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MARID /SMTP D. Crocker Internet-Draft Brandenburg InternetWorking Expires: July 27, 2005 January1,26, 2005July 3, 2004Internet Mail Architecturedraft-crocker-email-arch-01draft-crocker-email-arch-02 Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft,I certifyeach author represents that any applicable patent or other IPR claims of whichI amhe or she is aware have been or will be disclosed, and any of whichIhe or she become aware will be disclosed, in accordance with RFC 3668. 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 onJanuary 1,July 27, 2005. Copyright Notice Copyright (C) The Internet Society(2004). All Rights Reserved.(2005). 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 Crocker Expires July 27, 2005 [Page 1] Internet-Draft EMail Architecture January 2005 stark relief. This draft offers clarifications and enhancements, to provide a more consistent base for community discussion of emailCrocker Expires January 1, 2005 [Page 1] Internet-Draft ID-Mail-Arch July 2004service problems and proposed email service enhancements. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Service Overview . . . . . . . . . . . . . . . . . . . . . . .34 1.2 Document Changes . . . . . . . . . . . . . . . . . . . . . . .45 1.3 Discussion venue . . . . . . . . . . . . . . . . . . . . . . .45 2. Email Actor Roles . . . . . . . . . . . . . . . . . . . . . . 5 2.1User . . . . . . .User-Level Actors . . . . . . . . . . . . . . . . . . . . . . 6 2.2Relay . . . . . . . .Transfer-Level Actors . . . . . . . . . . . . . . . . . . . . 8 2.3Provider . . . . . . .Administrative Actors . . . . . . . . . . . . . . . . . . . .811 3. Email Identities . . . . . . . . . . . . . . . . . . . . . . .811 3.1 Mailbox Addresses . . . . . . . . . . . . . . . . . . . . . .812 3.2 Domain Names . . . . . . . . . . . . . . . . . . . . . . . . .913 3.3 Message Identifers . . . . . . . . . . . . . . . . . . . . . .1013 3.4 IdentityReferenceReferencing Convention . . . . . . . . . . . . . . .. 1013 4.Email System ArchitectureProtocols and Services . . . . . . . . . . . . . . . . . . .10. 13 4.1ArchitecturalService Components . . . . . . . . . . . . . . . . . . .11. . . 15 4.2 Operational Configuration . . . . . . . . . . . . . . . . . .1921 4.3 Layers of Identity References . . . . . . . . . . . . . . . .2021 5. Message Data . . . . . . . . . . . . . . . . . . . . . . . . .2022 5.1 Envelope . . . . . . . . . . . . . . . . . . . . . . . . . . .2022 5.2 MessageHeaders . . .Header Fields . . . . . . . . . . . . . . . . . . . .2122 5.3 Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2123 6. Two Levels of Store-And-Forward . . . . . . . . . . . . . . .2123 6.1 MTA Relaying . . . . . . . . . . . . . . . . . . . . . . . . .2123 6.2 MUA Forwarding . . . . . . . . . . . . . . . . . . . . . . . .2223 7. Security Considerations . . . . . . . . . . . . . . . . . . .3230 8. References . . . . . . . . . . . . . . . . . . . . . . . . . .3231 Author's Address . . . . . . . . . . . . . . . . . . . . . . .3433 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .3433 Intellectual Property and Copyright Statements . . . . . . . .3534 Crocker ExpiresJanuary 1,July 27, 2005 [Page 2] Internet-DraftID-Mail-Arch July 2004EMail Architecture January 2005 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.TheHowever the basic style and use of names, addresses and message structure have remained remarkably constant.However each has benefitedThere are two, basic categories of participants in Internet Mail. Users are customers of the Mail Handling Service (MHS). They represent the sources and sinks of that service. The Mail Handling Service is responsible for accepting a message fromsignificant elaborations.one user and delivering to one or more others. +--------+ +---------------->| User | | +--------+ | . +--------+ | +--------+ . | User +--+--------->| User | . +--------+ | +--------+ . | . . . | +--------+ . . . +-->| User | . . . +--------+ . . . . . . . . . . . . . . +--------------------------------------+ | | | Mail Handling Service (MHS) | | | +--------------------------------------+ Figure 1: Basic Email Service Model Public discussion and agreement aboutthe natureterms ofthese changes hasreference have not keptpace,pace with the changes, and abuses of the Internet mail service have broughtthese issuesthis into stark relief.The current draft seeks to: 1. Document changesSo, it is necessary to produce a revised architecture. However it is important thathave taken placethe original distinction between user-level