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About Email

Electronic mail, abbreviated email or e-mail, is a method of composing, sending, and receiving messages over electronic communication systems. The term email applies both to the Internet email 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 email. Some organizations may use the internet protocols for internal email service.

Origins of email

Despite common belief, email actually predates the Internet; in fact, existing e-mail systems were a crucial tool in creating the Internet.

Email 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.

Email was quickly extended to become network email, allowing users to pass messages between different computers. The early history of network email 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 (SAGE) system had something similar some time before.

The ARPANET computer network made a large contribution to the evolution of email. 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 email 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"². The ARPANET significantly increased the popularity of email, and it became the Killer application of the ARPANET.

Growing popularity

As the utility and advantages of email on the ARPANET became more widely known, the popularity of email 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 email system.

Since not all computers or computer network were directly inter-networked, email 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. Email 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 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 email to other users of the same system. These systems communicated with other, non-like, systems via specialized email gateways which translated one vendor's (usually proprietary) email format into a form understandable by another vendor.

The ITU-T developed the X.400 standard in the 1980's to allow different email 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 email transfer on the Internet. With the advent of widespread use of home personal computers connected to the Internet, interoperability via SMTP-based Internet email has become a critical feature for all email 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 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 email

How Internet email works

The diagram above shows a stereotypical sequence of events that takes place when Alice and Bob sends an email to Bob .

Alice composes a message using her Email client (MUA). She types in, or selects from an address book, the email address of her correspondent. She hits the "send" button. Her MUA formats the message in Internet email 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 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 server s 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 email 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 email system, such as IBM's Lotus Notes or Microsoft's Microsoft Exchange Server. These systems often have their own internal email format and their clients typically communicate with the email server using a vendor-specific, proprietary, protocol. The server sends or receives email 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 email 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 email spam/unsolicited bulk email 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 email 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:

* Headers - Message summary, sender, receiver, and other information about the email

* Body - The message itself, sometimes containing a signature block at the end

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 MIMEEncoded-Word. Messages usually have at least four headers:

From: The email address, and optionally name, of the sender of the message

To: The email 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 email 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 email. 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 email, 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 typewriter s 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.

Email content encoding

Email was only designed for 7-bit ASCII . While a lot of email 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 arbitary 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.

Saved Message Extension

Different applications save email files with different file extensions.

* .eml

This is used by Outlook Express , and is the default email extension for Mozilla Thunderbird .

* .emlx

Used by Apple Mail

Messages and mailboxes

Messages are exchanged between hosts using the SMTP/Simple Mail Transfer Protocol with software like Sendmail . Users download their messages from servers usually with either the Post Office Protocol -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 (computing)/client or on the server side. Standard formats for mailboxes include Maildir and mbox . Several prominent email clients use their own, proprietary format, and require conversion software to transfer email 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 email worms

The usefulness of email is being threatened by three phenomena, Email spam, phishing and email worms.

Spamming is unsolicited commercial email. Because of the very low cost of sending email, spammers can send hundreds of millions of email 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 emails each day.

Email worms use email as a way of replicating themselves into vulnerable computers. Although the first e-mail 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 email, which reduces the usefulness of e-ail as a practical tool.

A number of stopping email 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 email

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Email privacy, without some security precautions, can be compromised because

* email messages are generally not encrypted;

* email 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 email 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 Network s, message encryption using Pretty Good Privacy/PGP or the GNU Privacy Guard , encrypted communications with the email servers using Transport Layer Security and Secure Sockets Layer, and/or encrypted authentication schemes such as Simple Authentication and Security Layer.