   +------+ |  *       |       |
      .         +---+--+-----*------------+  *       |       |
      .             |  |     *****************       |       |
      .     pop} +--+  +---+                         |       |
      .    imap} |         | {local                  |       |
      .  ******************V********                 |       |
      .  *       |       +------+  * rMS        //===+===\\  |
      .  *       |       | srMS |  *            || sieve ||  |
      .  *       V       +--+-+-+  *            \\=======//  |
      .  *  +------+   pop} | |    *                 ^       |
      .  *  | urMS |<-------+ |    *                 |       |
      .  *  +--+---+  imap}   |    *                 |       |
      .  ***************************                 |       |
      .  local}|  +------+       |{pop,imap          |       |
      .        +->|      |<------+                   |       |
      ...........>| rMUA +---------------------------+       |
                  |      +-----------------------------------+
                  +------+

4.1.  Message Data

The purpose of the Mail Handling Service (MHS) is to exchange a message object among participants [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.), [RFC0822] (Crocker, D., “Standard for the format of ARPA Internet text messages,” August 1982.). Hence all of its underlying mechanisms are merely in the service of getting that message from its Author to its Recipients. A message can be explicitly labeled as to its nature [RFC3458] (Burger, E., Candell, E., Eliot, C., and G. Klyne, “Message Context for Internet Mail,” January 2003.).

A message comprises a transit handling envelope and the message content. The envelope contains information used by the MHS. The content is divided into a structured header and the body. The header comprises transit trace information and end-user structured fields. The body may be unstructured simple lines of text, or it may be a MIME tree of multi-media subordinate objects, called body-parts, or attachments [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.), [RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.), [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.), [RFC4288] (Freed, N., Klensin, J., and J. Postel, “Media Type Specifications and Registration Procedures,” December 2005.), [RFC4289] (Freed, N., Klensin, J., and J. Postel, “Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures,” December 2005.), [RFC2049] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples,” November 1996.).

In addition, Internet Mail has a few conventions for special control data --

Delivery Status Notification (DSN):

A Delivery Status Notification (DSN) is a message that can be generated by the MHS (MSA, MTA or MDA) and sent to the RFC2821.MailFrom address. The mailbox for this is shown as Returns in Figure 5 (Protocols and Services). DSNs provide information about message transit, such as transmission errors or successful delivery [RFC3461] (Moore, K., “Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs),” January 2003.).

Message Disposition Notification (MDN):

A Message Disposition Notification (MDN) is a message that provides information about user-level, Recipient-side message processing, such as indicating that the message has been displayed [RFC3798] (Hansen, T. and G. Vaudreuil, “Message Disposition Notification,” May 2004.) or the form of content that can be supported [RFC3297] (Klyne, G., Iwazaki, R., and D. Crocker, “Content Negotiation for Messaging Services based on Email,” July 2002.). It can be generated by an rMUA and is sent to the Disposition-Notification-To address(es). The mailbox for this is shown as Disp in Figure 5 (Protocols and Services).

Message Filtering (SIEVE):

SIEVE is a scripting language that permits specifying conditions for differential handling of mail, typically at the time of delivery [RFC3028] (Showalter, T., “Sieve: A Mail Filtering Language,” January 2001.). It can be conveyed in a variety of ways, as a MIME part. Figure 5 (Protocols and Services) shows a Sieve specification going from the rMUA to the MDA. However filtering can be done at many different points along the transit path and any one or more of them might be subject to Sieve directives, especially within a single ADMD. Hence the Figure shows only one relationship, for (relative) simplicity.

4.1.1.  Envelope

Internet Mail has a fragmented framework for transit-related "handling" information. Information that is directly used by the MHS is called the "envelope". It directs handling activities by the transfer service as is carried in transfer service commands. That is, the envelope exists in the transfer protocol SMTP [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.).

Trace information records handling activity and is recorded in the message Header. [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.)

