E-mail, or email, is short for "electronic 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 the most popular use of the Internet.
E-mail before the Internet
E-mail actually pre-dates the widespread use of the Internet. Since not all networks were directly internetworked, 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.
A modern Internet E-mail address is a string of the form [email protected]. 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:
- From - The e-mail address of the sender of the message
- To - The e-mail address of the receiver of the message
- Subject - A brief summary of the contents of the message
- Body - The message itself, usually containing a Signature block at the end
Other fields include:
- Cc - Carbon copy (because typewriters used carbon film to copy what was written on them.
- Bcc - Blind carbon copy (the recipient of this copy will know who was in the To: field, but the To:-recipients cannot see who is on the BCC: list)
- Attachments - Files that complement the message
- Date - Date in which the e-mail was sent
- Content-Type - Information about how the message has to be displayed, usually a MIME type.
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.
Social issues about e-mail: Netiquette, Internet humor, Internet slang, spam, virus.
See also: Email client - Webmail
This article, or an earlier version, contains content derived from FOLDOC, used by permission.