RFC 5598
Internet Mail Architecture
Internet Mail Architecture 💌✨
okay bestie, let's start with the basics 🎀
- RFC number: 5598
- Series: Network Working Group
- Author: D. Crocker, from Brandenburg InternetWorking
- Category: Informational
- Date: July 2009
- Obsoletes / Updates: the title block doesn't list anything it obsoletes or updates
Abstract 🌸
okay so the vibe is that Internet Mail has been around for thirty-five years 😭 and in that time it went from a small thing to a global infrastructure service 🌍 that everybody depends on. The changes were evolutionary, not revolutionary, because everyone really wanted to keep the existing installed base working and keep it useful 🫶
here's the tea though: to work on this huge, complicated system together, everyone needs the same mental picture AND the same vocabulary. right now the different perspectives make it super hard to know what another person actually means when they say something 💀 so this doc is the shared reference point 💅 it describes the enhanced Internet Mail architecture as it works right now 🦋
Status of This Memo 🎀
this memo gives information to the Internet community 💜 it is not an Internet standard of any kind, so don't go treating it like one 🙈 and distribution is unlimited, so share it with everyone ✨
Copyright Notice 💖
Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. 🔒
this document is covered by BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents, as they stood on the date it was published. you can find those at http://trustee.ietf.org/license-info. please actually read them, because they spell out your rights and restrictions for this doc 🧐
also, this doc might have material from IETF Documents or IETF Contributions that were published or made publicly available before November 10, 2008. the people who control the copyright on some of that stuff might not have given the IETF Trust permission to allow changes outside the IETF Standards Process 😬 so unless you get a proper license from those copyright holders, this document can't be modified outside the IETF Standards Process, and you can't make derivative works of it outside that process either. the exceptions are formatting it for publication as an RFC, or translating it into languages other than English 🌈
Table of Contents 🧚
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. The Role of This Architecture . . . . . . . . . . . . . . 6
1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 7
2. Responsible Actor Roles . . . . . . . . . . . . . . . . . . . 7
2.1. User Actors . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Message Handling Service (MHS) Actors . . . . . . . . . . 11
2.3. Administrative Actors . . . . . . . . . . . . . . . . . . 14
3. Identities . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1. Mailbox . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2. Scope of Email Address Use . . . . . . . . . . . . . . . . 18
3.3. Domain Names . . . . . . . . . . . . . . . . . . . . . . . 19
3.4. Message Identifier . . . . . . . . . . . . . . . . . . . . 19
4. Services and Standards . . . . . . . . . . . . . . . . . . . . 21
4.1. Message Data . . . . . . . . . . . . . . . . . . . . . . . 24
4.1.4. Identity References in a Message . . . . . . . . . . . 25
4.2. User-Level Services . . . . . . . . . . . . . . . . . . . 29
4.3. MHS-Level Services . . . . . . . . . . . . . . . . . . . . 31
4.4. Transition Modes . . . . . . . . . . . . . . . . . . . . . 34
4.5. Implementation and Operation . . . . . . . . . . . . . . . 35
5. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.2. ReSender . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.3. Mailing Lists . . . . . . . . . . . . . . . . . . . . . . 39
5.4. Gateways . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.5. Boundary Filter . . . . . . . . . . . . . . . . . . . . . 42
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 42
6.1. Security Considerations . . . . . . . . . . . . . . . . . 42
6.2. Internationalization . . . . . . . . . . . . . . . . . . . 43
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7.1. Normative References . . . . . . . . . . . . . . . . . . . 45
7.2. Informative References . . . . . . . . . . . . . . . . . . 47
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 50
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511. Introduction ✨
okay so same energy as the abstract, but now with the receipts 🧾 Internet Mail has a thirty-five-year history, and it's changed a LOT in scale and complexity as it turned into a global infrastructure service 🌏 the changes were evolutionary, not revolutionary, because everyone was protecting the installed base and keeping it useful 💜 today it's run by lots of independent operators, with lots of different components serving Users, plus lots of different components that move messages around 🚚
the underlying technical standards are a whole rich buffet of features 🍓 the core specs are:
- Simple Mail Transfer Protocol (SMTP) ([RFC0821], [RFC2821], [RFC5321]) is what moves a message through the Internet 📨
- Internet Mail Format (IMF) ([RFC0733], [RFC0822], [RFC2822], [RFC5322]) defines what a message object actually is 📦
- Multipurpose Internet Mail Extensions (MIME) [RFC2045] is the upgrade to the message object that lets you add multimedia attachments 📎🖼️
public collaboration on the technical, operations, and policy side of email has been a big deal 🙌 this includes all the work that responds to email abuse. it's brought a MUCH wider range of people into the technical community, which is iconic and also a little chaotic 💀 to collaborate on this huge system, everyone needs the same shared view and the same language for its parts and how they interact. but because perspectives differ so much, it's hard to tell what someone else means 😭
that's why this document exists 🦋 it describes the realities of the current system. Internet Mail is always getting new technical, operations, and policy work, and the discussions keep getting stuck because people use different models of email service design and different meanings for the same terms 🙃
so, to be the shared frame of reference, this document describes the enhanced Internet Mail architecture as it currently works. it focuses on:
- Capturing refinements to the email model 🪄
- Clarifying functional roles for the architectural components 🎭
- Clarifying identity-related issues across the email service 🪪
- Defining terminology for architectural components and their interactions 📖
⚠️ bestie reminder: some of those citations point at older RFCs, so keep the numbers handy when you go looking. ([RFC0733] and friends are all in the references 🫶)
1.1. History ✨
Okay so the very first standardized architecture for networked email split things into two worlds 💅: the user world, made of Message User Agents (MUAs), and the transfer world, the Message Handling Service (MHS), which is made up of Message Transfer Agents (MTAs) [RFC1506]. The MHS takes a message from one User and delivers it to one or more other Users, which basically creates a virtual MUA-to-MUA exchange environment 🦋
Figure 1 shows that this architecture has two logical layers of interoperability 🌸. One is directly between Users. The other is among the components along the transfer path. On top of that, there's interoperability between the layers too, first when a message gets posted from the User to the MHS, and later when it's delivered from the MHS back to the User.
The operational service has glowed up a lot 💖, but core stuff like mailbox addressing and message format style is still remarkably constant (iconic 🙈). The original split between user level and transfer level is still there, just with elaborations in each. "Internet Mail" is the term for the whole collection of user and transfer components and services.
For Internet Mail, "end-to-end" usually refers to a single posting and the set of deliveries that result from a single transit of the MHS. The big exception is group dialogue mediated through a Mailing List 📬. There, two postings happen before the intended Recipients receive an Author's message, see Section 2.1.4 for more on that. Also, some uses of email treat the entire email service, including Author and Recipient, as a subordinate component 👀. For those, "end-to-end" refers to points outside the email service. Examples are voicemail over email [RFC3801], EDI (Electronic Data Interchange) over email [RFC1767], and facsimile over email [RFC4142] 📠
+--------+
++================>| User |
|| +--------+
|| ^
+--------+ || +--------+ .
| User +==++=========>| User | .
+---+----+ || +--------+ .
. || ^ .
. || +--------+ . .
. ++==>| User | . .
. +--------+ . .
. ^ . .
. . . .
V . . .
+---+-----------------+------+------+---+
| . . . . |
| .................>. . . |
| . . . |
| ........................>. . |
| . . |
| ...............................>. |
| |
| Message Handling Service (MHS) |
+---------------------------------------+
Legend: === lines indicate primary (possibly indirect)
transfers or roles
... lines indicate supporting transfers or roles
Figure 1: Basic Internet Mail Service ModelEnd-to-end Internet Mail exchange is accomplished by using a standardized infrastructure with these components and characteristics:
- An email object
- Global addressing
- An asynchronous sequence of point-to-point transfer mechanisms
- No requirement for prior arrangement between MTAs or between Authors and Recipients
- No requirement for prior arrangement between point-to-point transfer services over the open Internet
- No requirement for Author, Originator, or Recipients 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 separates control information, used for handling, from the Author's content 📦
A precept for designing mail over the open Internet is permitting User-to-User and MTA-to-MTA interoperability without prior, direct arrangement between the independent administrative authorities responsible for handling a message 🤝. All participants rely on the core services being universally supported and accessible, either directly or through Gateways that act as translators between Internet Mail and email environments conforming to other standards. Given how important spontaneity and serendipity are in interpersonal communication, not requiring that prearrangement is a core benefit of Internet Mail, and it remains a core requirement for it 💜
Within localized networks at the edge of the public Internet, prior administrative arrangement often is required. That can include access control, routing constraints, and configuration of the information query service 🔐. Recipient authentication has usually been required for message access since the beginning of Internet Mail. In recent years, it also has been required for message submission. In these cases, a server validates the client's identity, either through explicit security protocols or through implicit infrastructure queries that identify "local" participants.
1.2. The Role of This Architecture 🧠
An Internet service is an integration of related capabilities among two or more participating nodes. Those capabilities are accomplished across the Internet by one or more protocols. What connects a protocol to a service is an architecture 🎀. An architecture specifies how the protocols implement the service by defining the logical components of a service and the relationships among them. So from that logical view, a service defines what is being done, an architecture defines where the pieces are (in relation to each other), and a protocol defines how particular capabilities are performed.