4.1.2.  Header Fields

Header fields are attribute name/value pairs covering an extensible range of email service, user content and user transaction meta-information. The core set of header fields is defined in [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.), [RFC0822] (Crocker, D., “Standard for the format of ARPA Internet text messages,” August 1982.). It is common to extend this set, for different applications. Procedures for registering header fields are defined in [RFC4021] (Klyne, G. and J. Palme, “Registration of Mail and MIME Header Fields,” March 2005.). An extensive set of existing header field registrations is provided in [RFC3864] (Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” September 2004.).

One danger with placing additional information in header fields is that Gateways often alter or delete them.

4.1.3.  Body

The body of a message might simply be lines of ASCII text or it might be hierarchically structured into a composition of multi-media body-part attachments, using MIME [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.), [RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.), [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.), [RFC4288] (Freed, N., Klensin, J., and J. Postel, “Media Type Specifications and Registration Procedures,” December 2005.), [RFC2049] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples,” November 1996.). MIME structures each body-part into a recursive set of MIME header field meta-data and MIME Content sections.

4.1.4.  Identity References in a Message

For a message in transit, the core uses of identifiers combine into:

The most common address-related fields are:

RFC2822.From:

Set by - Author

Names and addresses for author(s) of the message content are listed in the From: field.

RFC2822.Reply-To:

Set by - Author

If a message Recipient sends a reply message that would otherwise use the RFC2822.From field address(es) that are contained in the original message, then they are instead to use the address(es) in the RFC2822.Reply-To field. In other words this field is a direct override of the From: field, for responses from Recipients.

RFC2822.Sender:

Set by - Source

This specifies the address responsible for submitting the message into the transfer service. For efficiency this field can be omitted if it contains the same address as RFC2822.From. However this does not mean there is no Sender specified. Rather it means that that header field is virtual and that the address in the From: field MUST be used.

Specification of the notifications Return addresses -- contained in RFC2821.MailFrom -- is made by the RFC2822.Sender. Typically the Return address is the same as the Sender address. However some usage scenarios require it to be different.

RFC2822.To/.CC:

Set by - Author

These specify MUA Recipient addresses. However some or all of the addresses in these fields might not be present in the RFC2821.RcptTo commands.

The distinction between To and CC is subjective. Generally a To addressee is considered primary and is expected to take action on the message. A CC addressee typically receives a copy only for their information.

RFC2822.BCC:

Set by - Author

A message might be copied to an addressee whose participation is not to be disclosed to the RFC2822.To or RFC2822.CC Recipients and, usually, not to the other BCC Recipients. The BCC header field indicates a message copy to such a Recipient.

Typically, the field lists no addresses or only lists the single address of the Recipient receiving this copy. 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 of the Author.

RFC2821.HELO/.EHLO:

Set by - Source

The MSA can specify its hosting domain identity for the SMTP HELO or EHLO command operation.

RFC3461.ENVID:

Set by - Source

The MSA can specify an opaque string, to be included in a DSN, as a means of assisting the Return address recipient in identifying the message that produced a DSN, or message tracking.

RFC2821.MailFrom:

Set by - Source

This is an end-to-end string that specifies an email address for receiving return control information, such as "bounces". The name of this field is misleading, because it is not required to specify either the author or the Actor responsible for submitting the message. Rather, the Actor responsible for submission specifies the RFC2821.MailFrom address. Ultimately the simple basis for deciding what address needs to be in the RFC2821.MailFrom is to determine what address needs to be informed about transmission-level problems (and, possibly, successes.)

RFC2821.RcptTo:

Set by - Author

This specifies the MUA mailbox address of a recipient. The string might not be visible in the message content header. For example, the message destination address header fields, such as RFC2822.To, might specify a mailing list mailbox, while the RFC2821.RcptTo address specifies a member of that list.

RFC2821.Received:

Set by - Source, Relay, Mediator, Dest

This indicates trace information, including originating host, relays, Mediators, and MSA host domain names and/or IP Addresses.

