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Electronic mail, abbreviated e-mail or
email, is a method of composing, sending, storing, and receiving
messages over electronic communication systems. The term e-mail applies
both to the Internet e-mail system based on the Simple Mail Transfer
Protocol (SMTP) and to intranet systems allowing users within one
company or organization to send messages to each other. Often these
workgroup collaboration systems natively use non-standard protocols but
have some form of gateway to allow them to send and receive Internet
e-mail. Some organizations may use the Internet protocols for internal
e-mail service. Origins of
e-mail E-mail predates the Internet; 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 made a large contribution to the evolution
of e-mail. There is one report [1] which indicates experimental
inter-system e-mail transfers on it shortly after its creation, in 1969.
Ray Tomlinson initiated the use of the @ sign to separate the names of
the user and their machine in 1971 [2]. The common report that he
"invented" e-mail is an exaggeration, although his early e-mail programs
SNDMSG and READMAIL were very important. The first message sent by Ray
Tomlinson is not preserved; it was "a message announcing the
availability of network email"[3]. The ARPANET significantly increased
the popularity of e-mail, and it became the killer app of the ARPANET.
Growing popularity As the utility and advantages of
e-mail on the ARPANET became more widely known, the popularity of e-mail
increased, leading to demand from people who were not allowed access to
the ARPANET. A number of protocols were developed to deliver e-mail
among groups of time-sharing computers over alternative transmission
systems, such as UUCP and IBM's VNET e-mail system.
Since not all computers or networks were directly inter-networked,
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" (the 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.
E-mail became an increasingly important feature of work group
collaboration products developed by vendors such as Wang, Lotus, IBM,
and Microsoft. These systems often provided enhanced e-mail features
(such as file attachments, Rich Text Format, and delivery confirmation),
but only when sending e-mail to other users of the same system. These
systems communicated with other, non-like, systems via specialized
e-mail gateways which translated one vendor's (usually proprietary)
e-mail format into a form understandable by another vendor.
The CCITT developed the X.400 standard in the 1980s to allow different
e-mail systems to interoperate. Roughly at the same time, the IETF
developed a much simpler protocol called the Simple Mail Transfer
Protocol (SMTP) which has become the de facto standard for e-mail
transfer on the Internet. With the advent of widespread use of home
personal computers connected to the Internet, interoperability via
SMTP-based Internet e-mail has become a critical feature for all e-mail
systems.
In 1969 US Air Force users were sending text messages by keypunching
cards with long text messages using one card for each 80 character line
and transmitting them as card decks from one computer to another. By
1979, US Air Force users were logging onto central computers and leaving
messages for government contractors and other US Air Force users to read
in special file areas where their replies were often received back
within hours. By the end of 1983 US Air Force users were using user
names like alclark@vax1.mil to send e-mail between a nationwide linkup
of VAX computers. By 1984 these same users were using personal computers
for same.
In 1979, the US Post Office bought a computer specifically for email,
but wound up selling it to private industry.
In 1982 the White House adopted a prototype e-mail system from IBM
called the Professional Office System, or PROFs for the National
Security Council (NSC) staff. By April 1985, the system was fully
operational within the NSC with home terminals for principals on the
staff. And by November of 1986 the rest of the White House came online,
first with the PROFs system, and later (by the end of the 1980s) through
a variety of systems including VAX A-1 ("All in One"), and ccmail.
Modern Internet e-mail
How Internet e-mail works
The diagram above shows a typical sequence of events that takes place
when Alice sends e-mail to Bob.
Alice composes a message using her mail user agent (MUA). She types in,
or selects from an address book, the e-mail address of her
correspondent. She hits the "send" button. Her MUA formats the message
in Internet 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 Alice's Internet Service Provider (ISP).
The MTA looks at the destination address provided in the SMTP protocol
(not from the message header), in this case bob@b.org. A modern Internet
e-mail address is a string of the form localpart@domain.example,
creating a Fully Qualified Domain Address (FQDA). 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. The MTA looks
up this domain name in the Domain Name System to find the mail exchange
servers accepting messages for that domain.
The DNS server for the b.org domain, ns.b.org, responds with an MX
record listing the mail exchange servers for that domain, in this case
mx.b.org, a server run by Bob's ISP.
smtp.a.org sends the message to mx.b.org using SMTP, which delivers it
to the mailbox of the user bob.
Bob presses the "get mail" button in his MUA, which picks up the message
using the Post Office Protocol (POP3).
This sequence of events applies to the majority of e-mail users.
However, there are many alternative possibilities and complications to
the e-mail system:
Alice or Bob may use a client connected to a corporate e-mail system,
such as IBM's Lotus Notes or Microsoft's Exchange. These systems often
have their own internal e-mail format and their clients typically
communicate with the e-mail server using a vendor-specific, proprietary,
protocol. The server sends or receives e-mail via the Internet through
the product's Internet mail gateway which also does any necessary
reformatting. If Alice and Bob work for the same company, the entire
transaction may happen completely within a single corporate e-mail
system.
Alice may not have a MUA on her computer but instead may connect to a
webmail service.
Alice's computer may run its own MTA, so avoiding the transfer at step
1.
Bob may pick up his e-mail in many ways, for example using the Internet
Message Access Protocol, by logging into mx.b.org and reading it
directly, or by using a webmail service.
Domains usually have several mail exchange servers so that they can
continue to accept mail when the main mail exchange server is not
available.
