E-mail

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.

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. "!". 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 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 typically consist of two major components:

   * Headers - Message summary, sender, receiver, and other information
     about the e-mail
   * Body - The message itself, usually containing a signature block at the end

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.