RFC2821.Return-Path:

Set by - Source

The MDA records the RFC2821.MailFrom address into the RFC2822.Return-Path field.

RFC2919.List-Id:

Set by - Mediator Author

This provides a globally unique mailing list naming framework that is independent of particular hosts. [RFC2919] (Chandhok, R. and G. Wenger, “List-Id: A Structured Field and Namespace for the Identification of Mailing Lists,” March 2001.)

The identifier is in the form of a domain name; however the string usually is constructed by combining the two parts of an email address and the result rarely is a true domain name, listed in the domain name service -- although it can be.

RFC2369.List-*:

Set by - Mediator Author

[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,” July 1998.) defines a collection of message header fields for use by mailing lists. In effect they supply list-specific parameters for common mailing list user operations. The identifiers for these operations are for the list, itself, and the user-as-subscriber [RFC2369] (Neufeld, G. and J. Baer, “The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields,” July 1998.).

RFC0791.SourceAddr:

Set by - The Client SMTP sending host immediately preceding the current receiving SMTP server.

[RFC0791] (Postel, J., “Internet Protocol,” 1981 September.) defines the basic unit of data transfer for the Internet, the IP Datagram. It contains a "Source Address" field that specifies the IP Address for the host (interface) from which the datagram was sent. This information is set and provided by the IP layer, and is therefore independent of mail-level mechanisms. As such, it is often taken to be authoritative, although it is possible to provide false addresses.

4.2.  User-Level Services

Interactions at the user level entail protocol exchanges, distinct from those that occur at lower layers of the Internet Mail architecture, which is above the Internet Transport layer. Because the motivation for email, and much of its use, is for interaction among humans, the nature and details of these protocol exchanges often are determined by the needs of human and group communication. In terms of efforts to specify behaviors, one effect of this is to require subjective guidelines, rather than strict rules, for some aspects of system behavior. Mailing Lists provide particularly salient examples of this.

4.2.1.  Mail User Agent (MUA)

A Mail User Agent (MUA) works on behalf of end-users and end-user applications. It is their "representative" within the email service.

The Origination-side MUA (oMUA) creates a message and performs initial "submission" into the transfer infrastructure, via a Mail Submission Agent (MSA). It can also perform any creation- and posting-time archival in its Message Store (oMS). An MUA's oMS will typically include a folder for messages under development (Drafts), a folder for messages waiting to be sent (Queued or Unsent) and a folder for messages that have been successfully posted for transmission (Sent).

The Recipient-side MUA (rMUA) works on behalf of the end-user Recipient to process received mail. This includes generating user-level return control messages, displaying and disposing of the received message, and closing or expanding the user communication loop, by initiating replies and forwarding new messages.

NOTE:

Although not shown in Figure 5 (Protocols and Services), an MUA can, itself, have a distributed implementation, such as 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] (Crispin, M., “Internet Message Access Protocol - Version 4rev1,” March 2003.) for most of the interactions between an MUA client and an MUA server. A standardized approach for such scenarios is defined by [RFC4550] (Maes, S., , S., and Isode Ltd., “Internet Email to Support Diverse Service Environments (Lemonade) Profile,” June 2006.).

A Mediator is special class of MUA. It performs message re-posting, as discussed in Section 2.1 (User Actors).

Identity fields relevant to a typical end-user MUA include:

RFC2822.From

RFC2822.Reply-To

RFC2822.Sender

RFC2822.To, RFC2822.CC

RFC2822.BCC

4.2.2.  Message Store (MS)

An MUA can employ a long-term Message Store (MS). Figure 5 (Protocols and Services) depicts an Origination-side MS (oMS) and a Recipient-side MS (rMS). There is a rich set of choices for configuring a store, because any MS may comprise a distributed set of component stores. In Figure 5 (Protocols and Services), the rMS demonstrates this by showing an rMS that is located on a remote server (srMS) and an rMS that is on the same machine as the MUA (urMS). The relationship between two message stores, themselves, can vary.

