Email agent (infrastructure)

Last updated
Schema of e-mail delivery Schema of e-mail delivery.svg
Schema of e-mail delivery

An e-mail agent is a program that is part of the e-mail infrastructure, from composition by sender, to transfer across the network, to viewing by recipient. The best-known are message user agents (MUAs, aka, e-mail clients) and message transfer agents (MTAs, programs that transfer e-mail between clients), but finer divisions exist.

Contents

More precisely, this is a technical way of referring to functions performed by various programs, considering them as software agents: a given program may perform several functions, but while it is performing a given function (such as message transfer), it is referred to as a message transfer agent. These terms are most often used within internet standards, and technical discussions of e-mail infrastructure, not by end-users.

While the individual terms are widely used in internet standards and RFCs, there is no widely used umbrella term for these programs, though such a program may informally be referred to generically as an MxA, 'x' being a wildcard, as the programs are referred to by acronyms of this form, such as MUA and MTA, with 'x' depending on role.

E-mail servers are built from one or more software packages, each of which carries out the functions of one or more MxA. [1] [2]

Classification

The finest and most expansive classification in current use is into five functions in addition to the mail exchanger (MX): [3]

The traditional division is into client-side (MUA) and server-side (MTA, notably sendmail), with the flow given as: [17]

MUA → MTA → … → MTA → MUA,

Other divisions have been made to draw distinctions that some have found useful, which are detailed as follows.

A detailed flow of a message through these various agents is given at , and may be summarized as

MUA → MSA → MTA → … → MTA → MDA →→ MRA →→ MUA,

with the arrow styles changing to distinguish between push steps (→) and pull steps (→→).

Another source gives the flow as: [18]

MUA → (MSA) → MTA → … → MX → MDA →→ MRA/MUA,

Programs such as fetchmail which retrieve mail from a server but do not provide a human interface for viewing or other client tasks are referred to as MRAs – they provide retrieval but no other client functions. Traditionally and in internet standards (such as the recent RFC 5598) these are referred to as a type of MUA, because they are client-side and hence outside the scope of internet standards, and indeed many MUAs perform MRA functions. However, traditional Unix mail readers such as elm, Pine, or mutt would often not include MRA functions (or only optionally), reading mail that had already been delivered to a mailbox file (formally, delivered by an MDA).

Broadly and traditionally, any program that transfers mail between the ends (all server-side functions) is an MTA. More finely and more recently, the endpoints of the chain have been distinguished, with the initial client-server step referred to as submission, and the final server-client step referred to as delivery. The motivation for distinguishing the MSA role has largely been security, with MUA–MSA interactions (initial submission) receiving greater scrutiny than MTA–MTA (server–server) transfers. The delivery (MDA) stage is where such tasks as filtering (of undesired messages) and filing (into separate folders) occur, and are the start of the user agent; traditionally this was done via procmail, while today it may be done via server-side programs, often using spam filters such as SpamAssassin. The MDA can be said to work "before the message hits the user's mailbox".

See also

Related Research Articles

Email Electronic mail

Electronic mail is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic (digital) version of, or counterpart to, mail, at a time when "mail" meant only physical mail. Email later became a ubiquitous communication medium, to the point that in current use, an e-mail address is often treated as a basic and necessary part of many processes in business, commerce, government, education, entertainment, and other spheres of daily life in most countries. Email is the medium, and each message sent therewith is called an email.

In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection. IMAP is defined by RFC 9051.

Within the Internet email system, a message transfer agent (MTA), or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using SMTP. The terms mail server, mail exchanger, and MX host are also used in some contexts.

In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. POP version 3 (POP3) is the version in common use.

The Simple Mail Transfer Protocol (SMTP) is an internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. For retrieving messages, IMAP is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.

Email client Computer program used to access and manage a users email

An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email.

Maildir E-mail format

The Maildir e-mail format is a common way of storing email messages in which each message is stored in a separate file with a unique name, and each mail folder is a file system directory. The local file system handles file locking as messages are added, moved and deleted. A major design goal of Maildir is to eliminate the need for program code to handle file locking and unlocking.

A message delivery agent (MDA), or mail delivery agent, is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's mailbox. It is also called a local delivery agent (LDA).

Mutt (email client) Text-based email client for Unix-like systems

Mutt is a text-based email client for Unix-like systems. It was originally written by Michael Elkins in 1995 and released under the GNU General Public License version 2 or any later version.

A bounce message or just "bounce" is an automated message from an email system, informing the sender of a previous message that the message has not been delivered. The original message is said to have "bounced".

