Main Page | See live article | Alphabetical index


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.

Table of contents
1 E-mail before the Internet
2 Modern internet e-mail
3 Messages and mailboxes
4 E-mail content encoding
5 Spamming and e-mail worms
6 Further Reading
7 External links

E-mail before the Internet

Despite common belief, e-mail actually pre-dates the Internet; in fact, existing e-mail systems were a crucial tool in creating the Internet.

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.

E-mail was quickly extended to become network e-mail, allowing users to pass messages between different computers. The early history of network e-mail is also murky; the AUTODIN system may have been the first allowing electronic text messages to be transferred between users on different computers, in 1966, but it is possible the SAGE system had something similar some time before.

The ARPANET computer network significantly increased the popularity of e-mail. There is one report [1] which indicates experimental inter-system e-mail transfers on it shortly after its creation, in 1969. The use of the "@" sign to separate the names of the user and their machine, was initiated by Ray Tomlinson in 1972; the common report that he "invented" email is an exaggeration, although his early e-mail programs SNDMSG and READMAIL were very important.

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 specified 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", i.e. Exclamation mark. 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 auto-routing 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 relayss.

A modern Internet e-mail address is a string of the form It should be read as "jsmith at". 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 typically consist of two major components:

The headers 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. Date - The local time and date when the message was originally sent

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 common 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. Received - Tracking information generated by mail servers that have previously handled a message
  4. 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, yet in a large corporate environment users are likely to use some proprietary protocol such as Lotus Notes or Microsoft Exchange Server's.

Mails can be stored either on the client or on the server side. 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.

Spamming and e-mail worms

The usefulness of e-mail is being threatened by two phenomena, spamming and e-mail worms.

Spamming is unsolicited commercial 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 many computer users receiving tens or even hundreds of junk e-mails each day.

E-mail worms use e-mail as a way of replicating themselves into vulnerable computers. Although the first e-mail worm (the Morris worm) affected early UNIX computers, this problem is today almost entirely confined to the Microsoft Windows oprerating 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 technology-based initiatives mitigate the impact of spam. Congress has also passed a law, the Can Spam Act of 2003, to regulate such e-mail.

Further Reading

See also:

External links

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