e-mail

Electronic mail, often abbreviated as email or e-mail, is a method of exchanging digital messages, designed primarily for human use. E-mail systems are based on a store-and-forward model in which e-mail computer server systems accept, forward, deliver and store messages on behalf of users, who only need to connect to the e-mail infrastructure, typically an e-mail server, with a network-enabled device (e.g., a personal computer) for the duration of message submission or retrieval. Rarely is e-mail transmitted directly from one user's device to another's.

An electronic mail message consists of two components, the message header, and the message body, which is the email's content. The message header contains control information, including, minimally, an originator's email address and one or more recipient addresses. Usually additional information is added, such as a subject header field.

Originally a text-only communications medium, email is extended to carry multi-media content attachments, which were standardized in with RFC 2045 through RFC 2049, collectively called, Multipurpose Internet Mail Extensions (MIME).

The foundation for today's global Internet e-mail service was created in the early ARPANET and standards for encoding of messages were proposed as early as, for example, in 1973 (RFC 561). An e-mail sent in the early 1970s looked very similar to one sent on the Internet today. Conversion from the ARPANET to the Internet in the early 1980s produced the core of the current service.

Network-based email was initially exchanged on the ARPANET in extensions to the File Transfer Protocol (FTP), but is today carried by the Simple Mail Transfer Protocol (SMTP), first published as Internet Standard 10 (RFC 821) in 1982. In the process of transporting email messages between systems, SMTP communicates delivery parameters using a message envelope separately from the message (headers and body) itself.

Spelling

There are several spelling variations, which are occasionally the the cause of vehement disagreement.^[1][2]

e-mail is recommended by several prominent journalistic and technical style guides. ^[3][4][5][6]

email is the form required by newer RFCs and IETF working groups ^[7] and is also recognized in many dictionaries.^{[8][9][10][11][12]}

Less common forms include eMail and simply mail.

mail was the form used in the original RFC - the service is referred to as mail, and a single piece of electronic mail is called a message.^[13][14][15]

eMail, with the letter M capitalized, was common with ARPANET users and early developers from Unix, CMS, AppleLink, eWorld, AOL, GEnie, and HotMail.^{[citation needed]}

EMail is the form which has always been used in RFCs for the "Author's Address"^[14][15], and is expressly required "...for historical reasons...".^[16]

Origin

This section requires expansion.

E-mail predates the inception of the Internet, and was in fact a crucial tool in creating the Internet.

MIT first demonstrated the Compatible Time-Sharing System (CTSS) in 1961.^[17] It allowed multiple users to log into the IBM 7094^[18] from remote dial-up terminals, and to store files online on disk. This new ability encouraged users to share information in new ways. E-mail started in 1965 as a way for multiple users of a time-sharing mainframe computer to communicate. Although the exact history is murky, among the first systems to have such a facility were SDC's Q32 and MIT's CTSS.

Host-based mailsystems

The original email systems allowed communication only between users who logged into the one host or "mainframe", but this could be hundreds or thousands of users within a company or university. By 1966 (or earlier, it is possible that the SAGE system had something similar some time before), such systems allowed email between different companies as long as they ran compatible operating systems, but not to other dissimilar systems.

Examples include BITNET, IBM PROFS, Digital All-in-1 and the original Unix mail.

LAN-based mailsystems

From the early 1980s networked personal computers on LANs became increasingly important - and server-based systems similar to the earlier mainframe systems developed, and again initially allowed communication only between users logged into the one server, but these also could generally be linked between different companies as long as they ran the same email system and (proprietary) protocol.

Examples include cc:Mail, WordPerfect Office, Microsoft Mail, Banyan VINES and Lotus Notes - with various vendors supplying gateway software to link these incompatible systems.

Attempts at Interoperability

• Novell briefly championed the open MHS protocol
• uucp was used as an open "glue" between differing mail systems
• X.400 in the early 1990s was mandated for government use under GOSIP but almost immediately abandoned by all but a few — in favour of ARPANET SMTP

The rise of ARPANET-based mail

The ARPANET computer network made a large contribution to the development of e-mail. There is one report that indicates experimental inter-system e-mail transfers began shortly after its creation in 1969.^[19] Ray Tomlinson initiated the use of the "@" sign to separate the names of the user and their machine in 1971.^[20] The ARPANET significantly increased the popularity of e-mail, and it became the killer app of the ARPANET.