An architecture describes a service more formally, at a relatively high level. A protocol that implements some portion of a service will conform to the architecture to a greater or lesser extent, depending on the pragmatic tradeoffs it makes when trying to implement the architecture under real-world constraints. Failing to precisely follow an architecture is not a failure of the protocol, and failing to precisely cast a protocol is not a failure of the architecture 🙅♀️. Where a protocol varies from the architecture, it's of course appropriate for it to explain the reason for the variance. Still, that variance is not a mark against a protocol. Happily, the IETF prefers running code to architectural purity 🏃♀️💨
In this particular case, this architecture tries to define the logical components of Internet email, and it does so post hoc, capturing the architectural principles that the current email protocols embody. Different email protocols conform to this architecture more or less well 🌈. Insofar as this architecture differs from those protocols, the reasons are generally well understood and are required for interoperation. Those differences are not a sign that protocols need to be fixed. However, this architecture is a best attempt at a logical model of Internet email. So insofar as new protocol development varies from this architecture, designers need to understand those differences and explain them carefully 📝
1.3. Document Conventions 📚
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 part is the name of the field. So <RFC5322.From> is the IMF From: header field in an email content header, and <RFC5321.MailFrom> is the address in the SMTP "Mail From" command 💌
When it appears without the IMF (RFC 5322) qualifier, a header field name is shown with a colon suffix. For example, From:.
References to labels for actors, functions, or components have the first letter capitalized ✨
2. Responsible Actor Roles 👑
Internet Mail is a highly distributed service, with a variety of Actors playing different roles. These Actors fall into three basic types 🫶:
- User
- Message Handling Service (MHS)
- ADministrative Management Domain (ADMD)
Although they're related to a technical architecture, the focus on Actors is about participant responsibilities, not the functionality of modules. That's why the labels are different from the ones used in classic diagrams of email architecture 🧚
2.1. User Actors 🦋
Users are the sources and sinks of messages. Users can be people, organizations, or processes 🙌. They can have an exchange that iterates, and they can expand or contract the set of Users that participate in a set of exchanges. In Internet Mail, there are four types of Users 💅:
- Authors
- Recipients
- Return Handlers
- Mediators
Figure 2 shows the primary and secondary flows of messages among them. As a pragmatic heuristic: User Actors can generate, modify, or look at the whole message 👀
++==========++
|| Author ||<..................................<..
++=++=++=++=++ .
|| || || ++===========++ .
|| || ++====>|| Recipient || .
|| || ++=====+=====++ .
|| || . .
|| || ..........................>.+
|| || .
|| || ................... .
|| || . . .
|| || V . .
|| || +-----------+ ++=====+=====++ .
|| ++========>| Mediator +===>|| Recipient || .
|| +-----+-----+ ++=====+=====++ .
|| . . .
|| ..................+.......>.+
|| .
|| ..............+.................. .
|| . . . .
\/ V V ' .
+-----------+ +-----------+ ++=====+=====++ .
| Mediator +===>| Mediator +===>|| Recipient || .
+-----+-----+ +-----+-----+ ++=====+=====++ .
. . . .
.................+.................+.......>..
Legend: === lines indicate primary (possibly indirect)
transfers or roles
... lines indicate supporting transfers or roles
Figure 2: Relationships among User ActorsFrom a User's perspective, all message-transfer activities are performed by a monolithic Message Handling Service (MHS), even though the actual service can be provided by many independent organizations. Users are customers of this unified service 🛍️
Whenever any MHS Actor sends information back to an Author or Originator in the sequence of handling a message, that Actor is a User 🔄
2.1.1. Author ✨
Okay so the Author is the one who creates the message, writes its contents, and makes the list of Recipient addresses 💌. The MHS takes the message from the Author and delivers it to the Recipients. 🦋 The MHS has an Originator role (Section 2.2.1) that lines up with the Author role.
2.1.2. Recipient 📬
The Recipient is the consumer of the delivered message. The MHS has a Receiver role (Section 2.2.4) that matches up with the Recipient role. This is labeled Recv in Figure 3.
Any Recipient can close the user-communication loop by creating and submitting a brand-new message that replies to the Author 🔁. An automated example of this is the Message Disposition Notification (MDN), which tells the Author how the Recipient handled the message. (See Section 4.1.)
2.1.3. Return Handler 🎀
Also called "Bounce Handler," the Return Handler is a special kind of Recipient whose whole job is servicing the notifications the MHS generates while it transfers or delivers the message. (See Figure 3.) These notices can be about failures or completions, and they go to an address that the Originator specifies. This Return Handling address (also known as a Return Address) might look nothing like the address of the Author or Originator 🙈.
2.1.4. Mediator 🧚
A Mediator receives, aggregates, reformulates, and redistributes messages among Authors and Recipients who are the principals in (potentially) long-running exchanges. This gets easily mixed up with the underlying MHS transfer exchanges. ngl, though, they have totally different purposes and work in totally different ways 💅.
When mail gets delivered to the Mediator named in the RFC5321.RcptTo command for the original message, the MHS handles it the same way it handles any other Recipient. The MHS sees each posting and delivery activity between sources and sinks as independent. It does not see later re-posting as a continuation of a process 👀. Because the Mediator originates messages, it can receive replies. So when a Mediator submits a reformulated message, it's an Author, even though it's really acting as an agent for one or more other Authors. A Mediator is literally a full-fledged User. Mediators get a deep dive in Section 5.
A Mediator tries to keep the original Author's information in the message it reformulates, but it's allowed to make meaningful changes to the message content or envelope. The MHS sees a brand-new message, but Users get something they interpret as coming from, or at least started by, the Author of the original message. The Mediator's role is not just about connecting other participants. The Mediator is responsible for the new message ✨.
A Mediator's role is complex and depends on the situation. For example, it might modify and add content, or regulate which Users are allowed to take part and when. The classic example is a group Mailing List 🌈. In a more complex setup, a sequence of Mediators could carry out a sequence of formal steps, like reviewing, modifying, and approving a purchase request.
A Gateway is a particularly interesting form of Mediator 🦋. It's a hybrid of User and Relay that connects heterogeneous mail services. Its purpose is to emulate a Relay. For a detailed discussion, see Section 2.2.3.
2.2. Message Handling Service (MHS) Actors 🌸
The Message Handling Service (MHS) does a single end-to-end transfer on behalf of the Author to reach the Recipient addresses given in the original RFC5321.RcptTo commands. Exchanges that are either mediated or iterative and long-running, like those used for collaborating over time, are handled by the User Actors, not the MHS Actors. As a pragmatic heuristic, MHS Actors generate, modify, or look at only transfer data, rather than the entire message 🧠.
Figure 3 shows how the transfer participants in Internet Mail relate to each other. Even though it shows the Originator (labeled Origin) as separate from the Author, and the Receiver (labeled Recv) as separate from the Recipient, each pair of roles usually has the same Actor 👯. Transfers typically involve one or more Relays. Direct delivery from the Originator to the Receiver is possible, though. Intra-organization mail services usually have only one Relay.
++==========++ ++===========++
|| Author || || Recipient ||
++====++====++ +--------+ ++===========++
|| | Return | /\
|| +-+------+ ||
\/ . ^ ||
+---------+ . . +---++---+
| | . . | |
/--+---------+----------------------------+--------+----\
| | | . . MHS | | |
| | Origin +<...... .................+ Recv | |
| | | ^ | | |
| +---++----+ . +--------+ |
| || . /\ |
| || ..............+.................. || |
| \/ . . . || |
| +-------+-+ +--+------+ +-+--++---+ |
| | Relay +=======>| Relay +=======>| Relay | |
| +---------+ +----++---+ +---------+ |
| || |
| || |
| \/ |
| +---------+ |
| | Gateway +-->... |
| +---------+ |
\-------------------------------------------------------/
Legend: === and || lines indicate primary (possibly
indirect) transfers or roles
... lines indicate supporting transfers or roles
Figure 3: Relationships among MHS Actors2.2.1. Originator 📝
The Originator makes sure a message is valid for posting, and then submits it to a Relay. A message is valid if it conforms to both Internet Mail standards and local operational policies. The Originator can just review the message for conformance and reject it if it finds errors, or it can create some or all of the information that's needed. Basically, the Originator is responsible for the functions of the Mail Submission Agent 🎀.
The Originator operates with dual allegiance 👑. It serves the Author, and the two can be the same entity. But its role in making sure a message is valid means it also represents the local operator of the MHS, that is, the local ADministrative Management Domain (ADMD).
The Originator also handles any post-submission, Author-related administrative tasks tied to message transfer and delivery. Notably, that means sending error and delivery notices, enforcing local policies, and dealing with messages from the Author that turn out to be problematic for the Internet 🙃. The Originator is accountable for the message content, even when it isn't the one responsible for it. The Author creates the message, but the Originator deals with any transmission issues that come up with it.
2.2.2. Relay 🚀
The Relay does MHS-level transfer-service routing and store-and-forward by transmitting or retransmitting the message to its Recipients. The Relay adds trace information [RFC2505] but does not modify the envelope information or the message content semantics. It can modify the message content representation, like changing the form of transfer encoding from binary to text, but only as required to meet the capabilities of the next hop in the MHS 🧩.
A Message Handling System (MHS) network consists of a set of Relays. This network sits above any underlying packet-switching network that might be used, and below any Gateways or other Mediators.
In other words, email scenarios can involve three distinct architectural layers, each providing its own type of store-and-forward service:
- User Mediators
- MHS Relays
- Packet Switches
The bottom layer is the Internet's IP service. The most basic email scenarios involve Relays and Switches 🌐.
