Electronic mail, most commonly abbreviated email and e-mail, is a method of exchanging digital messages.
Electronic mail (or e-mail or email) is an Internet service that allows those people who have an e-mail address (accounts) to send and receive electronic letters. Those are much like postal letters, except that they are delivered much faster than snail mail when sending over long distances, and are usually free. To send or receive an email, you need a gadget (computer, phone etc) connected to the Internet and an e-mail program (simply called mailer). Several formats exist for email addresses. The most common, called RFC 2822, looks like firstname.lastname@example.org. E-mail messages are sent mostly by text, and sometimes by HTML style.
Like with regular mail, users may get a lot of unwanted mail. With e-mail, this is called spam. Some programs used for sending and receiving mail can detect spam and filter it out nearly completely.
Some companies let you send and receive emails for free from a remote website. Gmail, Hotmail and Yahoo! do this kind of service: known as "web mail".
Microsoft Outlook Express is a email program made by Microsoft. It is a free version of Outlook, also made by Microsoft. Microsoft's Outlook Express was discontinued in 2006 with the release of it's new operating system, Windows Vista, which has Windows Mail instead.
Hotmail is a free email service from MSN. It started in 1995. It was bought by Microsoft in 1997. Microsoft Hotmail accounts are used to log in to MSN Messenger.
Topics of Interest
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 for the duration of message submission or retrieval. Originally, e-mail was always transmitted directly from one user's device to another's; nowadays this is rarely the case.
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.
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 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.
Origin: Electronic 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. It allowed multiple users to log into the IBM 7094 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 Equipment Corporation 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.
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. Ray Tomlinson is credited by some as having sent the first email, initiating the use of the "@" sign to separate the names of the user and the user's machine in 1971, when he sent a message from one Digital Equipment Corporation DEC-10 computer to another DEC-10. The two machines were placed next to each other. The ARPANET significantly increased the popularity of e-mail, and it became the killer app of the ARPANET.
- A message is composed using a mail user agent (MUA) (email client, a computer program used to manage email) then the e-mail address of the correspondent is entered, and the "send" button is hit.
- Here 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 an Internet Service Provider (ISP).
- The MTA looks at the destination address provided in the SMTP protocol (not from the message header), for example email@example.com. 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.
- 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 the correspondent's ISP.
- smtp.a.org sends the message to mx.b.org using SMTP, which delivers it to the mailbox of the mail receiver.
- The correspondent presses the "get mail" button in his MUA, which picks up the message using the Post Office Protocol (POP3).
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. Certain abbreviations are commonly used in the subject, including "RE:" and "FW:".
- Date: The local time and date when the message was written. Like the From: field, many email clients fill this in automatically when sending. The recipient's client may then display the time in the format and time zone local to her.
- Message-ID: Also an automatically generated field; used to prevent multiple delivery and for reference in In-Reply-To.
Common header fields for email include:
- 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 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.
Plain text and HTML: Most modern graphic e-mail clients allow the use of either plain text or HTML for the message body at the option of the user. HTML e-mail messages often include an automatically-generated plain text copy as well, for compatibility reasons.
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. 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.
Mailing lists commonly insist that all posts to be made in plain-text for all the above reasons, but also because they have a significant number of readers using text-based e-mail clients such as Mutt.
Some Microsoft e-mail clients have allowed richer formatting by using RTF rather than HTML, but unless the recipient is guaranteed to have a compatible e-mail client this should be avoided.
- .eml Used by many e-mail clients including Microsoft Outlook Express, Windows Mail and Mozilla Thunderbird. The files are plain text in MIME format, containing the e-mail header as well as the message contents and attachments in one or more of several formats.
- .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.
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.
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, but its impact has been minimal since most spam comes from regimes that seem reluctant to regulate the sending of spam.
E-mail spoofing is a term used to describe (usually fraudulent) e-mail activity in which the sender address and other parts of the e-mail header are altered to appear as though the e-mail originated from a different source. E-mail spoofing is a technique commonly used for spam e-mail and phishing to hide the origin of an e-mail message. By changing certain properties of the e-mail, such as the From, Return-Path and Reply-To fields (which can be found in the message header), ill-intentioned users can make the e-mail appear to be from someone other than the actual sender. The result is that, although the e-mail appears to come from the address indicated in the From field (found in the e-mail headers), it actually comes from another source.
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.
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.
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.
E-mail encryption refers to encryption, and often authentication, of e-mail messages. E-mail encryption can rely on public-key cryptography.
E-mail spam, also known as junk e-mail, is a subset of spam that involves nearly identical messages sent to numerous recipients by e-mail. A common synonym for spam is unsolicited bulk e-mail (UBE). Definitions of spam usually include the aspects that email is unsolicited and sent in bulk. "UCE" refers specifically to unsolicited commercial e-mail.
E-mail authentication is the effort to equip messages of the e-mail transport system with enough verifiable information, so that recipients can recognize the nature of each incoming message automatically. It differs from content filtering.
Webmail (or Web-based e-mail) is an e-mail service intended to be primarily accessed via a web browser, as opposed to through a desktop e-mail client (such as Microsoft Outlook, Pegasus Mail, Mozilla's Thunderbird, or Apple Inc.'s Mail). Very popular webmail providers include Gmail, Yahoo! Mail, Hotmail, and AOL.
An electronic mailing list is a special usage of email that allows for widespread distribution of information to many Internet users. It is similar to a traditional mailing list — a list of names and addresses — as might be kept by an organization for sending publications to its members or customers, but typically refers to four things: a list of email addresses, the people ("subscribers") receiving mail at those addresses, the publications (e-mail messages) sent to those addresses, and a reflector, which is a single e-mail address that, when designated as the recipient of a message, will send a copy of that message to all of the subscribers.
Causes of a bounce message
There are many reasons why an e-mail may bounce. One reason is if the recipient address is misspelled, or simply does not exist on the receiving system. This is a user unknown condition. Other reasons include resource exhaustion — such as a full disk — or the rejection of the message due to spam filters. In addition, there are MUAs that allow users to bounce a message on demand.
Bounce messages in SMTP are sent with the envelope sender address <>, known as the null sender address. They are frequently sent with a From: header address of MAILER-DAEMON at the recipient site.
Typically, a bounce message will contain several pieces of information to help the original sender in understanding the reason his message was not delivered:
- The date and time the message was bounced,
- The identity of the mail server that bounced it,
- The reason that it was bounced (e.g. user unknown or mailbox full),
- The headers of the bounced message, and
- Some or all of the content of the bounced message.
RFC 3463 describes the codes used to indicate the bounce reason. Common codes are 5.1.1 (Unknown user), 5.2.2 (Mailbox full) and 5.7.1 (Rejected by security policy/mail filter).
The Post Office Protocol (POP) 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 (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both. The POP protocol has been developed through several versions, with version 3 (POP3) being the current standard.
Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 15), and last updated by RFC 5321 (2008) which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today. SMTP is specified for outgoing mail transport and uses TCP port 25.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)