E-mail client - AbsoluteAstronomy.com

An email client, email reader, or more formally mail user agent (MUA), is a computer program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

used to manage a user's email

Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

.

The term can refer to any system capable of accessing the user's email mailbox, regardless of it being a mail user agent, a relaying server, or a human typing on a terminal. In addition, a web application that provides message management, composition, and reception functions is sometimes also considered an email client, but more commonly referred to as webmail.

Popular email clients include Microsoft Outlook

Microsoft Outlook

Microsoft Outlook is a personal information manager from Microsoft, available both as a separate application as well as a part of the Microsoft Office suite...

, Pegasus Mail

Pegasus Mail

Pegasus Mail is a donationware , proprietary, email client that is developed and maintained by David Harris and his team. It was originally released in 1990 for internal and external mail on Netware networks with MS-DOS clients, and was subsequently ported to Microsoft Windows...

, Mozilla

Mozilla Foundation

The Mozilla Foundation is a non-profit organization that exists to support and provide leadership for the open source Mozilla project. The organization sets the policies that govern development, operates key infrastructure and controls trademarks and other intellectual property...

's Thunderbird

Mozilla Thunderbird

Mozilla Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a web browser...

, and Apple Inc.'s Mail

Mail (application)

Mail is an email program included with Apple Inc.'s Mac OS X operating system. Originally developed by NeXT as NeXTMail, a part of their NeXTSTEP operating system, it was adapted, following Apple's acquisition of NeXT, to become OS X's Mail application.Mail uses the SMTP, POP3, and IMAP protocols,...

Retrieving messages from a mailbox

Like most client programs, an email client is only active when a user runs it. The most common arrangement is for a remote Mail Transfer Agent

Mail transfer agent

Within Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...

(MTA) server, using a suitable mail delivery agent

Mail delivery agent

A mail delivery agent or message delivery agent is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's mailbox...

(MDA), to add email messages to a client's storage as they arrive. The remote mail storage is referred to as the user's mailbox. The default setting on many Unix systems is for the mail server to store formatted messages in mbox

Mbox

mbox is a generic term for a family of related file formats used for holding collections of electronic mail messages. All messages in an mbox mailbox are concatenated and stored as plain text in a single file...

, within the user's HOME directory. Of course, users of the system can log-in and run a mail client on the same computer that hosts their mailboxes. In the latter case, the server is not actually remote; it is remote in the most common cases, though.

Emails are stored in the user's mailbox on the remote server until the user's email client requests them to be downloaded to the user's computer, or can otherwise access the user's mailbox on the possibly remote server. The email client can be set up to connect to multiple mailboxes at the same time and to request the download of emails either automatically, such as at pre-set intervals, or the request can be manually initiated by the user.

A user's mailbox can be accessed in two dedicated ways. The Post Office Protocol

Post Office Protocol

In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...

(POP) allows the user to download messages one at a time and only deletes them from the server after they have been successfully saved on local storage. It is possible to leave messages on the server to permit another client to access them. However, there is no provision for flagging a specific message as seen, answered, or forwarded, thus POP is not convenient for users who access the same mail from different machines.

Alternatively, the Internet Message Access Protocol

Internet Message Access Protocol

Internet message access protocol is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol...

(IMAP) allows users to keep messages on the server, flagging them as appropriate. IMAP provides folders and sub-folders, that can be shared among different users with possibly different access rights. Typically, the Sent, Drafts, and Trash folders are created by default. IMAP features an idle extension

IMAP IDLE

In e-mail technology, IDLE is an IMAP feature described in RFC 2177 that allows a client to indicate to the server that it is ready to accept real-time notifications.- Significance :...

for real time updates, providing faster notification than polling, where long lasting connections are feasible.

In addition, the mailbox storage can be accessed directly by programs running on the server or via shared disks. Direct access can be more efficient but is less portable as it depends on the mailbox format; it is used by some email clients, including some webmail applications.

Message composition

Email clients usually contain user interface

Graphical user interface

In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...

s to display and edit text. Some applications permit the use of program-external editor.

The email clients will perform formatting according to RFC 5322 for headers and body, and MIME

MIME

Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...

for non-textual content and attachments. Headers include the destination fields, To, Cc, and Bcc, and the originator fields From which is the message's author(s), Sender in case there are more authors, and Reply-To in case responses should be addressed to a different mailbox. To better assist the user with destination fields, many clients maintain one or more address books and/or are able to connect to an LDAP directory server. For originator fields, clients may support different identities.

Client settings require the user's real name and email address for each user's identity, and possibly a list of LDAP servers.

