E-mail
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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.
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Origins of e-mail
E-mail predates the Internet; existing
e-mail systems were a crucial tool in creating the Internet. 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. |
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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. 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.
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: Carbon copy (because typewriters use carbon paper to make copies of letters)
- 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 carbon 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 only designed for 7-bit ASCII. While a lot of e-mail software was in fact 8-bit clean this couldn't be relied upon on open interchange. 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, possibly due to the complication of having to do content
transformations when forwarding to a mailserver that doesn't support it.
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. This is totally bogwash
Privacy problems regarding e-mail
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:
One may also use encrypted authentication schemes such as SASL.
Source -
Wikipedia, the free encyclopedia