When a Relay stops trying to transfer a message, it becomes an Author, because it sends an error message to the Return Address. Looping is avoided by leaving a Return Address out of this message 🔄.
2.2.3. Gateway 🌉
A Gateway is a hybrid of User and Relay that connects heterogeneous mail services. Its purpose is to emulate a Relay, and the closer it comes to this, the better 💯. A Gateway operates as a User when it needs the ability to modify message content.
Differences between mail services can be as small as minor syntax variations, but they usually involve significant semantic distinctions. One difference could be email addresses that are hierarchical and machine-specific, rather than a flat, global namespace. Another could be support for text-only content versus multimedia 🎨. So the Relay function in a Gateway is a real design challenge if the result is supposed to feel nearly seamless. The challenge is to make sure User-to-User functionality works between the services, despite differences in their syntax and semantics.
The basic test of Gateway design is 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 components in the Author's or Recipient's mail services other than adding the Gateway. To each of these otherwise independent services, the Gateway looks like a native participant 🫶. But the ultimate test of Gateway design is whether the Author and Recipient can keep up a dialogue. In particular: can a Recipient's MUA automatically formulate a valid Reply that will reach the Author? 🤔
2.2.4. Receiver 📥
The Receiver does final delivery or sends the message to an alternate address. It can also do filtering and other policy enforcement immediately before or after delivery 🛡️.
2.3. Administrative Actors ✨
Okay so, admin actors are the folks who run the infrastructure, and they can belong to totally different organizations, each with its own admin authority 🏢. Because they're all independent but still need to talk to each other, we need a way to draw lines between them. That's where the ADministrative Management Domain (ADMD) comes in 💅. Each ADMD can have wildly different operating policies and different ways of deciding what to trust 🔥. The classic example is mail that stays inside one organization versus mail that goes between independent organizations. Those two kinds of traffic get handled very differently, so we need the ADMD construct to mark where one zone ends and the next begins 🦋.
Internet Mail is run by different providers (or operators), and each one can be its own independent ADMD 🌸. That independence in decision-making is what draws the boundaries between the different parts of the Internet Mail service. Take a department running a local Relay, an IT team running an enterprise Relay, and an ISP running a public shared email service. They can be wired together in lots of combinations of admin and operational relationships. Each one is its own distinct ADMD, and each might have a complicated pile of functional components inside it 🧚. Figure 4 shows how ADMDs relate to each other. The big win of the ADMD idea is that it gives us a way to talk about designs, policies, and operations that need to tell internal issues apart from external ones 💜.
The architectural impact of needing boundaries between ADMDs is covered in [Tussle]. The biggest deal here is that the entities talking across an ADMD boundary usually have the extra job of enforcing their organization's policies on outside communication 😭. On a more everyday level, routing mail between ADMDs can get tricky. For example, you might need to send mail between organizational partners over specially trusted paths 🛤️.
There are three basic types of ADMDs 👑:
- Edge: Independent transfer services in networks at the edge of the open Internet Mail service.
- Consumer: Might be a type of Edge service, as is common for web-based email access.
- Transit: Mail Service Providers (MSPs) that offer value-added capabilities for Edge ADMDs, such as aggregation and filtering.
Mail-level transit is different from packet-level switching, so don't mix them up 🙈. End-to-end packet transfers usually go through intermediate routers. Email across the open Internet, though, can go directly between the Boundary MTAs of Edge ADMDs. This difference between direct and indirect interaction is what highlights the stuff we talk about in Section 2.2.2 ✨.
+--------+ +---------+ +-------+ +-----------+
| ADMD1 |<===>| ADMD2 |<===>| ADMD3 |<===>| ADMD4 |
| ----- | | ----- | | ----- | | ----- |
| | | | | | | |
| Author | | | | | | Recipient |
| . | | | | | | ^ |
| V | | | | | | . |
| Edge..+....>|.Transit.+....>|-Edge..+....>|..Consumer |
| | | | | | | |
+--------+ +---------+ +-------+ +-----------+
Legend: === lines indicate primary (possibly indirect)
transfers or roles
... lines indicate supporting transfers or roles
Figure 4: Administrative Domain (ADMD) ExampleEdge networks can use proprietary email standards internally, totally their call 🎀. The line between Transit and Edge transfer services still matters a lot, though, because it's a reminder to think hard about interaction and protection between independent administrations 🔒. This distinction specifically calls for extra care when you're looking at who takes over responsibility at each step, plus the accountability and authorization relationships among everyone involved in moving a message 💀.
The way ADMD components interact is governed by that domain's policies, and those policies cover things like these 📋:
- Reliability
- Access control
- Accountability
- Content evaluation and modification
These policies can live in different functional components, depending on what the ADMD needs. For an example, check out [RFC5068] 🌈.
Consumer, Edge, and Transit services can all be offered by providers who run component services or sets of services 🫶. It's also possible for one ADMD to host services for other ADMDs, which is kind of iconic 🌟.
Here are some common examples of ADMDs 🍓:
- Enterprise Service Providers: These ADMDs operate the internal data and/or the mail services within an organization.
- Internet Service Providers (ISP): These ADMDs operate the underlying data communication services, which are used by one or more Relay and User. ISPs are not responsible for performing email functions, but they can provide an environment in which those functions can be performed.
- Mail Service Providers: These ADMDs operate email services, such as for consumers or client companies.
Practical operational concerns mean providers often have to get involved in administration and enforcement issues. That involvement can even stretch down to the operators of lower-level packet services 🔥.
3. Identities 🦋
The forms of identity used by Internet Mail are: mailbox, domain name, message-ID, and ENVID (envelope identifier). Each one is globally unique 💖.
3.1. Mailbox 📬
"A mailbox receives mail. It is a conceptual entity that does not necessarily pertain to file storage." [RFC5322]
A mailbox is specified as an Internet Mail address <addr-spec> 💌. It has two distinct parts, separated by an at-sign (@). The right side is a globally interpreted domain name tied to an ADMD. We'll get into domain names in Section 3.3. The addressing syntax can support source routes, which show the path a message ought to take. Source routes aren't common, and [RFC5321] has deprecated them, so please don't go using them 🙅♀️.
The left side of the at-sign holds a string that's globally opaque, called the <local-part> 🤫. Only the entity named by the address's domain can interpret it. Except for the exceptions covered later in this section, everyone else treats the <local-part> as an uninterpreted literal string and keeps all of its original details intact. So putting it out there publicly is basically like sending a web browser "cookie" that only means something when it gets returned to whoever made it 🍪.
Some <local-part> values have been standardized for contacting people at an organization. These names cover common operations and business functions, and they're defined in [RFC2142] 🏢.
Sites often have their own local structuring conventions for the left-hand side, <local-part>, of an <addr-spec>. This lets them do sub-addressing, like telling apart different discussion groups used by the same person 🧩. Just know that these conventions are strictly private to the User's organization and aren't interpreted by any domain except the one on the right side of the <addr-spec> 🙃. The only exceptions are specialized services that follow public, standardized conventions, which we'll mention below.
Basic email addressing defines the <local-part> as globally opaque. Some uses of email, though, add a standardized, global schema to that value. One example is between an Author and a Gateway. The <local-part> details stay invisible to the public email transfer infrastructure, but they give addressing and handling instructions for further processing by the Gateway ✨. Standardized examples of these conventions include the telephone numbering formats for the Voice Profile for Internet Mail (VPIM) [RFC3801], like this:
+16137637582@vpim.example.comAnd iFax ([RFC3192], [RFC4143]) looks like this:
FAX=+12027653000/T33S=1387@ifax.example.com3.2. Scope of Email Address Use 🌍
Email addresses have gone way beyond their original job of email transfer and delivery 😭. Practically speaking, an email address string has turned into the common identifier for online identity. So it's super important to be clear about both the nature and role of an identity string in a given context, and about which entity is responsible for setting that string 🫣. For example, see Sections 4.1.4, 4.3.3, and 5.
3.3. Domain Names 🌐
A domain name is a global reference to an Internet resource, like a host, a service, or a network 💻. A domain name usually maps to one or more IP Addresses. Conceptually, a name can represent an organization, a collection of machines working together as one homogeneous service, or a single machine. A domain name can be administered to refer to an individual User, but that isn't common practice 🙅♀️. The name is a hierarchical sequence of labels separated by dots (.), with the top of the hierarchy on the right end of the sequence. There can be lots of names in the sequence, so the depth of the hierarchy can get pretty substantial 📚. Domain names are defined and operated through the Domain Name System (DNS) ([RFC1034], [RFC1035], [RFC2181]) 🧠.
When a domain name isn't part of a mailbox address, Internet Mail uses it to refer to the ADMD or to the host that acted on the message. For example, it might provide the administrative scope for a message identifier or handle transfer processing 🎯.
3.4. Message Identifier 🏷️
There are two standardized tags for identifying messages: Message-ID: and ENVID 💜. A Message-ID: is about content, and an ENVID is about transfer. Keep those two straight and you're golden 👑.