Submitting messages to a server

When a user wishes to create and send an email, the email client will handle the task. The email client is usually set up automatically to connect to the user's mail server, which is typically either an MSA

Mail submission agent

A mail submission agent is a computer program or software agent that receives electronic mail messages from a mail user agent and cooperates with a mail transfer agent for delivery of the mail...

or an MTA, two variations of the SMTP protocol. The email client which uses the SMTP protocol creates an authentication extension, which the mail server uses to authenticate the sender. This method eases modularity and nomadic computing. The older method was for the mail server to recognize the client's IP address, e.g. because the client is on the same machine and uses internal address 127.0.0.1, or because the client's IP address is controlled by the same internet service provider

Internet service provider

An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...

that provides both internet access and mail services.

Client settings require the name or IP address of the preferred outgoing mail server, the port number (25 for MTA, 587 for MSA), and the user name and password for the authentication, if any. There is a non-standard port 465 for SSL

Transport Layer Security

Transport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...

encrypted SMTP sessions, that many clients and servers support for backward compatibility.

Encryption

With no encryption, much like for postcards, email activity is plainly visible by any occasional eavesdropper. Email encryption enables to safeguard privacy by encrypting the mail sessions, the body of the message, or both. Without it, anyone with network access and the right tools can monitor email and obtain login passwords. Examples of concern include the government (warrantless wiretapping, great firewall of China) and fellow wireless network users such as at an Internet cafe

Internet cafe

An Internet café or cybercafé is a place which provides internet access to the public, usually for a fee. These businesses usually provide snacks and drinks, hence the café in the name...

Encryption of mail sessions

All relevant email protocols have an option to encrypt the whole session, to prevent a user's name and password from being sniffed

Packet sniffer

A packet analyzer is a computer program or a piece of computer hardware that can intercept and log traffic passing over a digital network or part of a network...

. They are strongly suggested for nomadic users and whenever the internet access provider is not trusted. When sending mail, users can only control encryption at the first hop from a client to its configured outgoing mail server. At any further hop, messages may be transmitted with or without encryption, depending solely on the general configuration of the transmitting server and the capabilities of the receiving one.

Encrypted mail sessions deliver messages in their original format, i.e. plain text or encrypted body, on a user's local mailbox and on the destination server's. The latter server is operated by an email hosting service provider, possibly a different entity than the internet access provider currently at hand.

Encryption of the message body

There are two models for managing cryptographic keys. S/MIME

S/MIME

S/MIME is a standard for public key encryption and signing of MIME data. S/MIME is on an IETF standards track and defined in a number of documents, most importantly RFCs. S/MIME was originally developed by RSA Data Security Inc...

employs a model based on a trusted certificate authority

Certificate authority

In cryptography, a certificate authority, or certification authority, is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate...

(CA) that signs users' public keys. OpenPGP employs a somewhat more flexible web of trust
Web of trust
In cryptography, a web of trust is a concept used in PGP, GnuPG, and other OpenPGP-compatible systems to establish the authenticity of the binding between a public key and its owner. Its decentralized trust model is an alternative to the centralized trust model of a public key infrastructure ,...

mechanism that allows users to sign one another's public keys. OpenPGP is also more flexible in the format of the messages, in that it still supports plain message encryption and signing as they used to work before MIME

MIME

standardization.

In both cases, only the message body is encrypted. Header fields, including originator, recipients, and subject, remain in plain text.

Webmail

In addition to the fat client

Fat client

A fat client is a computer in client–server architecture or networks that typically provides rich functionality independent of the central server...

email clients and small email clients, there are also Web

World Wide Web

The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...

-based email applications called webmail. Webmail has several advantages, including an ability to send and receive email away from the user's normal base using a web browser

Web browser

A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...

, thus eliminating the need for an email client.

Some websites are dedicated to providing email services, including Hotmail

Hotmail

Windows Live Hotmail, formerly known as MSN Hotmail and commonly referred to simply as Hotmail, is a free web-based email service operated by Microsoft as part of its Windows Live group. It was founded by Sabeer Bhatia and Jack Smith and launched in July 1996 as "HoTMaiL". It was one of the first...

, Gmail

Gmail

Gmail is a free, advertising-supported email service provided by Google. Users may access Gmail as secure webmail, as well via POP3 or IMAP protocols. Gmail was launched as an invitation-only beta release on April 1, 2004 and it became available to the general public on February 7, 2007, though...

, AOL, and Yahoo

Yahoo! Mail

Yahoo! Mail is a web mail service provided by Yahoo!. It was inaugurated in 1997, and, according to comScore, Yahoo! Mail was the second largest web-based email service with 273.1 million users as of November 2010....

; but there are many internet service provider

Internet service provider

s which provide webmail services as part of their internet service package. The main limitations of webmail are that user interactions are subject to the website's operating system and the general inability to download email messages and compose or work on the messages offline, although Gmail does offer Offline Gmail through the installation of Gears

Gears (software)

Gears, formerly Google Gears, is software offered by Google that "enables more powerful web applications, by adding new features to your web browser. It allows some online files to be used offline"...