In email, a return receipt is an acknowledgment by the recipient's email client to the sender of receipt of an email message. What acknowledgment, if any, is sent by the recipient to the sender is dependent on the email software of the recipient.

Email authentication, or validation, is a collection of techniques aimed at providing verifiable information about the origin of email messages by validating the domain ownership of any message transfer agents (MTA) who participated in transferring and possibly modifying a message.

Message submission agent

A message submission agent (MSA), or mail submission agent, is a computer program or software agent that receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail. It uses ESMTP, a variant of the Simple Mail Transfer Protocol (SMTP), as specified in RFC 6409.

Dovecot (software)

Dovecot is an open-source IMAP and POP3 server for Unix-like operating systems, written primarily with security in mind. Timo Sirainen originated Dovecot and first released it in July 2002. Dovecot developers primarily aim to produce a lightweight, fast and easy-to-set-up open-source email server.

For a RFC 5321 mail transfer agent (MTA), the Sender Rewriting Scheme (SRS) is a scheme for rewriting the envelope sender address of an email message, in view of remailing it. In this context, remailing is a kind of email forwarding. SRS was devised in order to forward email without breaking the Sender Policy Framework (SPF), in 2003.

The UW IMAP server was the reference server implementation of the Internet Message Access Protocol. It was developed at the University of Washington by Mark Crispin and others.

Push email is an email system that provides an always-on capability, in which new email is actively transferred (pushed) as it arrives by the mail delivery agent (MDA) to the mail user agent (MUA), also called the email client. Email clients include smartphones and, less strictly, IMAP personal computer mail applications.

Email forwarding generically refers to the operation of re-sending an email message delivered to one email address to one or more different email addresses.

A mail retrieval agent (MRA) is a computer application that retrieves or fetches e-mail from a remote mail server and works with a mail delivery agent to deliver mail to a local or remote email mailbox. MRAs may be external applications by themselves or be built into a bigger application like an MUA. Significant examples of standalone MRAs include fetchmail and getmail.

A mailbox is the destination to which electronic mail messages are delivered. It is the equivalent of a letter box in the postal system.