It used to be the case that many MTAs would accept messages for any
recipient on the Internet and do their best to deliver them. Such MTAs
are called open mail relays. This was important in the early days of the
Internet when network connections were unreliable. If an MTA couldn't
reach the destination, it could at least deliver it to a relay that was
closer to the destination. The relay would have a better chance of
delivering the message at a later time. However, this mechanism proved
to be exploitable by people sending unsolicited bulk e-mail and as a
consequence very few modern MTAs are open mail relays, and many MTAs
will not accept messages from open mail relays because such messages are
very likely to be spam.
Note that the people, e-mail addresses and domain names in this
explanation are fictional: see Alice and Bob.
Internet e-mail format The format of Internet e-mail
messages is defined in RFC 2822 and a series of RFCs, RFC 2045 through
RFC 2049, collectively called Multipurpose Internet Mail Extensions
(MIME). Although as of July 13, 2005 (see [4]) RFC 2822 is technically a
proposed IETF standard and the MIME RFCs are draft IETF standards, these
documents are the de facto standards for the format of Internet e-mail.
Prior to the introduction of RFC 2822 in 2001 the format described by
RFC 822 was the de facto standard for Internet e-mail for nearly two
decades; it is still the official IETF standard. The IETF reserved the
numbers 2821 and 2822 for the updated versions of RFC 821 (SMTP) and RFC
822, honoring the extreme importance of these two RFCs. RFC 822 was
published in 1982 and based on the earlier RFC 733.
Internet e-mail messages consist of two major sections:
Header - Structured into fields such as summary, sender, receiver, and
other information about the e-mail
Body - The message itself as unstructured text; sometimes containing a
signature block at the end
The header is separated from the body by a blank line.
Internet e-mail header The message header consists of
fields. Each header field has a name and a value. RFC 2822 specifies the
precise syntax. Informally, the field name starts in the first character
of a line, followed by a ":", followed by the value which is continued
on non-null subsequent lines that have a space or tab as their first
character. Field names and values are restricted to 7-bit ASCII
characters. Non-ASCII values may be represented using MIME encoded
words. Messages usually have at least four fields in the header:
From: The e-mail address, and optionally name, of the sender of the
message
To: The e-mail addresses, and optionally names, of the receiver of the
message
Subject: A brief summary of the contents of the message
Date: The local time and date when the message was originally sent
Note however that the "To" field in the header is not necessarily
related to the addresses to which the message is delivered. The actual
delivery list is supplied in the SMTP protocol, not extracted from the
header content. The "To" field is similar to the greeting at the top of
a conventional letter which is delivered according to the address on the
outer envelope. Also note that the "From" field does not have to be the
real sender of the e-mail message. It is very easy to fake the "From"
field and let a message seem to be from any mail address. It is possible
to digitally sign e-mail, which is much harder to fake. Some Internet
service providers do not relay e-mail claiming to come from a domain not
hosted by them, but very few (if any) check to make sure that the person
or even e-mail address named in the "From" field is the one associated
with the connection.
Other common header fields include:
Cc: Courtesy copy (See also carbon copy.)
Received: Tracking information generated by mail servers that have
previously handled a message
Content-Type: Information about how the message has to be displayed,
usually a MIME type
Many e-mail clients present "Bcc" (Blind courtesy copy, recipients not
visible in the "To" field) as a header field. Since the entire header is
visible to all recipients, "Bcc" is not included in the message header.
Addresses added as "Bcc" are only added to the SMTP delivery list, and
do not get included in the message data.
E-mail content encoding E-mail was originally designed
for 7-bit ASCII. Much e-mail software is 8-bit clean but must assume it
will be communiticating with 7-bit servers and mail readers. The MIME
standard introduced charset specifiers and two content transfer
encodings to encode 8 bit data for transmission: quoted printable for
mostly 7 bit content with a few characters outside that range and base64
for arbitrary binary data. The 8BITMIME extension was introduced to
allow transmission of mail without the need for these encodings but many
mail transport agents still don't support it fully. For international
character sets, Unicode is growing in popularity.
Saved message file extension Different applications save
e-mail files with different file extensions.
.eml
This is used by Microsoft Office Outlook, Outlook Express, and is the
default e-mail extension for Mozilla Thunderbird.
.emlx
Used by Apple Mail.
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, though in a large
corporate environment users are likely to use some proprietary protocol
such as Lotus Notes or Microsoft Exchange Server's.
Mail 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 e-mail between them.
When a message cannot be delivered, the recipient MTA must send a bounce
message back to the sender, indicating the problem.
Spamming and e-mail worms The
usefulness of e-mail is being threatened by three phenomena: spamming,
phishing 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 information
overload for many computer users who receive tens or even hundreds of
junk messages 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 technology-based initiatives 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.
Privacy problems regarding e-mail
Main article: e-mail privacy
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 your e-mail
messages on their mail servers before they are delivered. The backups of
these can remain up to several months on their server, even if you
delete them in your mailbox.
There are cryptography applications that can serve as a remedy to the
above, such as Virtual Private Networks, message encryption using PGP or
the GNU Privacy Guard, encrypted communications with the e-mail servers
using:
Transport Layer Security
Secure Sockets Layer
Tor
Another risk is that e-mail passwords might be intercepted during
sign-in. One may use encrypted authentication schemes such as SASL to
help prevent this.
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