. The advantage of webmail provided by a regular mail server is that email remains on the mail server until the user can return to the base computer, when they can be downloaded. Users may be able to choose whether to leave a copy of the email on the server for a backup.

A major disadvantage of webmail is that the hosting corporation or institution retains control over the individual's email as it is performing a storage function in addition to the service function. Since the sole storage location is hosted and controlled by the corporation or institution the individual does not "have" their email but only has "access" to it and that access is under the sole control of the corporation or institution. This becomes a problem when users loses their email account through hacking or malice and are unable to retrieve the only copies of their stored email. Webmail will also be affected by the speed and quality of the internet connection and this may be a problem for dial-up connection users. A major advantage of webmail is that the individual's email is available everywhere there is an internet connection and a browser and the individual does not need a computer with their mail application installed in it. With webmail the users' email is usually backed up with multiple redundancy and corporations and institutions usually provide extremely reliable service as well as excellent spam filtering services. Privacy concerns have been raised about webmail as corporations are storing large amounts of personal information.

Protocols

While popular protocols for retrieving mail include POP3 and IMAP4, sending mail is usually done using the SMTP protocol.

Another important standard supported by most email clients is MIME

MIME

, which is used to send binary file

Binary file

A binary file is a computer file which may contain any type of data, encoded in binary form for computer storage and processing purposes; for example, computer document files containing formatted text...

email attachments. Attachments are files that are not part of the email proper, but are sent with the email.

Most email clients use a User-Agent header field

Header (information technology)

In information technology, header refers to supplemental data placed at the beginning of a block of data being stored or transmitted. In data transmission, the data following the header are sometimes called the payload or body....

to identify the software used to send the message. According to RFC 2076, this is a common but non-standard header field.

RFC 6409, Message Submission for Mail, details the role of the Mail submission agent

Mail submission agent

A mail submission agent is a computer program or software agent that receives electronic mail messages from a mail user agent and cooperates with a mail transfer agent for delivery of the mail...

.

RFC 5068, Email Submission Operations: Access and Accountability Requirements, provides a survey of the concepts of MTA, MSA, MDA, and MUA. It mentions that "Access Providers MUST NOT block users from accessing the external Internet using the SUBMISSION port 587" and that "MUAs SHOULD use the SUBMISSION port for message submission."

Port numbers

Email servers and client use the following TCP port numbers by convention, but customized configuration exist:

protocol	use	plain text or encrypt sessions	plain text sessions only	encrypt sessions only
POP3 Post Office Protocol In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...	incoming mail	110		995
IMAP4 Internet Message Access Protocol Internet message access protocol is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol...	incoming mail	143		993
SMTP Simple Mail Transfer Protocol Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...	outgoing mail	25		(unofficial) 465
MSA Mail submission agent A mail submission agent is a computer program or software agent that receives electronic mail messages from a mail user agent and cooperates with a mail transfer agent for delivery of the mail...	outgoing mail	587
HTTP	webmail		80	443

Note that while webmail obeys the earlier HTTP disposition of having separate ports for encrypt and plain text sessions, mail protocols use the STARTTLS

STARTTLS

STARTTLS is an extension to plain text communication protocols, which offers a way to upgrade a plain text connection to an encrypted connection instead of using a separate port for encrypted communication....

technique, thereby allowing encryption to start on an already established TCP connection. RFC 2595 discourages the use of the previously established ports 995 and 993.

Proprietary client protocols

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

mail systems define the proprietary

Proprietary software

Proprietary software is computer software licensed under exclusive legal right of the copyright holder. The licensee is given the right to use the software under certain conditions, while restricted from other uses, such as modification, further distribution, or reverse engineering.Complementary...

Messaging Application Programming Interface

Messaging Application Programming Interface is a messaging architecture and a Component Object Model based API for Microsoft Windows. MAPI allows client programmes to become messaging-enabled, -aware, or -based by calling MAPI subsystem routines that interface with certain messaging servers...

(MAPI) that is used in client applications, such as Microsoft Outlook

Microsoft Outlook

Microsoft Outlook is a personal information manager from Microsoft, available both as a separate application as well as a part of the Microsoft Office suite...

, to access Microsoft Exchange

Microsoft Exchange Server

Microsoft Exchange Server is the server side of a client–server, collaborative application product developed by Microsoft. It is part of the Microsoft Servers line of server products and is used by enterprises using Microsoft infrastructure products...

electronic mail servers.

History

See for early history of email clients. Email clients pre-date networks, with early email providing only local delivery on mainframe

Mainframe computer

Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

computers in the 1960s.

Retrieving messages from a mailbox

Message composition

Submitting messages to a server

Encryption

Encryption of mail sessions

Encryption of the message body

Webmail

Protocols

Port numbers

Proprietary client protocols

History

See also