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E-mail, or email, is short for "electronic mail" (as opposed to conventional mail, in this context also called snail mail) and refers to composing, sending, and receiving messages over electronic communication systems. Most e-mail systems today use the Internet, and e-mail is one of the most popular uses of the Internet.

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E-mail before the Internet

Despite common belief, E-mail actually pre-dates the widespread use of the Internet. It was invented by Ray Tomlinson[?] in 1971.

E-mail started as a way for multiple users of a mainframe computer to communicate. Soon, e-mail was being extended to pass messages between different sites.

Since not all computers or networks were directly inter-networked, e-mail was forwarded between sites using protocols such as UUCP, and e-mail addresses had to include the "route" of the message, that is, a path between the computer of the sender and the computer of the receivers. E-mail could be passed this way between a number of networks, including the ARPANET, BITNET and NSFNET, as well as to hosts connected directly to other sites via UUCP.

The route was specfied using so-call "bang path" addresses, specifying hops to get from some assumed-reachable location to the addressee, so called because each hop is signified by a bang sign. Thus, for example, the path ...!bigsite!foovax!barbox!me directs people to route their mail to machine bigsite (presumably a well-known location accessible to everybody) and from there through the machine foovax to the account of user me on barbox.

Before autorouting mailers became commonplace, people often published compound bang addresses using the { } convention (see glob[?]) to give paths from several big machines, in the hopes that one's correspondent might be able to get mail to one of them reliably (example: ...!{seismo, ut-sally, ihnp4}!rice!beta!gamma!me). Bang paths of 8 to 10 hops were not uncommon in 1981. Late-night dial-up UUCP links would cause week-long transmission times. Bang paths were often selected by both transmission time and reliability, as messages would often get lost. See the network and sitename.

Modern Internet E-mail

Nowadays, almost all e-mail is delivered directly to Internet-connected hosts, using DNS MX records and SMTP (Simple Mail Transfer Protocol). Very few modern servers allow routing (automatic or manual) any more due the potential for abuse by people sending unsolicited bulk email. Those that do allow it are called open relays[?].

A modern Internet e-mail address is a string of the form jsmith@corporation.com. It should be read as "jsmith at corporation.com". The first part is the username[?] of the person, and the second part is the hostname[?] of the computer in which that person has an e-mail account.

The format of internet e-mail messages is defined in RFC 2822. Prior to the introduction of RFC 2822 the format was described by RFC 822.

Internet e-mail messages usually have at least four fields:

  1. From - The e-mail address of the sender of the message
  2. To - The e-mail address of the receiver of the message
  3. Subject - A brief summary of the contents of the message
  4. Body - The message itself, usually containing a signature block at the end

Note however that the "To" field does not necessarily have the email address of the recipient. The information supplied in the headers on the recipients computer is similar to that found on top of a conventional letter. The actual information such as who the message was addressed to is removed by the mail server after it assigns it to the correct user's mailbox.

Other fields include:

  1. Cc - Carbon copy (because typewriters used carbon film[?] to copy what was written on them)
  2. Bcc - Blind carbon copy (the recipient of this copy will know who was in the To: field, but the recipients cannot see who is on the BCC: list)
  3. Attachments - Files that complement the message
  4. Date - Date in which the e-mail was sent
  5. Content-Type - Information about how the message has to be displayed, usually a MIME type

Messages and mailboxes

Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software like Sendmail. Users download their messages from servers usually with either the POP or IMAP protocols.

Standard formats for mailboxes include Maildir and mbox. Several prominent e-mail clients use their own, proprietary format, and require conversion software to transfer email between them.

E-mail content encoding

E-mail is only defined to carry 7-bit ASCII messages. Although many e-mail transports are in fact "8-bit clean", this cannot be guaranteed. For this reason, e-mail has been extended by the MIME standard to allow the encoding of binary attachments including images, sounds and HTML attachments.

E-mail social issues: netiquette, Internet humor, Internet slang, spam, virus.

See also: email client, electronic mailing list, mailing list archive, webmail, mail transfer agent, e-mail address[?], Hotmail.com

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This article, or an earlier version, contains content derived from FOLDOC, used by permission.



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