As discussed in [RFC1733] (Crispin, M., “Distributed Electronic Models in IMAP4,” December 1994.) the operational relationship among 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 completely moved from any remote store, rather than (also) being retained there.

Disconnected:

An rMS and a uMS are kept synchronized, for all or part of their contents, while there is a connection between them. While they are disconnected, mail can continue to arrive at the rMS and the user may continue to make changes to the uMS. Upon reconnection, the two stores are re-synchronized.

4.3.  MHS-Level Services

4.3.1.  Mail Submission Agent (MSA)

A Mail Submission Agent (MSA) accepts the message submission from the oMUA and enforces the policies of the hosting ADMD and the requirements of Internet standards. An MSA represents an unusual functional dichotomy. A portion of its task is to represent MUA (uMSA) interests during message posting, to facilitate posting success, and another portion is to represent MHS (hMSA) interests. This is best modeled, as shown in Figure 5 (Protocols and Services), with two sub-components, one for the oMUA (oMSA) and one for the MHS (hMSA)

The hMSA's function is to take transit responsibility for a message that conforms to the relevant Internet standards and to local site policies. It rejects messages that are not in conformance. The oMSA's is to perform final message preparation for submission and to effect the transfer of responsibility to the MHS, via the hMSA. The amount of preparation will depend upon the local implementations. Examples of oMSA tasks could be to add header fields, such as Date: and Message-ID, to modify portions of the message from local notations to Internet standards, such as expanding an address to its formal RFC2822 representation.

Historically, standards-based MUA/MSA interactions have used SMTP [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.). A recent alternative is SUBMISSION [RFC4409] (Gellens, R. and J. Klensin, “Message Submission for Mail,” April 2006.). Although SUBMISSION derives from SMTP, it uses a separate TCP port and imposes distinct requirements, such as access authorization.

Identities relevant to the MSA include:

RFC2821.HELO/.EHLO

RFC3461.ENVID

RFC2821.MailFrom

RFC2821.RcptTo

RFC2821.Received

4.3.2.  Mail Transfer Agent (MTA)

A Mail Transfer Agent (MTA) relays mail for one application-level "hop". It is like a packet-switch or IP router in that its job is to make routing assessments and to move the message closer to the Recipient(s). Relaying is performed by a sequence of MTAs, until the message reaches a destination MDA. Hence an MTA implements both client and server MTA functionality. It does not make changes to addresses in the envelope or reformulate the editorial content. Hence a change in data form, such as to the MIME Content-Transfer-Encoding, is within the purview of an MTA, whereas removal or replacement of body content is not. Also it can add trace information. Of course email objects are typically much larger than the payload of a packet or datagram, and the end-to-end latencies are typically much higher.

Internet Mail primarily uses SMTP [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.), [RFC0821] (Postel, J., “Simple Mail Transfer Protocol,” August 1982.) to effect point-to-point transfers between peer MTAs. Other transfer mechanisms include Batch SMTP [RFC2442] (, “The Batch SMTP Media Type,” November 1998.) and ODMR [RFC2645] (, “On-Demand Mail Relay (ODMR) SMTP with Dynamic IP Addresses,” August 1999.). As with most network layer mechanisms, Internet Mail's SMTP supports a basic level of reliability, by virtue of providing for retransmission after a temporary transfer failure. Contrary to typical packet switches (and Instant Messaging services) Internet Mail MTAs typically store messages in a manner that allows recovery across service interruptions, such as host system shutdown. However the degree of such robustness and persistence by an MTA can be highly variable.

The primary "routing" mechanism for Internet Mail is the DNS MX record [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.), which specifies a host through which the queried domain can be reached. This presumes a public -- or at least a common -- backbone that permits any attached host to connect to any other.

Identities relevant to the MTA include:

RFC2821.HELO/.EHLO

RFC3461.ENVID

RFC2821.MailFrom

RFC2821.RcptTo

RFC2822.Received

Set by - Relay Server

4.3.3.  Mail Delivery Agent (MDA)

A Mail Delivery Agent (MDA) delivers email to the Recipient's mailbox. It 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 elsewhere in the Recipient's ADMD, such as at an organizational border (Boundary) Relay. However it is required for the MDA, if only because the MDA must know where to deliver the message.