3.4.1. Message-ID ✨
okay so bestie, IMF only allows one Message-ID: per message 💖 It's a user-level tag that's good for a bunch of things: threading, spotting duplicates, and DSN (Delivery Status Notification) tracking 🧵. The Originator is the one who assigns it, and the Recipient's ADMD is the main consumer, though literally any Actor along the transfer path can use it 🫶
The Message-ID: is globally unique 🌍 It looks a lot like a mailbox, with two parts split by an at-sign (@). Usually the right side names the ADMD or host that made the ID, and the left side is a string that's globally opaque, so it just exists to make the message unique inside the domain on the right 🔐 How long that uniqueness lasts is undefined, so don't count on forever 💀
Now the tricky part: what happens when a message gets revised? Deciding whether to make a new Message-ID: is kind of a vibe check 😭 It takes a subjective call on whether the editorial content changed enough to count as a new message. [RFC5322] says, "a message identifier pertains to exactly one version of a particular message; subsequent revisions to the message each receive new message identifiers." But real life is messy, so some flexibility is needed 🦋 The impossible test is whether the Recipient will think the new one is equivalent to the old one. For most parts of Internet Mail there's no way to predict what a specific Recipient wants here 🙈 And honestly, both making a new ID and not making one have their downsides 💅
Here are some guidelines and examples:
- If a message is changed only in form, such as character encoding, it 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 RFC5322.Subject header field, or some Mailing List administrative information added to the end of the primary body part text, it is probably the same message.
- If a message has viruses deleted from it, it is probably the same message.
- If a message has offensive words deleted from it, some Recipients will consider it the same message, but some will not.
- If a message is translated into a different language, some Recipients will consider it the same message, but some will not.
- If a message is included in a digest of messages, the digest constitutes a new message.
- If a message is forwarded by a Recipient, what is forwarded is a new message.
- If a message is "redirected", such as using IMF "Resent-*" header fields, some Recipients will consider it the same message, but some will not.
The thing is, there are no objective, precise rules for regenerating a Message-ID:, and the string doesn't get strong protection either 😤 So an ID can give you an assessment that's a little better than a guess, but it has no real value on its own for strict formal reference or comparison 💜 That's exactly why the Message-ID: is not intended to be used for any function that has security implications 🚨
3.4.2. ENVID
The ENVID (envelope identifier) is used for message-tracking purposes ([RFC3885], [RFC3464]) about a single posting/delivery transfer 📦 It labels one single transit of the MHS by one specific message. So the ENVID is used for a single posting of the message until that message is delivered 🚚 If the message gets re-posted, like by a Mediator, that doesn't reuse the ENVID. It can get a new one, even though the message might still legitimately keep its original Message-ID: 🦋
The format of an ENVID is free form 🎀 The creator can add structure to the string if they want, but no Internet standards impose any. So by implication, the scope of the string is defined by the domain name of the Return Address 🌸
4. Services and Standards ✨
Okay, now we're getting into the big picture 👑 The Internet Mail architecture has six basic types of functionality, all arranged to support a store-and-forward service. As Figure 5 shows, each type can have multiple instances, and some of those represent specialized roles 🧚 This section looks at how these components act and relate to each other, plus the Internet Mail standards that apply to them 💖
Message
Message User Agent (MUA)
Author MUA (aMUA)
Recipient MUA (rMUA)
Message Submission Agent (MSA)
Author-focused MSA functions (aMSA)
MHS-focused MSA functions (hMSA)
Message Transfer Agent (MTA)
Message Delivery Agent (MDA)
Recipient-focused MDA functions (rMDA)
MHS-focused MDA functions (hMDA)
Message Store (MS)
Author MS (aMS)
Recipient MS (rMS)This figure shows function modules and the standardized protocols used between them 🫶
++========++
|| || +-------+
...........++ aMUA ||<............................+ Disp |
. || || +-------+
. ++=+==+===++ ^
. local,imap}| |{smtp,submission .
. +-----+ | | +--------+ .
. | aMS |<---+ | ........................>| Return | .
. +-----+ | . +--------+ .
. | . ***************** ^ .
. +-----V-.----*------------+ * . .
. MSA | +-------+ * +------+ | * . .
. | | aMSA +-(S)->| hMSA | | * . .
. | +-------+ * +--+---+ | * . .
V +------------*------+-----+ * . .
//==========\\ * V {smtp * . .
|| MESSAGE || * +------+ * //===+===\\ .
||----------|| MHS * | MTA | * || dsn || .
|| ENVELOPE || * +--+---+ * \\=======// .
|| smtp || * V {smtp * ^ ^ .
|| CONTENT || * +------+ * . . //==+==\\
|| imf || * | MTA +....*...... . || mdn ||
|| mime || * +--+---+ * . \\=====//
\\==========// * smtp}| {local * . ^
. MDA * | {lmtp * . .
. +----------------+------V-----+ * . .
. | +----------+ * +------+ | * . .
. | | | * | | +..*.......... .
. | | rMDA |<-(D)--+ hMDA | | * .
. | | | * | | |<.*........ .
. | +-+------+-+ * +------+ | * . .
. +------+---------*------------+ * . .
. smtp,local}| ***************** . .
. V . .
. +-----+ //===+===\\ .
. | rMS | || sieve || .
. +--+--+ \\=======// .
. |{imap,pop,local ^ .
. V . .
. ++==========++ . .
. || || . .
.......>|| rMUA ++........................... .
|| ++...................................
++==========++
Legend: --- lines indicate primary (possibly indirect)
transfers or roles
=== boxes indicate data objects
... lines indicate supporting transfers or roles
*** lines indicate aggregated service
Figure 5: Protocols and Services4.1. Message Data
So the whole point of the Message Handling System (MHS) is to swap an IMF message object between participants [RFC5322] 📨 All of its underlying mechanisms are there to get that message from its Author to its Recipients. A message can also be explicitly labeled as to its nature [RFC3458] 🏷️
A message is made up of a transit-handling envelope and the message content 💌 The envelope holds info the MHS uses. The content splits into a structured header and the body. The header has transit-handling trace information and structured fields that are part of the Author's message content 🎀 The body can be unstructured lines of text, or a tree of multimedia subordinate objects, called "body-parts" or, popularly, "attachments" 📎 [RFC2045], [RFC2046], [RFC2047], [RFC4288], [RFC4289], [RFC2049].
Internet Mail also has a few conventions for special control data, and the big ones are:
Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) is a message that the MHS (MSA, MTA, or MDA) can generate, and it gets sent to the RFC5321.MailFrom address 📬 MDA and MTA show up as DSN sources in Figure 5, and the destination is marked as Returns. DSNs give you info about message transit, like transfer errors or successful delivery ✅ [RFC3461]
Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) is a message that tells you about post-delivery processing, like that the message has been displayed [RFC3798] or what form of content can be supported [RFC3297] 👀 An rMUA can generate it, and it's sent to the Disposition-Notification-To addresses. The mailbox for this is shown as Disp in Figure 5.
Message Filtering (SIEVE):
Sieve is a scripting language used to set conditions for differential handling of mail, usually at delivery time [RFC5228] 🧹 Scripts can be shipped around in a bunch of ways, like as a MIME part in a message. Figure 5 shows a Sieve script going from the rMUA to the MDA. Filtering can actually happen at lots of different points along the transit path, though, and any one or more of them might be subject to Sieve directives, especially inside a single ADMD 🌈 Figure 5 only shows one relationship, just to keep things (relatively) simple 🙈
4.1.1. Envelope
Internet Mail has a fragmented framework for transit-related handling info 🧩 Info that the MHS uses directly is called the "envelope". It directs handling activities by the transfer service and gets carried in transfer-service commands. So basically, the envelope lives in the transfer protocol SMTP [RFC5321] 💜
Trace info, like RFC5322.Received, gets recorded in the message header and isn't changed afterward [RFC5322] 🕰️
4.1.2. Header Fields
Header fields are attribute name/value pairs 🎀 They cover an extensible range of email-service parameters, structured user content, and user transaction meta-information. The core set is defined in [RFC5322], and it's common practice to extend that set for different applications 🌸 Procedures for registering header fields are defined in [RFC3864]. There's also an extensive set of existing header field registrations in [RFC4021] 📚
One danger of putting extra info in header fields is that Gateways often alter or delete them 😬
4.1.3. Body 🎀
okay so the body of a message can be just plain lines of ASCII text, OR it can be a hierarchically structured composition of multimedia body part attachments using MIME ([RFC2045], [RFC2046], [RFC2047], [RFC4288], and [RFC2049]) 🦋 picture a whole nested stack of attachments, ngl it's giving organized chaos ✨
4.1.4. Identity References in a Message 💜
Table 1 lists the core identifiers that show up in a message while it's in transit 🧚
+----------------------+----------------+---------------------------+
| Layer | Field | Set By |
+----------------------+----------------+---------------------------+
| Message Body | MIME Header | Author |
| Message header | From: | Author |
| fields | | |
| | Sender: | Originator |
| | Reply-To: | Author |
| | To:, CC:, BCC: | Author |
| | Message-ID: | Originator |
| | Received: | Originator, Relay, |
| | | Receiver |
| | Return-Path: | MDA, from MailFrom |
| | Resent-*: | Mediator |
| | List-Id: | Mediator |
| | List-*: | Mediator |
| SMTP | HELO/EHLO | Latest Relay Client |
| | ENVID | Originator |
| | MailFrom | Originator |
| | RcptTo | Author |
| | ORCPT | Originator |
| IP | Source Address | Latest Relay Client |
+----------------------+----------------+---------------------------+Here's the legend so you can read the table like a pro 📚
Layer means the part of the email architecture that uses the identifier 🌸
Field means the protocol construct that contains the identifier 💖