concerns and transfer-level concerns be retained. This becomes challenging when the user-level exchange is, itself, a sequence, such as with group dialogue or organizational message flow, as occurs with a purchase approval process. It is easy to confuse this user-level activity with the underlying mail transmission service exchanges. Crocker Expires July 27, 2005 [Page 3] Internet-Draft EMail Architecture January 2005 For Internet mail, the term "end-to-end" usually refers to single posting and the set of deliveries resulting from a single transiting of the MHS. However, note that specialized uses of email consider the entire email service -- including Originator 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 and EDI over email. 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.Crocker Expires January 1, 2005 [Page 3] Internet-Draft ID-Mail-Arch July 2004A precept to the design of Internet mail is to permit user-to-user and MTA-to-MTA interoperability with no prior, direct administrativearrangement between the participants.arrangement. 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. Crocker Expires July 27, 2005 [Page 4] Internet-Draft EMail Architecture January 2005 For localized environments (edge networks) prior, administrative arrangement can include access control, routing constraints 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 theseOverall: Clarify roleshas also been added to the tables showing architectural function. The distinction of Actors, versusand responsibilities Diagrams: Revised diagrams and tightened things up Distinct architecturalsystem 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'sections': Added concept ofthe Message Store has been added. This change is intended to reflect the consensus viewADMDs, as operational layer, separate fromonline discussion, rather than being the editor's view, which has in any event changed... However it is likely that it will need significant revisionfunctional orreplacement. Please review it carefully! Message Identifiers: Discussion of message identifiers has been added to the section on Email Identities.architectural layer. Added user "layer", as distinct from transfer. Introduced 'mediator'. 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>Crocker Expires January 1, 2005 [Page 4] Internet-Draft ID-Mail-Arch July 2004mailing list. The IETF-SMTP mailing list <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 eachperforms inperforms. The best way to maintain theoveralldistinction between user activity and handlingof mail. For this level of discussion "the service" has the task of performingactivities is to depict their details in separate diagrams. Current Internet mail provides only asingle, end-to-end transfer. Protracted, iterative exchanges, such as those usedsmall set of capabilities forcollaboration over time, are beyond the scopesupporting different kinds 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.ongoing, user-level Crocker ExpiresJanuary 1,July 27, 2005 [Page 5] Internet-DraftID-Mail-Arch July 2004 The following depicts the relationships among participants in Internet Mail.EMail Architecture January 2005 exchanges. Although related to a technical architecture,itsthe focus of a discussions on Actors is on participant responsibilities, rather than functional modules. Hence the labels used are different than for classic emailarchitecture. This figure depictsarchitecture diagrams. The figures depict the relationships among theactors. It shows the Submitter as distinct from the Originator, although it is common for them toActors. Actors often will bethe same actor. The figure also shows multiple Relaysassociated with entirely independent organizations from other Actors who are participating in thesequence. It is legal toemail service. 2.1 User-Level Actors Users are the sources and sinks of messages. They may haveonly one,an exchange that iterates andfor intra-organizationthey may expand or contract the set of users participating in a set of exchanges. In Internet Mail there are three, basic types of user-level Actors: Originators, Recipients, and Mediators. Fromhe t User-level perspective all mailservices, this is common. User (Originator, Author) | -+ Submitter | Provider | -+ | -+ Relaytransfer activities are performed by a monolithic, shared handling service. Users are customers of this service. The following depicts the relationships among them. +------------+ | Originator |<--------------+ +-+---+----+-+ | | |Provider|Relay| | |-+V | |-+| +-----------+ |-+| |User (Forwarder)| Recipient | |[ Intermediate ]||Provider|[ Recipient ]+-----------+ | | |[ Originator ]|Provider| |[ Submitter ]+----------+ | | | |-+| |Relay| V V | | |-++-----------+ +---+---+---+ | |-+ RelayMediator +--->| Recipient | | +-----------+ +-----------+ |Provider User (Recipient)V +-----------+ +-----------+ +-----------+ | Mediator +--->| Mediator +--->| Recipient |-+ 2.1 User Users are customers of the email relaying service. They represent the sources and sinks of that service. Three types+-----------+ +-----------+ +-----------+ The functions ofusers are distinguished:these Actors are: 2.1.1 Originator Also called "Author", this isathe user-level participant responsible Crocker Expires July 27, 2005 [Page 6] Internet-Draft EMail Architecture January 2005 for creating original content and requesting its transmission. Theemail serviceMail Handling Service operates to send and deliver mail among Originators andCrocker Expires January 1, 2005 [Page 6] Internet-Draft ID-Mail-Arch July 2004Recipients. 2.1.2SubmitterRecipient TheSubmitterRecipient isresponsible for ensuring thatamessage is valid for postingconsumer of delivered content. A recipient may close the user-level communication loop by creating andthensubmittingit into the mail transfer service. It primarily servesa new message that replies to an originator. An automated, or semi-automated form of reply informs the Originatorand often it is the same entity. The Submitter hasabout theresponsibility 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 thinkRecipient's disposition of theSubmittermessage. 2.1.3 Mediator A Mediator receives, aggregates, reformulates and distributes messages asmore like the editor or publisherpart of aperiodical, rather than simply the administrative assistant for the Originator. Hence, the Submitterpotentially-protracted, higher-level exchange among users. A Mediator isbest held accountable forviewed by themessage content, evenMail Handling Service, whenthey did not create any or most of it. 2.1.3 Recipient The Recipient is a consumer of delivered content. The recipientthe Mediator's address is specifiedas an addressee,in the envelope.2.1.4 Forwarder Email often transits intermediate, user-level points, called Forwarders. The task of a ForwarderWhen submitting messages, the Mediator isto 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 performsan Originator. What is distinctive is that anatural sequenceMediator preserves Originator information ofemail steps: o Servicethemailbox specified inmessage(s) it reformulates, but makes meaningful changes to theenvelope and accept arriving messages. o Reformulatecontent. Hence the Mail Handling Service sees a new message, but Users receive a messagecontent and addressing, according tothat is interpreted as primarily being from thepoliciesauthor of theadministratororiginal message. The role of a Mediator permits distinct, active creativity, rather than being limited theForwarder. Request (further) message transmission. Notemore passive job of merely connecting together other participants. Hence it is really the Mediator thatan Intermediate Originator operates with dual allegiance, notably its operating authority,is responsible for the new message. A Mediator's task may be complex, contingent and creative, such astheby modifying and adding content or regulating which users may participate and when. The popular example of this role is a group mailinglist administrator,list. A sequence of mediators may even perform a series of formal steps, such aswellreviewing, modifying and approving a purchase request. Because a Mediator originates messages, it might also receive replies. That is, a Mediator is a full-fledged User. Specialized Mediators include: Forwarder: A new message encapsulates the original message and is seen as strictly "from" the"original" originator. o PerformMediator. However the Mediator might add commentary and certainly has the opportunity to modify theusual Submitter tasks.original message content. Crocker ExpiresJanuary 1,July 27, 2005 [Page 7] Internet-DraftID-Mail-Arch July 2004 2.2 Relay A mail relay performs email transfer-service routing and store-and-forward. It (re-)transmitsEMail Architecture January 2005 Redirector: Redirection differs from Forwarding by virtue of having themessage on towards it recipient(s). A basic transfer operation isMediator "splice" communication betweena clientthe Originator of the original message anda server Relay. A setthe Recipient ofRelays composes a mail handling service network.the new message. Hence the new Recipient sees the message as being From the original