E-mail

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.

Modern Internet e-mail

How Internet e-mail works

The diagram above shows a stereotypical sequence of events that takes place when Alice sends an e-mail to Bob.

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• 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 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 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.

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• 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:

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• 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.

Related Topics:
Open mail relay - Unsolicited bulk e-mail

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Note that the people, email addresses and domain names in this explanation are fictional: see Alice and Bob.

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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.

Related Topics:
Multipurpose Internet Mail Extensions - July 13 - 2005

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Internet e-mail messages consist of two major sections:

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

The header section is separated from the body by a blank line.

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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 encoded words. Messages usually have at least four headers:

Related Topics:
ASCII - Encoded words

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• 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 digitally sign an e-mail, which is much harder to fake. Some Internet service providers 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.

Related Topics:
Digitally sign - Internet service providers

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Other common headers include:

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• 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.

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E-mail 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 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.

Related Topics:
ASCII - MIME - Quoted printable - Base64 - 8BITMIME

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Saved Message Extension

Different applications save email files with different file extensions.

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• .eml

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

Related Topics:
Outlook Express - Mozilla Thunderbird

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• .mbox

Entourage

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• .mbox

Eudora

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