Set By means the Actor role responsible for specifying the identifier value. This can be different from the Actor that does the fill-in function for the protocol construct, so don't mix them up 🙈
Table 1: Layered IdentitiesThese are the most common address-related fields, bestie 👑
RFC5322.From: Set by - Author
Names and addresses for Authors of the message content go in the From: field ✍️
RFC5322.Reply-To: Set by - Author
So if a Recipient sends a reply that would otherwise use the RFC5322.From addresses from the original message, the RFC5322.Reply-To addresses get used instead. In other words, this field overrides the From: field for responses from Recipients. main character energy 💅
RFC5322.Sender: Set by - Originator
This field specifies the address responsible for submitting the message to the transfer service. It can be left out if it has the same address as RFC5322.From. Omitting it doesn't mean there's no Sender, though! It means the header field is virtual and the address in the From: field gets used 🫶
Specifying the notifications Return Addresses, which live in RFC5321.MailFrom, is done by the RFC5322.Sender. Usually the Return address matches the Sender address, but some usage scenarios need it to be different 🌈
RFC5322.To/.CC: Set by - Author
These fields specify MUA Recipient addresses. BUT some or all of the addresses in these fields might not show up in the RFC5321.RcptTo commands 👀
The difference between To and CC is kinda subjective. Generally, a To addressee is considered primary and is expected to take action on the message. A CC addressee typically gets a copy as a courtesy 💌
RFC5322.BCC: Set by - Author
A copy of the message might get sent to an addressee whose participation is not supposed to be disclosed to the RFC5322.To or RFC5322.CC Recipients, and usually not to the other BCC Recipients either 🤫 The BCC: header field indicates a message copy to that Recipient. Use of this field is discussed in [RFC5322] ✨
RFC5321.HELO/.EHLO: Set by - Originator, MSA, MTA
Any SMTP client, including an Originator, MSA, or MTA, can specify its hosting domain identity for the SMTP HELO or EHLO command operation 🍓
RFC3461.ENVID: Set by - Originator
The MSA can specify an opaque string, to be included in a DSN, as a way of helping the Return Address Recipient figure out which message produced a DSN or message tracking. iykyk 🦋
RFC5321.MailFrom: Set by - Originator
This field is an end-to-end string that specifies an email address for receiving return control information, like returned messages. The name is kinda misleading, tbh, because it isn't required to specify either the Author or the Actor responsible for submitting the message. Instead, the Actor responsible for submission specifies the RFC5321.MailFrom address. Ultimately, the simple basis for deciding which address goes in RFC5321.MailFrom is figuring out which address should be informed about transfer-level problems (and possibly successes) 💀
RFC5321.RcptTo: Set by - Author, Final MTA, MDA
This field specifies the MUA mailbox address of a Recipient. The string might not be visible in the message content header. For example, the IMF destination address header fields, like RFC5322.To, might specify a Mailing List mailbox, while the RFC5321.RcptTo address specifies one member of that list 🌸
RFC5321.ORCPT: Set by - Originator.
This is an optional parameter to the RCPT command. It indicates the original address that the current RCPT TO address corresponds to, after a mapping was performed during transit. An ORCPT is the only reliable way to correlate a DSN from a multi-Recipient message transfer with the intended Recipient 🔥
RFC5321.Received: Set by - Originator, Relay, Mediator, Dest
This field contains trace information, including the originating host, Relays, Mediators, and MSA host domain names and/or IP Addresses 🕵️♀️
RFC5321.Return-Path: Set by - Originator
The MDA records the RFC5321.MailFrom address into the RFC5321.Return-Path field 📬
RFC2919.List-Id: Set by - Mediator, Author
This field provides a globally unique Mailing List naming framework that is independent of particular hosts [RFC2919] 🌟
The identifier is in the form of a domain name. BUT the string is usually built by combining the two parts of an email address, so the result is rarely a true domain name listed in the domain name service, although it can be 🤷♀️
RFC2369.List-*: Set by - Mediator, Author
[RFC2369] 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 for the user-as-subscriber [RFC2369] 💜
RFC0791.SourceAddr: Set by - The Client SMTP sending host immediately preceding the current receiving SMTP server
[RFC0791] 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, which makes it 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 involve protocol exchanges that are different from the ones at lower layers of the Internet Mail MHS architecture, which sits above the Internet Transport layer. Because email is mostly about people talking to each other, the nature and details of these protocol exchanges are often shaped by the needs of interpersonal and group communication. To handle the idiosyncratic behavior that comes with that, only subjective guidelines, rather than strict rules, can be offered for some aspects of system behavior. Mailing Lists are particularly salient examples 🦋
4.2.1. Message User Agent (MUA) 💅
A Message User Agent (MUA) works on behalf of User Actors and User applications. It's their representative within the email service 🎀
The Author MUA (aMUA) creates a message and does the initial submission into the transfer infrastructure via a Mail Submission Agent (MSA). It can also handle any creation- and posting-time archiving in its Message Store (aMS). An MUA aMS can organize messages in lots of different ways. A common model uses aggregations, called "folders"; in IMAP they're called "mailboxes". This model allows 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 transfer (Sent). BUT none of these folders is required. For example, IMAP allows drafts to be stored in any folder, so no Drafts folder needs to be present 🌈
The Recipient MUA (rMUA) works on behalf of the Recipient to process received mail. This processing includes generating user-level disposition control messages, displaying and disposing of the received message, and closing or expanding the user-communication loop by starting replies and forwarding new messages 💖
NOTE: Although not shown in Figure 5, an MUA itself can have a distributed implementation, such as a "thin" user-interface module on a constrained device such as a smartphone, with most of the MUA functionality running 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. An approach for such scenarios is defined by [RFC4550]. 🧚
A Mediator is a special class of MUA. It performs message re-posting, as discussed in Section 2.1 ✨
An MUA can be automated, acting on behalf of a User who isn't present at the time the MUA is active. One example is a bulk sending service that has a timed-initiation feature. These services aren't to be confused with a Mailing List Mediator, since there's no incoming message triggering the activity of the automated service 🍓
A popular and problematic MUA is an automatic responder, like one that sends out-of-office notices. This behavior might get confused with a Mediator's, BUT this MUA is generating a new message. Automatic responders can annoy Users of Mailing Lists unless they follow [RFC3834] 🙈
The identity fields that are relevant to a typical MUA are:
4.2.2. Message Store (MS) 📦
okay so an MUA can hang onto a long-term Message Store (MS) 💖 Figure 5 shows an Author's MS (aMS) and a Recipient's MS (rMS) 🦋 An MS can live on a remote server or on the very same machine as the MUA, whichever works for you 💻
An MS can pick up messages from an MDA in a couple of ways. It can do it proactively through a local mechanism, or even through a standardized mechanism like SMTP(!) 😭 Or it can do it reactively, using POP or IMAP 🧚 The MUA gets into the MS either through a local mechanism or by using POP or IMAP. Using POP for individual message accesses, instead of bulk transfer, is relatively rare and inefficient 🙈 not the vibe, bestie.
4.3. MHS-Level Services 🎀
4.3.1. Mail Submission Agent (MSA) 📮
A Mail Submission Agent (MSA) takes in the message that the aMUA submits and enforces the policies of the hosting ADMD along with the requirements of Internet standards 👑 The MSA has a super unusual double life, though 😳 it looks out for the Author (aMUA) during posting so the post actually succeeds, and it also looks out for the MHS. In the architecture, Figure 5 splits the MSA into two sub-components to show this, aMSA and hMSA, respectively 🌈 Handing responsibility for one message from an Author's environment over to the MHS is called "posting", and it's marked as the (S) transition inside the MSA 💅
The hMSA takes transit responsibility for a message that conforms to the relevant Internet standards and to local site policies. It rejects messages that don't conform 🙅♀️ The MSA does the final message preparation before submission and handles the transfer of responsibility to the MHS, through the hMSA. How much prep happens depends on the local implementation 🍓 Examples of aMSA tasks include adding header fields like Date: and Message-ID:, and changing parts of the message from local notations into Internet standard forms, like expanding an address into its formal IMF representation ✨
Historically, standards-based MUA/MSA message postings have used SMTP [RFC5321]. The one currently preferred is SUBMISSION [RFC4409]. SUBMISSION comes from SMTP, but it uses a separate TCP port and has its own requirements, like access authorization 🔐
These identities are relevant to the MSA:
RFC5321.HELO/.EHLO
RFC3461.ENVID
RFC5321.MailFrom
RFC5321.RcptTo
RFC5321.Received
RFC0791.SourceAddr4.3.2. Message Transfer Agent (MTA) 🚚
A Message Transfer Agent (MTA) relays mail for one application-level "hop" 🦋 It's a lot like a packet switch or IP router, because its whole job is making routing decisions and moving the message closer to the Recipients 🛣️ Email objects are usually way bigger than a packet or datagram payload, though, and the end-to-end latencies are usually much higher 🐌 Relaying is done by a chain of MTAs until the message reaches a destination MDA. So an MTA runs both client and server MTA functionality 🫶 It doesn't change addresses in the envelope or rewrite the editorial content. Changing the data form, such as to MIME Content-Transfer-Encoding, is fair game for an MTA, but removing or replacing body content is not 🙅♀️ An MTA also adds trace information [RFC2505].