As with an MSA, an MDA serves two roles, as depicted in Figure 5 (Protocols and Services). Formal transfer of responsibility, called "delivery" is effected between the two components that embody these roles. The MHS portion (hMDA) primarily functions as a server SMTP engine. A common additional role is to re-direct the message to an alternative address, as specified by the recipient addressee's preferences. The job of the recipient portion of the MDA (rMDA) is to perform any delivery-actions are desired by the recipient.

Using Internet protocols, delivery can be effected by a variety of standard protocols. When coupled with an internal local mechanism, SMTP [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.) and LMTP [RFC2033] (Myers, J., “Local Mail Transfer Protocol,” October 1996.) permit "push" delivery to the Recipient system, at the initiative of the upstream email service. POP [RFC1939] (Myers, J. and M. Rose, “Post Office Protocol - Version 3,” May 1996.) and IMAP [RFC3501] (Crispin, M., “Internet Message Access Protocol - Version 4rev1,” March 2003.) are used for "pull" delivery at the initiative of the Recipient system. POP and IMAP can also be used for repeated access to messages on a remote MS.

Identities relevant to the MDA include:

RFC2821.Return-Path:

Set by - Author Source or Mediator Source

The MDA records the RFC2821.MailFrom address into the RFC2822.Return-Path field.

RFC2822.Received:

Set by - MDA server

An MDA can record a Received header field to indicate trace information, including source host and receiving host domain names and/or IP Addresses.

5.  Mediators

Basic email transfer from an Author to the specified Recipients is accomplished by using an asynchronous, store-and-forward communication infrastructure, in a sequence of independent transmissions through some number of MTAs. A very different task is a User-level sequence of postings and deliveries, through Mediators. A Mediator forwards a message, through a re-posting process. The Mediator does share some functionality with basic MTA relaying, but it enjoys a degree of freedom with both addressing and content that is not available to MTAs.

RFC2821.HELO/.EHLO:

Set by - Mediator Source

RFC3461.ENVID

Set by - Author Source or Mediator Source

RFC2821.MailFrom:

Set by - Author Source or Mediator Source

RFC2821.RcptTo:

Set by - Mediator Author

RFC2821.Received:

Set by - Mediator Dest

The salient aspect of a Mediator, that distinguishes it from any other MUA creating an entirely new message, is that a Mediator preserves the integrity and tone of the original message, including the essential aspects of its origination information. The Mediator might also add commentary.

Examples of MUA message creation NOT performed by Mediators include --

New message that forwards an existing message:

This action rather curiously provides a basic template for a class of Mediators. However for its typical occurrence it is not itself an example of a Mediator. The new message is viewed as being from the Actor doing the forwarding, rather than being from the original Author.

A new message encapsulates the original message and is seen as strictly "from" the Mediator. The Mediator might add commentary and certainly has the opportunity to modify the original message content. The forwarded message is therefore independent of the original message exchange and creates a new message dialogue. However the final Recipient sees the contained message as from the original Author.

Reply:

When a Recipient formulates a response back to the original message's author, the new message is not typically viewed as being a "forwarding" of the original. Its focus is the new content, although it might contain all or part of the material in the original message. Therefore the earlier material is merely contextual and secondary.

Annotation:

The integrity of the original message is usually preserved, but one or more comments about the message are added in a manner that distinguishes commentary from original text. The tone of the new message is that it is primarily commentary from a new Author, similar to a Reply.

The remainder of this section describes common examples of Mediators.

5.1.  Aliasing

Aliasing is a simple re-addressing facility that is available in most MDA implementations. It is performed just before placing a message into the specified Recipient's mailbox. Instead the message is submitted back to the transfer service, for delivery to one or more alternate addresses. Although typically implemented as part of an MDA, this facility is strictly a Recipient user function. It resubmits the message, replacing the envelope address, on behalf of the mailbox address that was listed in the envelope.