References

  1. ↑ Schroder 2004, p. 362: "There are several ways to build a Linux mail server. Most admins take the modular approach and build it from a collection of specialized programs... Another approach is to use the Courier package, because it is a complete package that contains an MTA, POP3, IMAP, and a mailing list manager. Or purchase a distribution that puts it all together for you, like SuSE OpenExchange."
  2. ↑ McBee 2009, p. 22: "Each e-mail system can use a wide variety of solutions to implement these functions. Some applications, such as Exchange, incorporate all of these functions into a single end-to-end offering, whereas others provide just one piece of the puzzle, relying on other applications to provide the missing functionality. Even when using a complete solution, however, you can always mix and match pieces to provide functionality (such as using a third-party client for MUA functionality, or an edge mail appliance as an MTA to other mail systems). To ensure that these implementations work together, a series of standards have been developed over time."
  3. ↑ Faircloth 2013, p. 51: "SMTP is comprised of a mail submission agent (MSA), a mail user agent (MUA), a mail retrieval agent (MRA), a mail exchanger (MX), a mail delivery agent (MDA), and potentially multiple servers in between known as mail transfer agents (MTAs)."
  4. ↑ McBee 2009, p. 21: "The mail user agent (MUA) is the component that the user directly interacts with. If I were to use a postal metaphor, the MUA is roughly the equivalent of your local mailbox at the end of the driveway. Traditionally, the MUA has been a stand-alone client application such as Outlook; however, a web-based client such as Outlook Web Access also offers MUA functionality, even though it is technically a server-side application."
  5. ↑ Schroder 2004, p. 361: "MUA: Mail user agent, also called 'mail client.' Mutt, Pine, Kmail, Evolution, and Balsa are MUAs. This is the user's program for composing, sending, and receiving e-mail. MUAs can fetch mail from a local folder, or from a remote server via POP and IMAP."
  6. ↑ Vakali 2006, p. 221: "Mail User Agent (MUA): It is responsible for helping the user to read and write e-mail messages. The MUA is usually implemented in software commonly referred to as an e-mail client. Two popular e-mail clients are Microsoft Outlook and Mozilla Thunderbird. These programs transform a text message into the appropriate Internet format in order for the message to reach its destination."
  7. ↑ McBee 2009, p. 22: "Just as the MRA is a variant role often performed by the MUA, the mail submission agent (MSA) is a specialized form of the MTA. It's adapted to accept mail submissions from the MUA, introduce them into the mail flow, and handle any specialized processing that may be required. In Exchange 2007, this function is handled both in the Mailbox role as well as the Client Receive Connector on the Hub Transport role."
  8. 1 2 Bauer 2003, p. 458: "[A] little background on IMAP's role in the e-mail food chain. IMAP, the Internet Message Access Protocol (specified in RFC 3501), is a protocol for mail delivery agents (MDAs). Whereas mail transport agents (MTAs), such as Postfix and Sendmail, move mail between networks, MDAs move mail from MTAs to destination mailboxes. To use a simile from my book Building Secure Servers With Linux, if an MTA is like a mail truck that moves mail between post offices, an MDA is like a letter carrier who delivers mail from the local post office to your house.
    An IMAP-based MDA system has two parts: an IMAP server, which houses user mailboxes and receives mail from some MTA, and a group of users running IMAP client software. The three most popular open-source IMAP servers are University of Washington IMAP (UW IMAP), Cyrus IMAP from Carnegie Mellon University and Courier IMAP from Inter7 Internet Technologies. Popular IMAP client applications include Netscape/Mozilla Communicator, Ximian Evolution, Microsoft Outlook Express, KMail, mutt, pine and Apple Mac OS X Mail."
  9. ↑ McBee 2009, pp. 21–22: "If the MUA is the local mailbox, the mail transport agent (MTA) is the Post Office infrastructure connecting different towns and cities with each other. The MTA is responsible for accepting messages from other systems such as MUAs and MTAs, routing them, and ensuring their delivery on to their recipients. Messages typically travel through two MTAs - the sender's and the recipient's (unless, of course, they share an MTA). In an Exchange 2007 system, the Hub Transport and Edge Transport roles fill the MTA role."
  10. ↑ Schroder 2004, p. 361: "MTA: Mail transfer agent. This moves e-mail between servers. Sendmail, Exim, qmail, and Postfix are MTAs. An MTA must support SMTP."
  11. ↑ Vakali 2006, p. 221: "Mail Transfer Agent (MTA): It accepts a message passed to it by either an MUA or another MTA, and then decides on the appropriate delivery method and the route that the mail should follow. It uses SMTP to send the message to another MTA or a mail delivery agent (MDA)."
  12. ↑ McBee 2009, p. 22: "What's missing from this picture? In this case, it's the equivalent of the local Post Office (or, if you prefer, the mail room in the big corporation) - the mail delivery agent (MDA) or local delivery agent (LDA). Once the incoming message has been delivered to the proper collection of systems, the MDA/LDA is responsible for ensuring it's been put into the correct mailbox."
  13. ↑ Schroder 2004, p. 361: "Mail delivery agent. This is an intermediary between an MTA and a MUA. Procmail and Fetchmail are two popular MDAs. An MDA is not required; it is used for extra functionality, such as filtering, sorting, and autoresponding."
  14. ↑ Vakali 2006, p. 221: "Mail Delivery Agent: It receives messages from MTAs and delivers them to the user's mail box in the user's mail server."
  15. ↑ McBee 2009, p. 21: "The mail retrieval agent, closely related to the MUA, is the component that handles retrieving messages from the main mail store. Depending on which protocols you are using, such as the Post Office Protocol (POP) or Internet Mail Access Protocol (IMAP), you can't just rely on new messages to be pushed to your MUA; something needs to pull them down for you. Typically, the MRA is not a separate component in modern systems, but a set of additional routines in the MUA that support message retrieval."
  16. ↑ Vakali 2006, p. 221: "Mail Retrieval Agent (MRA): It fetches mail from the user's mail server to the user's local in-box. MRAs are often embedded in e-mail clients."
  17. ↑ See Figure 1. Life cycle of an e-mail in Vakali 2006, p. 221
  18. ↑ Faircloth 2013, p. 51: "The process flow for sending an e-mail goes as follows:
    1. The MUA (client) sends the properly formatted mail to the MSA or directly to an MTA
    2. The MSA sends the mail to its MTA
    3. Additional MTAs may be routed through until the e-mail is on a 'boundary MTA'
    4. The boundary MTA performs a query using DNS to identify the MX for the domain the e-mail is intended for
    5. The MTA connects to the MX and transfers the e-mail
    6. The MX transfers the e-mail to the MDA
    7. At this point, the e-mail is transferred to the appropriate internal mail server and stored until the MUA or MRA connects to it and retrieves the e-mail on behalf of the user (usually using the POP or IMAP protocols)"

Bibliography

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.