NOTE: Within a destination ADMD, email-relaying modules can make a variety of changes to the message before delivery. When they do, they're acting as Gateways, not MTAs 🧐
Internet Mail uses SMTP ([RFC5321], [RFC2821], [RFC0821]) mainly to do point-to-point transfers between peer MTAs 📬 Other transfer mechanisms include Batch SMTP [RFC2442] and On-Demand Mail Relay (ODMR) SMTP [RFC2645]. Like most network-layer mechanisms, Internet Mail SMTP has a basic level of reliability, because it provides for retransmission after a temporary transfer failure 🔁 Unlike typical packet switches (and Instant Messaging services), Internet Mail MTAs are expected to store messages in a way that lets them recover across service interruptions, like a host-system shutdown 💾 How robust and persistent an MTA is can vary. The base SMTP specification gives a framework for protocol response codes, and an extensible enhancement to that framework is defined in [RFC5248] 🧩
Even though it's pretty basic, the dominant routing mechanism for Internet Mail is the DNS MX record [RFC1035]. It names an MTA through which the queried domain can be reached 🗺️ This mechanism assumes a public, or at least common, backbone that lets any attached MTA connect to any other 🌐
MTAs can play any of these well-established roles:
- Boundary MTA: An MTA that is part of an ADMD and interacts with MTAs in other ADMDs. This is also called a Border MTA. There can be different Boundary MTAs, depending on the direction of mail-flow.
- Outbound MTA: An MTA that relays messages to other ADMDs.
- Inbound MTA: An MTA that receives inbound SMTP messages from MTA Relays in other ADMDs, for example, an MTA running on the host listed as the target of an MX record.
- Final MTA: The MTA that transfers a message to the MDA.
These identities are relevant to the MTA:
RFC5321.HELO/.EHLO
RFC3461.ENVID
RFC5321.MailFrom
RFC5321.RcptTo
RFC5322.Received: Set by - Relay Server
RFC0791.SourceAddr4.3.3. Mail Delivery Agent (MDA) 📫
When responsibility moves from the MHS to a Recipient's environment (mailbox), that's called "delivery" 💌 In the architecture, shown in Figure 5, delivery happens inside a Mail Delivery Agent (MDA). It shows up as the (D) transition from the MHS-oriented MDA component (hMDA) to the Recipient-oriented MDA component (rMDA) 💅
An MDA can offer distinctive, address-based functionality, because it has detailed information about the properties of the destination address 🔍 That information might also exist elsewhere in the Recipient's ADMD, like at an organizational border (Boundary) Relay. It is required for the MDA anyway, if only because the MDA has to know where to deliver the message 📍
Like an MSA, an MDA has two roles, as Figure 5 shows 🧁 The formal transfer of responsibility, called "delivery", happens between the two components that carry out those roles, and it's shown as "(D)" in Figure 5. The MHS portion (hMDA) mainly works as a server SMTP engine 🖥️ A common extra job is redirecting the message to an alternative address, based on the Recipient addressee's preferences. The Recipient portion of the MDA (rMDA) performs whatever delivery actions the Recipient has specified 🎯
Transfer into the MDA happens through a normal MTA transfer mechanism. Transfer from an MDA to an MS uses an access protocol, like POP or IMAP 🔄
NOTE: The term "delivery" can mean the formal MHS function described here, or the first time a message is displayed to a Recipient. A simple, practical test for whether the MHS-based definition applies is whether a DSN can be generated 🧠
These identities are relevant to the MDA:
RFC5321.Return-Path: Set by - Author Originator or Mediator
OriginatorThe MDA records the RFC5321.MailFrom address into the RFC5321.Return-Path field 📝
RFC5322.Received: Set by - MDA serverAn MDA can record a Received: header field to show trace information, including source host and receiving host domain names and/or IP Addresses 🕵️♀️
4.4. Transition Modes 🌸
From the origination site to the point of delivery, Internet Mail usually follows a "push" model 🚀 That means the Actor holding the message starts the transfer to the next venue, typically with SMTP [RFC5321] or the Local Mail Transfer Protocol (LMTP) [RFC2033]. With a "pull" model, the Actor holding the message waits for the Actor in the next venue to start a request for the transfer 🎣 Standardized mechanisms for pull-based MHS transfer are ETRN [RFC1985] and ODMR [RFC2645].
After delivery, the Recipient's MUA (or MS) can get access in one of two ways: the message can be pushed to it, or the receiver can pull the message, such as by using POP [RFC1939] and IMAP [RFC3501] 💜
4.5. Implementation and Operation 🛠️
A discussion of any interesting system architecture often gets bogged down when architecture and implementation get confused 😵💫 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, depending on what a particular operational environment needs. For example, a software system that mainly does message relaying is an MTA, but it might also include MDA functionality. That same MTA system might be able to interface with non-Internet email services, and so perform as both an MTA and a Gateway 🌈
Similarly, implemented modules might be configured to form elaborations of the architecture. An interesting example is a distributed MS, where one portion is a remote server and another is local to the MUA 🧩 As discussed in [RFC1733], there are three operational relationships among such MSs:
- Online: The MS is remote, and messages are accessible only when the MUA is attached to the MS, so the MUA will re-fetch 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 they are connected. When they are disconnected, mail can arrive at the rMS and the User can make changes to the uMS. The two stores are re-synchronized when they are reconnected 🦋
5. Mediators 🦋
Okay so basic message transfer from Author to Recipients is handled by an asynchronous store-and-forward setup, where it's a bunch of separate little transmissions hopping through however many MTAs 🚚. A very different job is a sequence of postings and deliveries through Mediators! A Mediator forwards a message through a re-posting process. It shares some stuff with basic MTA relaying, but it has way more flexibility with both addressing and content than MTAs get 💅
This is the core set of message information that basically every type of Mediator sets:
RFC5321.HELO/.EHLO: Set by - Mediator OriginatorRFC3461.ENVID: Set by - Mediator OriginatorRFC5321.RcptTo: Set by - Mediator AuthorRFC5321.Received: Set by - Mediator Dest- The Mediator can record received information to show the delivery to the original address and the submission to the alias address. The trace of
Received:header fields can cover everything from the original posting, through relaying, all the way to final delivery 🧾
- The Mediator can record received information to show the delivery to the original address and the submission to the alias address. The trace of
So what makes a Mediator different from any other MUA that's creating a message? A Mediator keeps the integrity and tone of the original message intact, including the important parts of its origination information 🫶 It might also add commentary, bestie.
Here are some examples of MUA messages that a Mediator does not create:
- New message that forwards an existing message: Even though this action gives a basic template for a class of Mediators, the usual forwarding itself isn't a Mediator example 🙈 The new message is seen as coming from the Actor who's doing the forwarding, not from the original Author. The new message wraps the original one and looks like it's from the new Originator. This Mediator Originator might add commentary and can change the original message content. Since the forwarded message sits inside the message the new Originator sends, the new message starts a new dialogue. Still, the final Recipient sees the contained message as coming from the original Author.