Most other networks had their own email protocols and address formats; as the influence of the ARPANET and later the Internet grew, central sites often hosted email gateways that passed mail between the Internet and these other networks. Internet email addressing is still complicated by the need to handle mail destined for these older networks. Some well-known examples of these were UUCP (mostly Unix computers), BITNET (mostly IBM and VAX mainframes at universities), FidoNet (personal computers), and DECNET (various networks).

An example of an Internet email address that routed mail to a user at a UUCP host:

hubhost!middlehost!edgehost!user@uucpgateway.somedomain.example.com

This was necessary because in early years UUCP computers did not maintain (or consult servers for) information about the location of all hosts they exchanged mail with, but rather only knew how to communicate with a few network neighbors; email messages (and other data such as Usenet News) were passed along in a chain among hosts who had explicitly agreed to share data with each other.

Operation overview

The diagram to the right shows a typical sequence of events^[21] that takes place when Alice composes a message using her mail user agent (MUA). She enters the e-mail address of her correspondent, and hits the "send" button.

1. Her MUA formats the message in e-mail format and uses the Simple Mail Transfer Protocol (SMTP) to send the message to the local mail transfer agent (MTA), in this case smtp.a.org, run by Alice's Internet Service Provider (ISP).
2. The MTA looks at the destination address provided in the SMTP protocol (not from the message header), in this case bob@b.org. An Internet e-mail address is a string of the form localpart@exampledomain. The part before the @ sign is the local part of the address, often the username of the recipient, and the part after the @ sign is a domain name or a fully qualified domain name. The MTA resolves a domain name to determine the fully qualified domain name of the mail exchange server in the Domain Name System.
3. The DNS server for the b.org domain, ns.b.org, responds with any MX records listing the mail exchange servers for that domain, in this case mx.b.org, a server run by Bob's ISP.
4. smtp.a.org sends the message to mx.b.org using SMTP, which delivers it to the mailbox of the user bob.
5. Bob presses the "get mail" button in his MUA, which picks up the message using the Post Office Protocol (POP3).

That sequence of events applies to the majority of e-mail users. However, there are many alternative possibilities and complications to the e-mail system:

• Alice or Bob may use a client connected to a corporate e-mail system, such as IBM Lotus Notes or Microsoft Exchange. These systems often have their own internal e-mail format and their clients typically communicate with the e-mail server using a vendor-specific, proprietary protocol. The server sends or receives e-mail via the Internet through the product's Internet mail gateway which also does any necessary reformatting. If Alice and Bob work for the same company, the entire transaction may happen completely within a single corporate e-mail system.
• Alice may not have a MUA on her computer but instead may connect to a webmail service.
• Alice's computer may run its own MTA, so avoiding the transfer at step 1.
• Bob may pick up his e-mail in many ways, for example using the Internet Message Access Protocol, by logging into mx.b.org and reading it directly, or by using a webmail service.
• Domains usually have several mail exchange servers so that they can continue to accept mail when the main mail exchange server is not available.
• E-mail messages are not secure if e-mail encryption is not used correctly.

Many MTAs use to accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called open mail relays. This was very important in the early days of the Internet when network connections were unreliable. If an MTA couldn't reach the destination, it could at least deliver it to a relay closer to the destination. The relay stood a better chance of delivering the message at a later time. However, this mechanism proved to be exploitable by people sending unsolicited bulk e-mail and as a consequence very few modern MTAs are open mail relays, and many MTAs don't accept messages from open mail relays because such messages are very likely to be spam.

Message format

The Internet e-mail message format is defined in RFC 5322 and a series of RFCs, RFC 2045 through RFC 2049, collectively called, Multipurpose Internet Mail Extensions, or MIME. Although as of July 13, 2005, RFC 2822 is technically a proposed IETF standard and the MIME RFCs are draft IETF standards,^[22] these documents are the standards for the format of Internet e-mail. Prior to the introduction of RFC 2822 in 2001, the format described by RFC 822 was the standard for Internet e-mail for nearly 20 years; it is still the official IETF standard. The IETF reserved the numbers 5321 and 5322 for the updated versions of RFC 2821 (SMTP) and RFC 2822, as it previously did with RFC 821 and RFC 822, honoring the extreme importance of these two RFCs. RFC 822 was published in 1982 and based on the earlier RFC 733 (see^[23]).