What is most distinctive about this forwarding mechanism is how closely it compares to normal MTA store-and-forward Relaying. Its only interesting difference is that it changes the RFC2821.RcptTo value. Having the change be this small makes it easy to view aliasing as a part of the lower-level mail relaying activity. However the small change has a large semantic impact: The designated recipient has chosen a new recipient. Hence that original recipient SHOULD become responsible for any handling issues. This change would be reflected by replacing the message's RFC2821.MailFrom address to be one within the scope of the ADMD doing the aliasing.

An MDA that is re-posting a message to an alias typically changes only envelope information:

RFC2822.To/.CC/.BCC:

Set by - Author

These retain their original addresses.

RFC2821.RcptTo:

Set by - Mediator Author

This field contains an alias address.

RFC2821.MailFrom:

Set by - Author Source or Mediator Source

The Actor responsible for submission to an alias address will often retain the original address to receive handling Returns. The benefit of retaining the original MailFrom value is to ensure that the origination-side Actor knows 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

Set by - Mediator Dest

The Actor can record Received information, to indicate the delivery to the original address and submission to the alias address. The trace of Received header fields can therefore include everything from original posting through final delivery to a final delivery.

5.2.  Re-Sending

Also called Re-Directing, Re-Sending differs from Forwarding by virtue of having the Mediator "splice" a message's addressing information, to connect the Author of the original message and the Recipient of the new message. This permits them to have direct exchange, using their normal MUA Reply functions. Hence the new Recipient sees the message as being From: the original Author, even if the Mediator adds commentary.

Identities specified in a resent message include

RFC2822.From:

Set by - original Author

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 Actor responsible for the redirection.

RFC2822.Reply-To:

Set by - original Author

If this field is present in the original message, it is retained in the Resent message.

RFC2822.Sender:

Set by - Author Source or Mediator Source.

RFC2822.To/.CC/.BCC:

Set by - original Author

These specify the original message Recipients.

RFC2822.Resent-From:

Set by - Mediator Author

The address of the original Recipient who is redirecting the message. Otherwise the same rules apply for the Resent-From: field as for an original RFC2822.From field.

RFC2822.Resent-Sender:

Set by - Mediator Source

The address of the Actor responsible for re-submitting the message. As with RFC2822.Sender, this field is often omitted when it would merely contain the same address as RFC2822.Resent-From.

RFC2822.Resent-To/-CC/-BCC:

Set by: Mediator Author

The addresses of the new Recipients who will now be able to reply to the original author.

RFC2821.MailFrom:

Set by - Mediator Source

The Actor responsible for re-submission (RFC2822.Resent-Sender) is also responsible for specifying the new MailFrom address.

RFC2821.RcptTo:

Set by - Mediator Author

This will contain the address of a new Recipient.

RFC2822.Received:

Set by - Mediator Dest

When resending a message the submission agent can record a Received header field, to indicate the transition from original posting to resubmission.

5.3.  Mailing Lists

Mailing lists have explicit email addresses and they re-post messages to a list of subscribed members. The Mailing List Actor performs a task that can be viewed as an elaboration of the Re-Director role. In addition to sending the new message to a potentially large number of new Recipients, the Mediator can modify content, such as deleting attachments, converting the format, and adding list-specific comments. In addition, archiving list messages is common. Still the message retains characteristics of being "from" the original Author.

Identities relevant to a mailing list processor, when submitting a message, include:

RFC2919.List-Id:

Set by - Mediator Author

RFC2369.List-*:

Set by - Mediator Author

RFC2822.From:

Set by - original Author

Names and email addresses for the original author(s) of the message content are specified -- or, rather, retained.

RFC2822.Reply-To:

Set by - original Author or Mediator Author

RFC2822.Sender:

Set by - Author Source or Mediator Source

This will usually specify the address of the Actor 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/.CC

Set by - original Author

These usually contain the original list of Recipient addresses.