- Reply: When a Recipient answers the Author of a message, the new message usually isn't seen as a forward of the original. The focus is the new content, even if it includes all or some of the original material. The older stuff is just context and secondary. This includes automated replies, like vacation out-of-office notices, which are covered in Section 4.2.1 ✈️
- Annotation: The original message's integrity is usually kept, but one or more comments get added in a way that makes the commentary stand out from the original text. The main point of the new message is to give commentary from a new Author, kind of like a Reply 💬
The rest of this section walks through some common examples of Mediators 🌸
5.1. Alias
One job of an MDA is figuring out the internal location of a mailbox so it can deliver there 📬 An Alias is a simple re-addressing facility that gives one or more new Internet Mail addresses instead of a single internal one. The message keeps going through the transfer service so it can reach one or more alternate addresses. Even though it's usually built into an MDA, this facility is a Recipient function. It resubmits the message, but keeps all the handling information except the envelope Recipient (rfc5321.RcptTo) address. Most importantly, the Return Address (rfc5321.MailFrom) doesn't change 🔒
This forwarding trick is seriously similar to normal MTA store-and-forward relaying. The only real difference is that it changes the RFC5321.RcptTo value. Because that change is so tiny, aliasing can be seen as part of the lower-level mail-relaying activity. BUT this tiny change has a huge semantic impact: the designated Recipient has picked a new Recipient 😭
NOTE: When the replacement list has more than one address, the alias is more and more likely to run into delivery problems. Any problem reports go to the original Author, not the administrator of the alias entry. That makes the problem harder to fix, since the original Author has no idea the Alias mechanism exists 💀
Along with the core set of message information from the start of this section, Alias usually changes these:
RFC5322.To/.CC/.BCC: Set by - Author- These fields keep their original addresses. 🎀
RFC5321.MailFrom: Set by - Author- Keeping the original MailFrom value helps make sure that an Actor tied to the originating ADMD knows there was a delivery problem 📨 On the other hand, once the message moves from the original Recipient mailbox to the alias mailbox, responsibility for handling problems usually sits with that original Recipient, since the Alias mechanism is totally under their control. Keeping the original MailFrom address is what stops that from happening 🙅♀️
5.2. ReSender
Also called the ReDirector, the ReSender is different from forwarding because the Mediator "splices" a message's addressing information to connect the Author of the original message with the Recipient of the new message 🪡 This connection lets them talk directly using their normal MUA Reply functions, and it also records full reference information about the Recipient who acted as a Mediator. So the new Recipient sees the message as coming from the original Author, even if the Mediator adds commentary 👑
Along with the core set of message information from the start of this section, these identities matter for a resent message:
RFC5322.From: Set by - original Author- Names and addresses for the original Author of the message content are kept. The free-form (display-name) part of the address might be changed to give an informal reference to the ReSender 🏷️
RFC5322.Reply-To: Set by - original Author- If this field is in the original message, it's kept in the resent message 💌
RFC5322.Sender: Set by - Author's Originator or Mediator OriginatorRFC5322.To/.CC/.BCC: Set by - original Author- These fields specify the original message Recipients 🎯
RFC5322.Resent-From: Set by - Mediator Author- This address belongs to the original Recipient who is redirecting the message. Otherwise, the same rules apply to the Resent-From: field as to an original
RFC5322.Fromfield 🔁
- This address belongs to the original Recipient who is redirecting the message. Otherwise, the same rules apply to the Resent-From: field as to an original
RFC5322.Resent-Sender: Set by - Mediator OriginatorRFC5322.Resent-To/-CC/-BCC: Set by - Mediator Author- These are the addresses of the new Recipients, who can now reply to the original Author 🥹
RFC5321.MailFrom: Set by - Mediator Originator- The Actor responsible for resubmission (
RFC5322.Resent-Sender) is also the one responsible for specifying the new MailFrom address 🫡
- The Actor responsible for resubmission (
5.3. Mailing Lists
A Mailing List takes in messages as an explicit addressee and then re-posts them to a list of subscribed members 📣 It's kind of like an elaborate version of the ReSender. Besides sending the new message to a potentially huge number of new Recipients, the Mailing List can change the content. For example, it can delete attachments, convert the format, and add list-specific comments 🌈 Mailing Lists also archive messages that Authors post. Even so, the message still keeps the characteristics of coming from the original Author 💜
Along with the core set of message information from the start of this section, these identities are relevant to a Mailing List processor when it submits a message:
RFC2919.List-Id: Set by - Mediator AuthorRFC2369.List-*: Set by - Mediator AuthorRFC5322.From: Set by - original Author- Names and email addresses for the original Author of the message content are kept 🍓
RFC5322.Reply-To: Set by - Mediator or original Author- It's problematic, but it's super common for a Mailing List to put its own addresses in the Reply-To: header field of the messages it posts 😬 This is meant to make sure replies go to all list members, instead of only the original Author. As a User Actor, a Mailing List is the Author of the new message and can legitimately set the Reply-To: value. But as a Mediator trying to represent the message on behalf of its original Author, creating or changing a Reply-To: field can count as going against that Author's intent. When the Reply-To is changed like this, a reply that was only meant for the original Author will go to the whole list instead 📢 When the Mailing List doesn't set the field, a reply meant for the whole list can end up going only to the original Author. Honestly, either choice is just a matter of group culture for that particular list 🤷♀️
RFC5322.Sender: Set by - Author Originator or Mediator Originator- This field usually has the address of the Actor who's in charge of Mailing List operations 🧑💻 Mailing Lists that work like a simple MTA Relay keep as much of the original handling information as they can, including the original
RFC5322.Senderfield. (Heads up that this way of operating makes the Mailing List act a lot like an Alias, the only difference being how many new addressees there are 🎀)
- This field usually has the address of the Actor who's in charge of Mailing List operations 🧑💻 Mailing Lists that work like a simple MTA Relay keep as much of the original handling information as they can, including the original
RFC5322.To/.CC: Set by - original Author- These fields usually hold the original list of Recipient addresses 💖
RFC5321.MailFrom: Set by - Mediator Originator- A Mailing List can change the message content in any way, so it's responsible for that content, which means it's an Author 👑 That means the Mailing List specifies the Return Address. It's possible for the Mailing List to reuse the Return Address the original Originator used, but notifications sent to that address after the Mailing List has processed a message could be a problem 😅
5.4. Gateways 🌉
okay so a Gateway does the same basic routing and transfer work as any message relay 📬, but it's also allowed to modify content, structure, address, or attributes 💅 when it needs to, so the message can get into a messaging environment that runs on different standards or potentially incompatible policies ✨. When a Gateway connects two different messaging services, its role is super easy to spot 👀. When it connects environments that follow similar technical standards but have significantly different administrative policies, it's easy to just think of the Gateway as merely an MTA 🤷♀️.
The critical distinction between an MTA and a Gateway is that a Gateway can make substantive changes to a message so it can map between the standards 🔁. Almost always, this mapping causes some degree of semantic loss 😬. So the big challenge of Gateway design is to minimize that loss 🎯. Standardized Gateways to Internet Mail include facsimile [RFC4143], voicemail [RFC3801], and the Multimedia Messaging Service (MMS) [RFC4356] 📠📞📱.
A Gateway can set any identity field that's available to an MUA 🪪. On top of the core set of message information listed at the beginning of this section, these identities are usually the ones that matter for Gateways:
RFC5322.From:Set by - original Author- Names and addresses for the original Author of the message content are kept 💖. Just like all the original addressing info in the message, the Gateway can translate addresses as needed so they stay useful in the target environment 🗺️.
RFC5322.Reply-To:Set by - original Author- It's best for a Gateway to keep this info if it's there 🙌. Being able to do a successful reply as a Recipient is a classic test of whether a Gateway is working ✅.
RFC5322.Sender:Set by - Author Originator or Mediator Originator- This field can keep the original value, or it can be set to a new address 🔄.
RFC5322.To/.CC/.BCC:Set by - original Recipient- These fields usually keep their original addresses 🫶.
RFC5321.MailFrom:Set by - Author Originator or Mediator Originator- The Actor handling the message can specify a new address to receive handling notices 📨.
5.5. Boundary Filter 🛡️
okay, security bestie moment 🥹. To enforce security boundaries, organizations can have messages analyzed for conformance with their safety policies 🔍. A classic example is detecting content classed as spam or a virus 🦠🚫. A filter might change the content to make it safe, like by removing content that's deemed unacceptable ✂️. Typically, these actions add content to the message that records what was done 📝.
6. Considerations 💅
6.1. Security Considerations 😱
this part is serious, so let's get into it 🫡. This document describes the existing Internet Mail architecture. It introduces no new capabilities 🙅♀️. The security considerations of this deployed architecture are documented extensively in the technical specifications referenced by this document 📚. These specifications cover classic security topics, like authentication and privacy 🔐. For example, email-transfer protocols can use standardized mechanisms for operation over authenticated and/or encrypted links, and message content has similar protection standards available 🔒. Examples of these mechanisms include SMTP-TLS [RFC3207], SMTP-Auth [RFC4954], OpenPGP [RFC4880], and S/MIME [RFC3851] ✨.
The core of the Internet Mail architecture does not impose any security requirements or functions on the end-to-end or hop-by-hop components 🙈. For example, it does not require participant authentication and does not attempt to prevent data disclosure 😭.
Particular message attributes might expose specific security considerations 👀. For example, the blind carbon copy feature of the architecture invites disclosure concerns, as discussed in Section 7.2 of [RFC5321] and Section 5 of [RFC5322] 🫣. Transport of text or non-text content in this architecture has security considerations that are discussed in [RFC5322], [RFC2045], [RFC2046], and [RFC4288]. Also, security considerations are present for some of the media types registered with IANA 🧾.
Agents that automatically respond to email raise significant security considerations, as discussed in [RFC3834] 🤖⚠️. Gateway behaviors affect end-to-end security services, as discussed in [RFC2480] 🌉🔐. Security considerations for boundary filters are discussed in [RFC5228] 🛡️.
See Section 7.1 of [RFC5321] for a discussion of origination validation 🕵️♀️. As mentioned in Section 4.1.4, it's common practice for components of this architecture to use the RFC0791.SourceAddr to make policy decisions [RFC2505], although the address can be "spoofed" 🫢. It's possible to use it without authorization 💀. SMTP and Submission authentication ([RFC4409], [RFC4954]) provide more secure alternatives 🔑✨.
The discussion of trust boundaries, ADMDs, Actors, roles, and responsibilities in this document highlights the relevance and potential complexity of security factors for operating an Internet Mail service 🧩. The core design of Internet Mail, which encourages open and casual exchange of messages, has met with scaling challenges 📈. The population of email participants has grown to include those with problematic practices 😤. For example, spam, as defined in [RFC2505], is a by-product of this architecture 🗑️. A number of Standards Track or BCP documents on the subject have been issued (see [RFC2505], [RFC5068], and [RFC5235]) 📜.
6.2. Internationalization 🌍
the core Internet email standards are based on US-ASCII -- that's SMTP [RFC5321] and IMF [RFC5322], plus their predecessors 🔤. They describe the transport and composition of messages as composed strictly of US-ASCII 7-bit encoded characters 🔡. The standards have been gradually upgraded to allow characters outside that limited set, while keeping backwards-compatibility mechanisms 🪄. Specifically:
- The MIME specifications ([RFC2045], [RFC2046], [RFC2047], [RFC2049]) allow for the use of coded character sets and character-encoding schemes ("charsets" in MIME terminology) other than US-ASCII 🎨. MIME's [RFC2046] lets the textual content of a message have a label that specifies the charset used in that content 🏷️. Equally, MIME's [RFC2047] lets the textual content of certain header fields be labeled in the same way 🏷️. However, since messages might be transported over SMTP implementations that can only handle 7-bit encoded characters, MIME's [RFC2045] also provides "content transfer encoding" so characters from other charsets can be re-encoded as an overlay to US-ASCII 🧁.