Internet e-mail messages consist of two major sections:

• Header — Structured into fields such as summary, sender, receiver, and other information about the e-mail.
• Body — The message itself as unstructured text; sometimes containing a signature block at the end. This is exactly the same as the body of a regular letter.

The header is separated from the body by a blank line.

Message header

Each message has exactly one header, which is structured into fields. Each field has a name and a value. RFC 5322 specifies the precise syntax.

Informally, each line of text in the header that begins with a printable character begins a separate field. The field name starts in the first character of the line and ends before the separator character ":". The separator is then followed by the field value (the "body" of the field). The value is continued onto subsequent lines if those lines have a space or tab as their first character. Field names and values are restricted to 7-bit ASCII characters. Non-ASCII values may be represented using MIME encoded words.

Header fields

The message header should include at least the following fields:

• From: The e-mail address, and optionally the name of the author(s). In many e-mail clients not changeable except through changing account settings.
• To: The e-mail address(es), and optionally name(s) of the message's recipient(s). Indicates primary recipients (multiple allowed), for secondary recipients see Cc: and Bcc: below.
• Subject: A brief summary of the topic of the message.
• Date: The local time and date when the message was written. Like the From: field, many email clients fill this in automatically.
• Message-ID: Also an automatically generated field; used to prevent multiple delivery and for reference in In-Reply-To: (see below).

Note that the "To:" field is not necessarily related to the addresses to which the message is delivered. The actual delivery list is supplied separately to the transport protocol, SMTP, which may or may not originally have been extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter which is delivered according to the address on the outer envelope. Also note that the "From:" field does not have to be the real sender of the e-mail message. One reason is that it is very easy to fake the "From:" field and let a message seem to be from any mail address. It is possible to digitally sign e-mail, which is much harder to fake, but such signatures require extra programming and often external programs to verify. Some Internet service providers do not relay e-mail claiming to come from a domain not hosted by them, but very few (if any) check to make sure that the person or even e-mail address named in the "From:" field is the one associated with the connection. Some Internet service providers apply e-mail authentication systems to e-mail being sent through their MTA to allow other MTAs to detect forged spam that might appear to come from them.

Other common header fields include (see RFC 4021, RFC 2076 or RFC 2822 for more):

• Bcc: Blind Carbon Copy; addresses added to the SMTP delivery list but not (usually) listed in the message data, remaining invisible to other recipients.
• Cc: Carbon copy; Many e-mail clients will mark e-mail in your inbox differently depending on whether you are in the To: or Cc: list.
• Content-Type: Information about how the message is to be displayed, usually a MIME type.
• In-Reply-To: Message-ID of the message that this is a reply to. Used to link related messages together.
• Precedence: commonly with values "bulk", "junk", or "list"; used to indicate that should automated "vacation" or "out of office" responses should not be returned for this mail, eg. to prevent vacation notices from being sent to all other subscribers of a mailinglist.
• Received: Tracking information generated by mail servers that have previously handled a message, in reverse order (last handler first).
• References: Message-ID of the message that this is a reply to, and the message-id of the message the previous was reply a reply to, etc.
• Reply-To: Address that should be used to reply to the message.
• Sender: Address of the actual sender acting on behalf of the author listed in the From: field (secretary, list manager, etc.).
• X-Face: Small icon.

IANA maintains a list of standard header fields.

Message body

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Content encoding

E-mail was originally designed for 7-bit ASCII.^[24] Much e-mail software is 8-bit clean but must assume it will communicate with 8-bit servers and mail readers. The MIME standard introduced character set specifiers and two content transfer encodings to enable transmission of non-ASCII data: quoted printable for mostly 7 bit content with a few characters outside that range and base64 for arbitrary binary data. The 8BITMIME extension was introduced to allow transmission of mail without the need for these encodings but many mail transport agents still do not support it fully. In some countries, several encoding schemes coexist; as the result, by default, the message in a non-Latin alphabet language appears in non-readable form (the only exception is coincidence, when the sender and receiver use the same encoding scheme). Therefore, for international character sets, Unicode is growing in popularity.