RFC2821.MailFrom

Set by - Author Source or Mediator Source

This can contain the original address to be notified of transmission issues, or the mailing list Actor can set it to contain a new Notification address. Typically the value is set to a new address, so that mailing list members and posters are not burdened with transmission-related Returns.

RFC2821.RcptTo

Set by - Mediator Author

This contains the address of a mailing list member.

RFC2821.Received

Set by - Mediator Dest

A Mailing List Actor can record a Received header field, to indicate the transition from original posting to mailing list forwarding. The Actor can choose to have the message retain the original set of Received header fields or can choose to remove them. In the latter case it can ensure that the original Received header fields are otherwise available, to ensure later accountability and diagnostic access to them.

5.4.  Gateways

A Gateway performs the basic routing and transfer work of message relaying, but it also may make any content, structure, address, or attribute modifications needed to send the message into a messaging environment that operates according to different standards or potentially incompatible policies. When a Gateway connects two differing messaging services, its role is easy to identify and understand. When it connects environments that have technical similarity, but can have significant administrative differences, it is easy to think that a Gateway is merely an MTA.

The critical distinction between an MTA and a Gateway is that the latter can make substantive changes to a message, in order to map between the standards of two, different messaging services. In virtually all cases, this mapping process results in some degree of semantic loss. The challenge of Gateway design is to minimize this loss.

A Gateway can set any identity field available to a regular MUA. Identities typically relevant to Gateways include:

RFC2822.From:

Set by - original Author

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 can translate addresses in whatever way will allow them continue to be useful in the target environment.

RFC2822.Reply-To:

Set by - original Author

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:

Set by - Author Source or Mediator Source

This can retain the original value or can be set to a new address.

RFC2822.To/.CC/.BCC

Set by - original Recipient

These usually retain their original addresses.

RFC2821.MailFrom

Set by - Author Source or Mediator Source

The Actor responsible for gatewaying the message can choose to specify a new address to receive handling notices.

RFC2822.Received

Set by - Mediator Dest

The Gateway can record a Received header field, to indicate the transition from the original posting environment to the new messaging environment.

5.5.  Boundary Filter

Organizations often enforce security boundaries by subjecting messages to analysis, for conformance with the organization's safety policies. An example is detection of content classed as spam or a virus. A Filter might alter the content, to render it safe, such as by removing content deemed unacceptable. Typically these actions will result in the addition of content that records the actions.

6.  Considerations

6.1.  Security Considerations

This document does not specify any new Internet Mail functionality. Consequently it is not intended to introduce any security considerations.

However its discussion of the roles and responsibilities for different mail service modules, and the information they create, highlights the considerable degree to which security issues are present when implementing any component of the Internet Mail service. In addition, email transfer protocols can operate over authenticated and/or encrypted links, and message content or authorship can be authenticated or encrypted.

6.2.  IANA Considerations

This document has no actions for IANA.

7.  References

7.1. Normative

7.2. Informative

Appendix A.  Acknowledgements

This work derives from a section in draft-hutzler-spamops [RFC5068] (Hutzler, C., Crocker, D., Resnick, P., Sanderson, R., and E. Allman, “Email Submission Operations: Access and Accountability Requirements,” Nov 2007.). Discussion of the Source actor role was greatly clarified during discussions in the IETF's Marid working group.

Graham Klyne, Pete Resnick and Steve Atkins provided thoughtful insight on the framework and details of the original drafts.

Later reviews and suggestions were provided by Eric Allman, Nathaniel Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ned Freed, Eric Hall, Tony Hansen, Willemien Hoogendoorn, Brad Knowles, John Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg, Alexey Melnikov, der Mouse, S. Moonesamy, Chris Newman, Daryl Odnert, Rahmat M. Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, Greg Vaudreuil.

Diligent proof-reading was performed by Bruce Lilly.

Index

Author's Address

Full Copyright Statement

Copyright © The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.