Originator. Mailing List: Thisis above any underlying packet-switching networkActor performs a task thattheycan be viewed as an elaboration of the Redirector role. In addition to sending the new message to a potentially large number of new Recipients, content might be modified, such as deletion of attachments, formatting conversion, and addition of list-specific comments. In additional, archival of list messages is common. Annotator: The integrity of the original message is preserved, but one or more comments about the message are added in a manner that distinguishes commentary from original text. Adaptor: {per Ned Freed} Security Filter: Organizations often enforce security boundaries by having message subjected to analysis for conformance with the organization's safety policies. Examples are detection of content classed as spam or a virus. A Security 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. 2.2 Transfer-Level Actors The Mail Handling Service has the task of performing a single, end-to-end transfer on behalf of the originator and reaching the recipient address(es) specified in the envelope. 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 service. Crocker Expires July 27, 2005 [Page 8] Internet-Draft EMail Architecture January 2005 The following depicts the relationships among transfer participants in Internet Mail. It shows Source 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. +------------+ +-----------+ | Originator | | Recipient | +-----+------+ +-----------+ | ^ | Mail Handling Service | +===================================================+ || | | || || | | || V | +---------+ +--------+ +----+----+ | | | |<------------+ | | Source +...>| Notice | | Dest | | | | |<---+ | | +----+----+ +--------+ | +---------+ | | ^ V | | +---------+ +----+----+ +----+----+ | Relay +-->.......-->| Relay +-->| Relay | +---------+ +----+----+ +---------+ | V +---------+ | Gateway +-->... +---------+ 2.2.1 Source The Source role is responsible for ensuring that a message is valid for posting and then submitting it to a mail relay. Validity includes conformance with Internet mail standards, as well as local operational policies. Source may simply review the message for conformance, and reject it if there are errors, or it may create some or all of the necessary information. Source operates with dual allegiance. It serves the Originator and often it is the same entity. However its role in assuring validity means that it must represent the local operator of the Mail Handling Service. Source also has the responsibility for any post-submission, originator-related administrative tasks associated with message Crocker Expires July 27, 2005 [Page 9] Internet-Draft EMail Architecture January 2005 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 Notices Handler Transfer efforts might result in the generation of service reporting information about failures or completions. These Transfer or Delivery notification messages are sent to an address that is specified by the Source. A Notices handling address (also known as Bounce or Return address) might have no characteristics in common the with address of the Originator or Source. 2.2.3 Relay A mail relay performs email transfer-service routing and store-and-forward. It adds envelope-related handling information and then (re-)transmits the message on towards its recipient(s). A Relay does not modify the message contents. 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. Aborting message transfer results in having the Relay become an Originator and send an error message to the Notifications (Bounce) address. (The potential for looping is avoided by having this message, itself, contain no Bounce address. 2.2.4 Gateway A Gateway is a special form of Relay that interconnects heterogeneous mail services. Differences between the services can be as small as minor syntax variations, but usually encompass much more basic, semantic distinctions. For example, the concept of an email address might beusing. 2.3as different as a hierarchical, machine-specific address versus a flat, global name space. Or between text-only and multi-media. Hence, the Relay function of a gateway is the minor component. The significant challenge is in 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 originator can send a message to a recipient, without requiring any changes to the components in the originator's mail service or the recipient's mail service, other than adding the gateway. To each of these otherwise independent services, the gateway will appear to be a Crocker Expires July 27, 2005 [Page 10] Internet-Draft EMail Architecture January 2005 "native" participant. However the ultimate test of a gateway's adequacy is whether the originator and recipient can sustain a dialogue. In particular, can a recipient formulate a Reply? 