- MIME's [RFC2045] allows the textual content of a message to be in an 8-bit character-encoding scheme 💫. To transport these without re-encoding them, the SMTP specification supports an option [RFC1652] that permits the transport of such textual content 🚚. However, the [RFC1652] option does not address the use of 8-bit content in message header fields, so [RFC2047] encoding is still required for those 🙃.
- A series of experimental protocols on Email Address Internationalization (EAI) have been released that extend SMTP and IMF so 8-bit encoded characters can appear in addresses and other information throughout the header fields of messages 🌏✨. [RFC5335] specifies the format of such message header fields (which encode the characters in UTF-8), and [RFC5336] specifies an SMTP option for transporting these messages 📦.
- MIME's [RFC2045] and [RFC2046] allow for the transport of true multimedia material 🎬🎵. This material enables internationalization because it isn't restricted to any particular language or locale 🗣️.
- The formats for Delivery Status Notifications (DSNs -- [RFC3462], [RFC3463], [RFC3464]) and Message Disposition Notifications (MDNs -- [RFC3798]) include both a structured and an unstructured representation of the notification 📬. If the unstructured representation is in the wrong language or is otherwise unsuitable for use, this lets an MUA build its own appropriately localized notification for display to the User 🌐💖.
- POP and IMAP have no trouble handling MIME messages, including ones containing 8bit, so they aren't a source of internationalization issues 🙌.
Hence, the use of UTF-8 is fully established in existing Internet Mail 🎉. However, support for long-standing encoding forms is retained and is still used 🫶.
7. References 📚
7.1. Normative References ✨
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.
[RFC2645] Gellens, R., "ON-DEMAND MAIL RELAY (ODMR) SMTP with
Dynamic IP Addresses", RFC 2645, August 1999.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC3192] Allocchio, C., "Minimal FAX address format in Internet
Mail", RFC 3192, October 2001.
[RFC3297] Klyne, G., Iwazaki, R., and D. Crocker, "Content
Negotiation for Messaging Services based on Email",
RFC 3297, July 2002.
[RFC3458] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
[RFC3462] Vaudreuil, G., "The Multipart/Report Content Type for the
Reporting of Mail System Administrative Messages",
RFC 3462, January 2003.
[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC3798] Hansen, T. and G. Vaudreuil, "Message Disposition
Notification", RFC 3798, May 2004.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, March 2005.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 4289, December 2005.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
[RFC4550] Maes, S. and A. Melnikov, "Internet Email to Support
Diverse Service Environments (Lemonade) Profile",
RFC 4550, June 2006.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248, June 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.7.2. Informative References 📚
[RFC0733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
"Standard for the format of ARPA network text messages",
RFC 733, November 1977.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1506] Houttuin, J., "A Tutorial on Gatewaying between X.400 and
Internet Mail", RFC 1506, August 1993.
[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
RFC 1652, July 1994.
[RFC1733] Crispin, M., "Distributed Electronic Mail Models in
IMAP4", RFC 1733, December 1994.
[RFC1767] Crocker, D., "MIME Encapsulation of EDI Objects",
RFC 1767, March 1995.
[RFC1985] De Winter, J., "SMTP Service Extension for Remote Message
Queue Starting", RFC 1985, August 1996.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[RFC2142] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997.
[RFC2442] Freed, N., Newman, D., and Hoy, M., "The Batch SMTP Media
Type", RFC 2442, November 1998.
[RFC2480] Freed, N., "Gateways and MIME Security Multiparts",
RFC 2480, January 1999.
[RFC2505] Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs",
BCP 30, RFC 2505, February 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
January 2003.
[RFC3801] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
Mail - version 2 (VPIMv2)", RFC 3801, June 2004.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC3885] Allman, E. and T. Hansen, "SMTP Service Extension for
Message Tracking", RFC 3885, September 2004.
[RFC4142] Crocker, D. and G. Klyne, "Full-mode Fax Profile for
Internet Mail (FFPIM)", RFC 4142, November 2005.
[RFC4143] Toyoda, K. and D. Crocker, "Facsimile Using Internet Mail
(IFAX) Service of ENUM", RFC 4143, November 2005.
[RFC4356] Gellens, R., "Mapping Between the Multimedia Messaging
Service (MMS) and Internet Mail", RFC 4356, January 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[RFC5068] Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
Finch, "Email Submission Operations: Access and
Accountability Requirements", BCP 134, RFC 5068,
November 2007.
[RFC5235] Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
Extensions", RFC 5235, January 2008.
[RFC5335] Abel, Y., "Internationalized Email Headers", RFC 5335,
September 2008.
[RFC5336] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email Addresses", RFC 5336, September 2008.
[Tussle] Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
"Tussle in Cyberspace: Defining Tomorrow's Internet",
ACM SIGCOMM, 2002.Appendix A. Acknowledgments 🫶
This work began in 2004 and has evolved through lots of rounds of community review, and it actually grew out of a section in an early version of [RFC5068]. Over its 5 years of development, the document went through 14 incremental versions, with seriously vigorous community review that led to a ton of substantive changes 🔥. Review happened in the IETF and in other email technical venues. Even though it wasn't a formal IETF activity, issues with the document's contents got resolved using the classic IETF style of open, group decision-making 💜. It's already cited in other work, like the IMAP and Sieve specifications and in academic classwork 📚. Standardizing it is useful because it gives the Internet's now-complex email service a solid, stable reference ✨.
The Originator Actor role got a lot clearer thanks to discussions in the IETF's Marid working group 🎀.
Graham Klyne, Pete Resnick, and Steve Atkins gave super thoughtful insight on the framework and details of the original drafts 🌸. Chris Newman did the same for the final versions, and he was also the cognizant Area Director for the document 👑. Tony Hansen served as document shepherd through the IETF process 🧚.
Later reviews and suggestions came from Eric Allman, Nathaniel Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch, Ned Freed, Eric Hall, Willemien Hoogendoorn, Brad Knowles, John Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg, Alexey Melnikov, der Mouse, S. Moonesamy, Daryl Odnert, Rahmat M. Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, Greg Vaudreuil, Patrick Cain, Paul Hoffman, Vijay Gurbani, and Hans Lachman 💖.
Diligent early proof-reading was done by Bruce Lilly 🍓, and diligent professional technical editing was provided by Susan Hunziker 💅.
The final stages of development were guided by a design team made up of Alexey Melnikov, Pete Resnick, Carl S. Gutekunst, Jeff Macdonald, Randall Gellens, Tony Hansen, and Tony Finch 🌈. Pete Resnick developed the final version of the section on internationalization 🦋.
Index ✨
okay bestie, here's the index, the cheat sheet for finding every term in this whole doc 🗂️💅
Index
7
7-bit 44
A
accountability 12
accountable 13-14
Actor
Administrative 14
Author 10
Consumer 15
Edge 15
Gateway 13
Originator 12
Recipient 10
Return Handler 10
Transit 15
actor 7, 19, 26, 28-29, 35-36, 38-40, 42-43, 49
Actors
MHS 11
addr-spec 17
address
addr-spec 17
local-part 18
ADMD 12, 14-15, 19, 25, 31, 37
Administrative Actors 14
Administrative Management Domain 12
aMSA 31
Author 10-11
author 35
B
body parts 24
bounce handler 10
boundary 15
C
charset 44
Consumer Actor 15
content 11, 13-14, 20, 24, 32
D
delivery 4, 10-11, 13-14, 18, 24-25, 35, 37-38
Discussion of document 7
domain name 17, 21, 28
DSN 44
E
EAI 44
Edge Actor 15
encoding 44
end-to-end 4-6, 11, 15, 28
envelope 10, 13, 21, 24-25, 32, 37
ETRN 35
G
Gateway 11, 13
gateway 6, 12-13, 18, 25, 32
H
header 24
hMSA 31
I
identifier 18-19, 21, 25, 29
IMAP 24, 31, 34-35, 44
IMF 19, 24, 44
Internet Mail 4
L
left-hand side 18
LMTP 24, 35
local-part 18
M
Mail 4
Mail From 37
Mail Submission Agent 12
mailbox 17, 19, 24, 28, 30, 33, 37-38
MDA 24, 37
MDN 10, 24, 44
message 6, 24
Message Disposition Notification 10
Message Handling Service 4
Message Handling System 11
Message Transfer Agent 4
Message User Agent 4
MHS 4, 10-13, 21-22, 24-25
Actors 11
MIME 24, 44
MS 24
MSA 12, 24, 31
MTA 4, 15
boundary 15
MUA 4, 14, 24, 30-31
O
ODMR 35
operations 3, 15, 18, 29, 40
Originator 10-12
P
POP 24, 31, 34-35, 44
posting 4, 10, 12, 21, 30-31, 35, 37
pull 35
push 35
R
RcptTo 11
Receiver 11
Recipient 10-11, 37
recipient 35
relay 11
responsibility 31
responsible 13-14
Return Address 37
Return Handler 10
role 10, 18
Author 10
Originator 12
Recipient 10
S
SIEVE 24-25
SMTP 24, 35, 44
T
transfer 11, 13-14
Transit Actor 15
transition 31
U
UA 4
User Agent 4Author's Address 💌
the author, Dave Crocker, is at Brandenburg InternetWorking, 675 Spruce Drive, Sunnyvale, CA 94086, USA 🌸
📞 Phone: +1.408.246.8253 📧 EMail: dcrocker@bbiw.net 🦋
📦 where this page comes from
- ietf: rfc5598 2009-07
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms