Electronic mail, abbreviated e-mail or email, is a method of composing, sending, 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 workgroup collaboration systems allowing users within one company or organization to send messages to each other. Often 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=
Despite common belief, e-mail actually predates 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 System Development Corporation s Q32 and Massachusetts Institute of Technology s CTSS.
According to some popular sources, email was invented as a way to ensure communications in case of a nuclear war during the Cold War.
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 Semi Automatic Ground Environment system had something similar some time before.
The 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 . 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 Laboratories, Lotus Software, 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 ITU-T developed the X.400 standard in the 1980 s 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 to send emails between a nationwide linkup of VAX computers. By 1984 these same users were using personal computers for same.
In 1982 the White House adopted a prototype email 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 stereotypical sequence of events that takes place when Alice and Bob sends an e-mail to Alice and Bob.
# Alice composes a message using her Email client (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 headers), in this case A modern Internet e-mail address is a string of the form 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:
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 e-mail spam 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, email 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 [http://www.ietf.org/iesg/1rfc_index.txt]) 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:
The header section is separated from the body by a blank line.
===Internet e-mail headers===
Each header has a name and a value. RFC 2822 specifies the precise syntax. Informally, the header 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. Header names and values are restricted to 7-bit ASCII characters. Non-ASCII values may be represented using MIME MIME#Encoded-Word. Messages usually have at least four headers:
# 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 header in the message is not necessarily related to the addresses to which the e-mail is delivered. The actual delivery list is supplied in the SMTP protocol, not extracted from the header content. The To header 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 header does not have to be the real sender of the e-mail. It is very easy to fake the From line and let an e-mail seem to be from any mail address. It is possible to Digital signature an e-mail, which is much harder to fake. Some Internet service provider do not relay e-mails 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 account named in the From header is the one associated with the connection.
Other common headers 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 header) as a header. Since all the headers are visible to all recipients, Bcc isn t actually a header. Addresses added as Bcc are only added to the SMTP delivery list.
===E-mail content encoding===
Email was only designed for 7-bit extention 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.
==Saved Message Extension==
Different applications save email files with different file extensions.
=Messages and mailboxes=
Messages are exchanged between hosts using the SMTP with software like Sendmail. Users download their messages from servers usually with either the Post Office Protocol 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 (computing) 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, E-mail spam, 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 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 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 stopping e-mail abuse mitigate the impact of spam. In the United States, Congress of the United States has also passed a law, the Can Spam Act of 2003, attempting to regulate such e-mail.
=Privacy problems regarding e-mail=
E-mail privacy, without some security precautions, can be compromised because
There are Cryptography applications that can serve as a remedy to the above, such as Virtual Private Networks, message encryption using Pretty Good Privacy or the GNU Privacy Guard, encrypted communications with the e-mail servers using Transport Layer Security and Secure Sockets Layer, and/or encrypted authentication schemes such as Simple Authentication and Security Layer.
=See also=
=Further reading=
=References=
=External links=
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