2.3 Administrative Actors Operation of Internet mail services is apportioned to different providers (or operators) each is composed of an independent Administrative Domain. 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 Administrative Domain potentially having a complex arrangement of functional components. The interactions between functional components within an Administrative Domain are subject to the policies of that domain. Policies can cover such things as reliability, access control, accountability and content evaluation and may be implemented in different functional components, according to the needs of the Administrative Domain. 2.3.1 Provider Providers operate component services or sets of services.As shown in the Figure, itIt 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 Mailservice.Service. 3. Email Identities Internet mail usestwothree forms of identity. The most common is the mailbox address <addr-spec> [RFC2822]. The otherform istwo forms are the Crocker Expires July 27, 2005 [Page 11] Internet-Draft EMail Architecture January 2005 <domain name>[RFC1034].[RFC1034] and message identifier [RFC2822]. 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 otherCrocker Expires January 1, 2005 [Page 8] Internet-Draft ID-Mail-Arch July 2004entities 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 structuring conventions forsub-structure tothe left-hand side (local-part) 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 Crocker Expires July 27, 2005 [Page 12] Internet-Draft EMail Architecture January 2005 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, orCrocker Expires January 1, 2005 [Page 9] Internet-Draft ID-Mail-Arch July 2004performing 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 IdentityReferenceReferencing 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 ArchitectureProtocols and Services 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 Crocker Expires July 27, 2005 [Page 13] Internet-Draft EMail Architecture January 2005 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 functional components, arranged to support a store-and-forward service architecture:Crocker Expires January 1, 2005 [Page 10] Internet-Draft ID-Mail-Arch July 2004 +-----------<-oMUA <-------------------------------++------+ .............+ oMUA |<------------------------------+ . +--+---+ | . |< smtp,{ smtp,submission |(envelope). Vsubmission|RFC2822 MSA <-------------------------+. +------+ |MIME. |< smtpMSA |<--------------------+ | . +--+---+ | |MTA dsn. | { smtp | |< smtp. V | | . +------+ +====+====+ | . | MTA->----------------------->+| || dsn || | +============+ +--+---+ +=========+ |<|| MESSAGE || . { smtp ^ ^ |||| || . | |. || ||(envelope)|| . | | | || || V | | | || RFC2822 || +------+ | | +===+===+ || || | MTA +-------------------+ | || mdn || || MIME || +--+---+ | +=======+ +============+ |<{ local, smtp, lmtp | | . V | | . +------+ | | . | +-----------------------+ | . | MDA<-------------------------+| | . | |<--------------------+ | . +-+--+-+ | | . local } | | | | . V | | | . +------+ | +====+====+ | . | MS-1 |< local,| || sieve || | . +-+--+-+ | +=========+ | . | | |<{ pop,sieveimap ^ | . | V V | | . |< imap,+------+ | | . | | MS-2< smtp, or|mdn| | . |< web+--+---+ | |V. | | { pop, imap, local | | . V V | | . +------+ | |+--------->.........>| rMUA->--------------------------+-----++------------------------+---------+ Crocker Expires July 27, 2005 [Page 14] Internet-Draft EMail Architecture January 2005 +------+ 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 thearchitectureservice are becoming more extensive. So, their separation is increasingly common. 4.1ArchitecturalService Components 4.1.1 Mail User Agent (MUA)An <MUA>A Mail User Agent (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>oMUA is used to create a message and perform initial "submission" into the transfer infrastructure, viaan <MSA>.a Mail Submission Agent (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>rMUA works on behalf of the end-user recipient 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 initiatingCrocker Expires January 1, 2005 [Page 11] Internet-Draft ID-Mail-Arch July 2004replies 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 MUAfunctions performperforms messageforwarding,re-posting, as discussed in the[2]<Mediator> section. Identity fields set by the MUA include:+----------------------+----------------------+---------------------+ | Identity | Actor | Description | +----------------------+----------------------+---------------------+ |RFC2822.From|Actor: Originator|Names and addresses| | | |for author(s) of| | | |the message content| | | |are listed in the| | | |Fromheader | |header field Crocker Expires July 27, 2005 [Page 15] Internet-Draft EMail Architecture January 2005 RFC2822.Reply-To|Actor: Originator|If a message| | | |recipient sends a| | | |reply message that would| | | |otherwise usethe | | | |RFC2822.From field| | | | informationaddress(es) contained in the| | | |original message,| | | |then they are instead to use the| | | | contents ofaddress(es) in the| | | |RFC2822.Reply-To| | | | field instead.field. In| | | |other words, this| | | |field is a direct| | | |override of the| | | |From field, for| | | |responses from| | | |recipients.| |RFC2822.Sender| Submitter |Actor: Source This specifies the| | | |address responsible| | | |for submission into| | | |the transfer| | | |service. For| | | |efficiency, this| | | |field should be| | | |omitted if it| | | |contains the same| Crocker Expires January 1, 2005 [Page 12] Internet-Draft ID-Mail-Arch July 2004 | | |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 error return| | | |addresses(the | | | | "bounces"-- the "notifications" (or "bounces") address,| | | |contained in| | | | RFC2821.MailFrom) | | | |RFC2821.MailFrom -- is made by the| | | | Sender.RFC2822.Sender. Typically| | | |thebouncenotifications address| | | |is the same as the| | | |Sender address.| | | |However some usage| | | |scenarios require| | | |it to be different.| |RFC2822.To,|RFC2822.CC Actor: Recipient|These specify MUA| | RFC2822.CC | | recipient | | | |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|Actor: Recipient|A message might be| | | |copied to an| | | |addresseewhowhose participation is| | | |not to be disclosed| | | |to theRFC2822.TO | | | |RFC2822.To or RFC2822.CC| | | |recipients. The BCC| Crocker Expires January 1, 2005 [Page 13] Internet-Draft ID-Mail-Arch July 2004 | | |header field 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| | | |usually 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| | | |Crocker Expires July 27, 2005 [Page 16] Internet-Draft EMail Architecture January 2005 addresses,| | | |depending upon the| | | |preferences or the| | | |Originator.| +----------------------+----------------------+---------------------+ Table 1: Message Identities4.1.2 Mail Submission Agent (MSA)An <MSA>A Mail Submission Agent (MSA) accepts the message submission from theoMUA and conditions it for insertion into the global email transfer network, according tooMUA and enforces the policies of the hosting network and the requirements of Internet standards.ItEnforcement might be passive, involving review and approval or rejection, or it might be active, involving direct modification of the message. An MSA 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 administrativeCrocker Expires January 1, 2005 [Page 14] Internet-Draft ID-Mail-Arch July 2004assistants, 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 |RFC2821.EHLO Actor: Source The MSA may specify| | RFC2821.EHLO | |its hosting domain| | | |identity for the| | | |SMTP HELO or EHLO| | | |command operation.| |RFC2821.MailFrom| Submitter |Actor: 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 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| Crocker Expires January 1, 2005 [Page 15] Internet-Draft ID-Mail-Arch July 2004 | | |RFC2821.MailFrom is| | | |to determine what| | | |address needs to be| | | |informed about| | | |transmission-level| | | |problems (and,| | | |possibly,| | | | successes.) | |successes. Crocker Expires July 27, 2005 [Page 17] Internet-Draft EMail Architecture January 2005 RFC2821.Rcpt-To|Actor: Recipient|This specifies the| | | |MUA inbox address| | | |of a recipient. The| | | |string might not be| | | |visible in the| | | |message content| | | | headers.header. For| | | |example, the| | | |message destination| | | |addressheaders, | | | |header fields, 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 |Actor: Source An MSA may record a| | | |Receivedheader,header field, to| | | |indicate initial| | | |submission trace| | | |information,| | | |including| | | |originating host| | | |and MSA host domain| | | |names and/or IP| | | |Addresses.| +----------------------+----------------------+---------------------+ Table 2: MSA Identities4.1.3 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.Crocker Expires January 1, 2005 [Page 16] Internet-Draft ID-Mail-Arch July 2004The 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 Crocker Expires July 27, 2005 [Page 18] Internet-Draft EMail Architecture January 2005 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|Actor: Relay|The MTA may specify| | | |its hosting domain| | | |identity for the| | | |SMTP HELO or EHLO| | | |command operation.| |RFC2821.Return-Path| Originator |Actor: Source The MDA records the| | | |RFC2821.MailFrom| | | |address into an| | | |RFC2822 header| | | |field named Return-Path.| |RFC2822.Received|Actor: Relay|An MTA must record| | | |a Receivedheader, | | | |header field, to indicate trace| | | |information,| | | |including source| Crocker Expires January 1, 2005 [Page 17] Internet-Draft ID-Mail-Arch July 2004 | | |host and receiving| | | |host domain names| | | |and/or IP| | | |Addresses.| +----------------------+----------------------+---------------------+ Table 3: MTA Identities4.1.4 Mail Delivery Agent (MDA) The <MDA> delivers email to the recipient's inbox.An MDAA Mail Delivery Agent (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 presentearlierelsewhere inan MTA relaying sequence that ends withtheMDA,recipient's Administrative Domain, such as at an organizational border 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],[RFC1939] or IMAP [RFC3501]. When coupled with an internal, local mechanism, SMTP permits "push" delivery to the recipient system, at theimitativeinitative of the upstream email service. POP is used for "pull" delivery at the initiative of the recipient system. Notably, SMTP and POP effect a Crocker Expires July 27, 2005 [Page 19] Internet-Draft EMail Architecture January 2005 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|RFC2821.EHLO Actor: Relay|The MDA may specify| | RFC2821.EHLO | |its hosting domain| | | |identity for the| | | |SMTP HELO or EHLO| | | |command operation.| |RFC2822.Received| h |Actors: Source, Relay, Dest An MTA must record| | | |a Receivedheader, | | | |header field, to indicate trace| | | |information,| | | |including source| | | |host and receiving| | | |host domain names| Crocker Expires January 1, 2005 [Page 18] Internet-Draft ID-Mail-Arch July 2004 | | |and/or IP| | | |Addresses.| +----------------------+----------------------+---------------------+ Table 4: MDA Identities4.1.5 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. Crocker Expires July 27, 2005 [Page 20] Internet-Draft EMail Architecture January 2005 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 independentCrocker Expires January 1, 2005 [Page 19] Internet-Draft ID-Mail-Arch July 2004administrations. 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 combineinto +-----------------+--------------+-----------------------------+into: Crocker Expires July 27, 2005 [Page 21] Internet-Draft EMail Architecture January 2005 +-----------------------+-------------+---------------------+ | Layer | Field | Set By |+-----------------+--------------+-----------------------------++-----------------------+-------------+---------------------+ | Message Content | MIMEHeadersHeader | Originator | | MessageHeadersheader fields | From | Originator | | | Sender |SubmitterSource | | | Reply-To | Originator | | | To, CC, BCC | Originator | | | Received |Submitter,Source, Relay,RecipientDest | | | Return-Path |MDAMDA, from MailFrom | | SMTP | HELO | Latest Relay Client | | | MailFrom |SubmitterSource | | | RCPT-TO |SubmitterOriginator | | 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 transferCrocker Expires January 1, 2005 [Page 20] Internet-Draft ID-Mail-Arch July 2004directives, are carried in-band by MTAs. All other envelope information, such as trace records, is carried within the contentheaders.header fields. Upon delivery, SMTP-level envelope information is typically encoded within additional contentheaders,header fields, such as Return-Path and Received (From and For). 