Plain text and HTML

Both plain text and HTML are used to convey e-mail. While text is certain to be read by all users without problems, there is a perception^{[citation needed]} that HTML-based e-mail has a higher aesthetic value, assuming the recipient uses an e-mail client that will properly show the HTML markup. This is not always the case, especially among the technological savvy and bandwidth-constrained users. Advantages of HTML include the ability to include inline links and images, set apart previous messages in block quotes, wrap naturally on any display, use emphasis such as underlines and italics, and change font styles. HTML e-mail messages often include an automatically-generated plain text copy as well, for compatibility reasons. Disadvantages include the increased size of the email, privacy concerns about web bugs, abuse of HTML email as a vector for phishing attacks and the spread of malicious software.^[25]

Servers and client applications

The interface of an e-mail client, Thunderbird.

Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software programs called mail transfer agents. Users can retrieve their messages from servers using standard protocols such as POP or IMAP, or, as is more likely in a large corporate environment, with a proprietary protocol specific to Lotus Notes or Microsoft Exchange Servers. Webmail interfaces allow users to access their mail with any standard web browser, from any computer, rather than relying on an e-mail client.

Mail can be stored on the client, on the server side, or in both places. Standard formats for mailboxes include Maildir and mbox. Several prominent e-mail clients use their own proprietary format and require conversion software to transfer e-mail between them.

Accepting a message obliges an MTA to deliver it, and when a message cannot be delivered, that MTA must send a bounce message back to the sender, indicating the problem.

Filename extensions

Upon reception of e-mail messages, e-mail client applications save message in operating system files in the filesystem. Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage. A historical standard of storage is the mbox format. The specific format used is often indicated by special filename extensions:

Please help improve this article or section by expanding it. Further information might be found on the talk page. (May 2009)

.eml

This is the default e-mail file extension for Mozilla Thunderbird and Windows Mail. It is also used by Microsoft Outlook Express.

.emlx

Used by Apple Mail.

.msg

Used by Microsoft Office Outlook.

.mbx

Used by Opera Mail, KMail, and Apple Mail based on the mbox format.

Some applications (like Apple Mail) also encode attachments into messages for searching while also producing a physical copy of the files on a disk. Others separate attachments from messages by depositing them into designated folders on disk.

URI scheme mailto:

The URI scheme, as registered with the IANA, defines the mailto: scheme for SMTP email addresses. Though its use is not strictly defined, URLs of this form are intended to be used to open the new message window of the user's mail client when the URL is activated, with the address as defined by the URL in the "To:" field. ^[26]

Use

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In society

There are numerous ways in which people have changed the way they communicate in the last 50 years; e-mail is certainly one of them. Traditionally, social interaction in the local community was the basis for communication – face to face. Yet, today face-to-face meetings are no longer the primary way to communicate as one can use a landline telephone or any number of the computer mediated communications such as e-mail.

Research has shown that people actively use e-mail to maintain core social networks, particularly when alters live at a distance. However, contradictory to previous research, the results suggest that increases in Internet usage are associated with decreases in other modes of communication, with proficiency of Internet and e-mail use serving as a mediating factor in this relationship. ^[27]

Flaming

Flaming occurs when a person sends a message with angry or antagonistic content. Flaming is assumed to be more common today because of the ease and impersonality of e-mail communications: confrontations in person or via telephone require direct interaction, where social norms encourage civility, whereas typing a message to another person is an indirect interaction, so civility may be forgotten.^{[citation needed]} Flaming is generally looked down upon by Internet communities as it is considered rude and non-productive.

E-mail bankruptcy

Also known as "e-mail fatigue", e-mail bankruptcy is when a user ignores a large number of e-mail messages after falling behind in reading and answering them. The reason for falling behind is often due to information overload and a general sense there is so much information that it is not possible to read it all. As a solution, people occasionally send a boilerplate message explaining that the e-mail inbox is being cleared out. Stanford University law professor Lawrence Lessig is credited with coining this term, but he may only have popularized it.^[28]

In business

E-mail was widely accepted by the business community as the first broad electronic communication medium and was the first ‘e-revolution’ in business communication. E-mail is very simple to understand and like postal mail, e-mail solves two basic problems of communication: logistics and synchronization (see below). LAN based email is also an emerging form of usage for business. It not only allows the business user to download mail when offline, it also provides the small business user to have multiple users e-mail ID's with just one e-mail connection.