5.2 MessageHeaders HeadersHeader Fields Header fields are attribute/value pairs covering an extensible range of email service, user content and user transaction meta-information. The core set ofheadersheader fields is defined in [RFC2822], [RFC0822]. It is common to extend this set, for different applications. A complete set of registeredheadersheader fields is being developed through [ID-hdr-reg]. One danger with placing additional information inheadersheader fields is that gateways often alter or delete them. Crocker Expires July 27, 2005 [Page 22] Internet-Draft EMail Architecture January 2005 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 MIMEHeaderheader field 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 messageCrocker Expires January 1, 2005 [Page 21] Internet-Draft ID-Mail-Arch July 2004payload 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><Mediator> 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 Crocker Expires July 27, 2005 [Page 23] Internet-Draft EMail Architecture January 2005 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.Crocker Expires January 1, 2005 [Page 22] Internet-Draft ID-Mail-Arch July 2004Identities specified in a resent message include+----------------------+----------------------+---------------------+ | Identity | Actor | Description | +----------------------+----------------------+---------------------+ |RFC2822.From|Actor: 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|Actor: Originator|If this field is| | | |present in the| | | |original message,| | | |it should be| | | |retained in the| | | |Re-sent message.| |RFC2822.Sender| Submitter |Actor: Source This field is| | | |expected to contain| | | |the original Sender| | | |value.| |RFC2822.TO,|RFC2822.CC, RFC2822.BCC Actor: Recipient|Crocker Expires July 27, 2005 [Page 24] Internet-Draft EMail Architecture January 2005 These specify the| | RFC2822.CC, | |original message| | RFC2822.BCC | |recipients.| |RFC2822.Resent-From| Intermediate |Actor: Mediating Originator 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 |RFC2822.Resent-Sender Actor: Mediating Source The address of the| | r | Submitter |agent responsible| | | |for re-submitting| | | |the message. For| | | |efficiency, this| | | |field should be| Crocker Expires January 1, 2005 [Page 23] Internet-Draft ID-Mail-Arch July 2004 | | |omitted if it| | | |contains the same| | | |address as| | | | RFC2822.Resent-From | | | | .RFC2822.Resent-From. However this does| | | |not mean there is| | | |no Resend-Sender| | | |specified. Rather,| | | |it means that that| | | |header field 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,|RFC2822.Resent-cc, RFC2822.Resent-bcc: Actor: 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 |Actor: Mediating Source The agent| | | Submitter |responsible for| | | | re-submission | | | | (RFC2822.Resent-Sen | | | | der)re-submission (RFC2822.Resent-Sender) is also| | | |responsible for| | | |specifying the new| | | |RFC2821.MailFrom| | | |address.| |RFC2821.Rcpt-to|Actor: Recipient|This will contain| | | |the address of a| | | |new recipient|RFC2822.Received Crocker ExpiresJanuary 1,July 27, 2005 [Page24]25] Internet-DraftID-Mail-Arch July 2004 | RFC2822.Received | Intermediate |EMail Architecture January 2005 Actor: Mediating Source When re-sending a| | | Submitter |message, the| | | |submission agent| | | |may record a| | | |Receivedheader,header field, to| | | |indicate the| | | |transition from| | | |original posting to| | | |resubmission.| +----------------------+----------------------+---------------------+ Table 6: ReSent Identities6.2.3 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.Crocker Expires January 1, 2005 [Page 25] Internet-Draft ID-Mail-Arch July 2004A gateway may set any identity field available to a regular MUA. Identities typically set by gatewaysinclude +----------------------+----------------------+---------------------+ | Identity | Actor | Description | +----------------------+----------------------+---------------------+ |include: RFC2822.From|Actor: 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|Actor: 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.| |Crocker Expires July 27, 2005 [Page 26] Internet-Draft EMail Architecture January 2005 RFC2822.Sender| Submitter |Actor: Source This may retain the| | | |original value or| | | |may be set to a new| | | |address| |RFC2822.TO,|RFC2822.CC, RFC2822.BCC Actor: Recipient|These usually| | RFC2822.CC, | | retain their | | RFC2822.BCC | |retain their original addresses.| |RFC2821.MailFrom| Submitter |Actor: Source The agent| | | |responsible for| | | |gatewaying the| | | |message may choose| | | |to specify a new| | | |address to receive| Crocker Expires January 1, 2005 [Page 26] Internet-Draft ID-Mail-Arch July 2004 | | |handling notices.| | RFC2822.Received | Forwarder |RFC2822.Receive Actors - Source, Relay, Dest The gateway may| | | |record a Received| | | | header,header field, to indicate| | | |the transition from| | | |original posting to| | | |the new messaging| | | |environment.| +----------------------+----------------------+---------------------+ Table 7: Gateway Identities6.2.5 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 deli