Pros

• The problem of logistics

Much of the business world relies upon communications between people who are not physically in the same building, area or even country; setting up and attending an in-person meeting, telephone call, or conference call can be inconvenient, time-consuming, and costly. E-mail provides a way to exchange information between two or more people with no set-up costs and that is generally far less expensive than physical meetings or phone calls.

• The problem of synchronization

With real time communication by meetings or phone calls, participants have to work on the same schedule, and each participant must spend the same amount of time in the meeting or call. E-mail allows asynchrony: each participant may control their schedule independently.

Cons

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Most business workers today spend from one to two hours of their working day on e-mail: reading, ordering, sorting, ‘re-contextualizing’ fragmented information, and writing e-mail.^[29] The use of e-mail is increasing due to increasing levels of globalization—labour division and outsourcing amongst other things. E-mail can lead to some well-known problems:

• Loss of Context: which means that the context is lost forever; there is no way to get the text back.

Information in context (as in a newspaper) is much easier and faster to understand than unedited and sometimes unrelated fragments of information. Communicating in context can only be achieved when both parties have a full understanding of the context and issue in question.

• Information overload: E-mail is a push technology—the sender controls who receives the information. Convenient availability of mailing lists and use of "copy all" can lead to people receiving unwanted or irrelevant information of no use to them.
• Inconsistency: E-mail can duplicate information. This can be a problem when a large team is working on documents and information while not in constant contact with the other members of their team.

Despite these disadvantages, e-mail has become the most widely used medium of communication within the business world.

Challenges

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Information overload

A December 2007 New York Times blog post described E-mail as "a $650 Billion Drag on the Economy"^[30], and the New York Times reported in April 2008 that "E-MAIL has become the bane of some people’s professional lives" due to information overload, yet "none of the current wave of high-profile Internet start-ups focused on e-mail really eliminates the problem of e-mail overload because none helps us prepare replies".^[31]

Technology investors reflect similar concerns.^[32]

Spamming and computer viruses

The usefulness of e-mail is being threatened by four phenomena: e-mail bombardment, spamming, phishing, and e-mail worms.

Spamming is unsolicited commercial (or bulk) e-mail. Because of the very low cost of sending e-mail, spammers can send hundreds of millions of e-mail messages each day over an inexpensive Internet connection. Hundreds of active spammers sending this volume of mail results in information overload for many computer users who receive voluminous unsolicited e-mail each day.^[33][34]

E-mail worms use e-mail as a way of replicating themselves into vulnerable computers. Although the first e-mail worm affected UNIX computers, the problem is most common today on the more popular Microsoft Windows operating system.

The combination of spam and worm programs results in users receiving a constant drizzle of junk e-mail, which reduces the usefulness of e-mail as a practical tool.

A number of anti-spam techniques mitigate the impact of spam. In the United States, U.S. Congress has also passed a law, the Can Spam Act of 2003, attempting to regulate such e-mail. Australia also has very strict spam laws restricting the sending of spam from an Australian ISP,^[35] but its impact has been minimal since most spam comes from regimes that seem reluctant to regulate the sending of spam.

E-mail spoofing

E-mail spoofing occurs when the header information of an email is altered to make the message appear to come from a known or trusted source. It is often used as a ruse to collect personal information.

E-mail bombing

E-mail bombing is the intentional sending of large volumes of messages to a target address. The overloading of the target email address can render it unusable and can even cause the mail server to crash.

Privacy concerns

E-mail privacy, without some security precautions, can be compromised because:

• e-mail messages are generally not encrypted;
• e-mail messages have to go through intermediate computers before reaching their destination, meaning it is relatively easy for others to intercept and read messages;
• many Internet Service Providers (ISP) store copies of e-mail messages on their mail servers before they are delivered. The backups of these can remain for up to several months on their server, despite deletion from the mailbox;
• the Received: fields and other information in the e-mail can often identify the sender, preventing anonymous communication.

There are cryptography applications that can serve as a remedy to one or more of the above. For example, Virtual Private Networks or the Tor anonymity network can be used to encrypt traffic from the user machine to a safer network while GPG, PGP, SMEmail ^[36] , or S/MIME can be used for end-to-end message encryption, and SMTP STARTTLS or SMTP over Transport Layer Security/Secure Sockets Layer can be used to encrypt communications for a single mail hop between the SMTP client and the SMTP server.

Additionally, many mail user agents do not protect logins and passwords, making them easy to intercept by an attacker. Encrypted authentication schemes such as SASL prevent this.

Finally, attached files share many of the same hazards as those found in peer-to-peer filesharing. Attached files may contain trojans or viruses.

Tracking of sent mail

The original SMTP mail service provides limited mechanisms for tracking a transmitted message, and none for verifying that it has been delivered or read. It requires that each mail server must either deliver it onward or return a failure notice (bounce message), but both software bugs and system failures can cause messages to be lost. To remedy this, the IETF introduced Delivery Status Notifications (delivery receipts) and Message Disposition Notifications (return receipts); however, these are not universally deployed in production.

US Government

The US Government has been involved in e-mail in several different ways.

Starting in 1977, the US Postal Service (USPS) recognized that electronic mail and electronic transactions posed a significant threat to First Class mail volumes and revenue. Therefore, the USPS initiated an experimental e-mail service known as E-COM. Electronic messages were transmitted to a post office, printed out, and delivered as hard copy. To take advantage of the service, an individual had to transmit at least 200 messages. The delivery time of the messages was the same as First Class mail and cost 26 cents. Both the Postal Regulatory Commission and the Federal Communications Commission opposed E-COM. The FCC concluded that E-COM constituted common carriage under its jurisdiction and the USPS would have to file a tariff.^[37] Three years after initiating the service, USPS canceled E-COM and attempted to sell it off.^{[38][39][40][41][42][43][44]}

The early ARPANET dealt with multiple e-mail clients that had various, and at times incompatible, formats. For example, in the system Multics, the "@" sign meant "kill line" and anything after the "@" sign was ignored.^[45] The Department of Defense DARPA desired to have uniformity and interoperability for e-mail and therefore funded efforts to drive towards unified interoperable standards. This led to David Crocker, John Vittal, Kenneth Pogran, and Austin Henderson publishing RFC 733, "Standard for the Format of ARPA Network Text Message" (November 21, 1977), which was apparently not effective. In 1979, a meeting was held at BBN to resolve incompatibility issues. Jon Postel recounted the meeting in RFC 808, "Summary of Computer Mail Services Meeting Held at BBN on 10 January 1979" (March 1, 1982), which includes an appendix listing the varying e-mail systems at the time. This, in turn, lead to the release of David Crocker's RFC 822, "Standard for the Format of ARPA Internet Text Messages" (August 13, 1982).^[46]

The National Science Foundation took over operations of the ARPANET and Internet from the Department of Defense, and initiated NSFNet, a new backbone for the network. A part of the NSFNet AUP forbade commercial traffic.^[47] In 1988, Vint Cerf arranged for an interconnection of MCI Mail with NSFNET on an experimental basis. The following year Compuserve e-mail interconnected with NSFNET. Within a few years the commercial traffic restriction was removed from NSFNETs AUP, and NSFNET was privatized.

In the late 1990s, the Federal Trade Commission grew concerned with fraud transpiring in e-mail, and initiated a series of procedures on spam, fraud, and phishing.^[48] In 2004, FTC jurisdiction over spam was codified into law in the form of the CAN SPAM Act.^[49] Several other US Federal Agencies have also exercised jurisdiction including the Department of Justice and the Secret Service.

See also

Internet portal

Enhancements

• E-mail encryption
• HTML e-mail
• Internet fax
• L- or letter mail, e-mail letter and letter e-mail
• Privacy-enhanced Electronic Mail
• Push e-mail
• Google Wave

E-mail social issues

Clients and servers

Mailing list

• Anonymous remailer
• Disposable e-mail address
• E-mail encryption
• E-mail tracking
• Electronic mailing list
• Mailer-Daemon
• Mailing list archive

Protocols

• IMAP
• POP3
• SMTP
• UUCP
• X400

References

Notes

Bibliography

• Free On-line Dictionary of Computing
• Microsoft Manual of Style for Technical Publications Version 3.0

External links

Look up email or outbox in Wiktionary, the free dictionary.

• IANA's list of standard header fields
• The History of Electronic Mail is a personal memoir by the implementer of an early e-mail system

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