Neither the users nor their computers are required to be online simultaneously; they need to connect, typically to a mail server or a webmail interface to send or receive messages or download it.
Originally a text-only ASCII communications medium, Internet email was extended by MIME to carry text in expanded character sets and multimedia content such as images.
Computer-based messaging between users of the same system became possible after the advent of time-sharing in the early 1960s, with a notable implementation by MIT's CTSS project in 1965.
[18] Over a series of RFCs, conventions were refined for sending mail messages over the File Transfer Protocol.
[nb 1][19][20][21] DEC's ALL-IN-1 and Hewlett-Packard's HPMAIL (later HP DeskManager) were released in 1982; development work on the former began in the late 1970s and the latter became the world's largest selling email system.
[28] In addition to this example, alternatives and complications exist in the email system: Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them.
Email pre-dates instant messaging, and transmission favors reliability over speed, in order to be able to cope with unreliable network links and busy servers (more common in the early days of the Internet).
[33] Messages are timestamped as they pass through each server, allowing for diagnosis of slow delivery, though analysis is complicated by time zones and computer clocks that are inaccurately set.
The delivery list is supplied separately to the transport protocol, SMTP, which may be extracted from the header content.
In some countries, e-mail software violates RFC 5322 by sending raw[nb 2] non-ASCII text and several encoding schemes co-exist; as a result, by default, the message in a non-Latin alphabet language appears in non-readable form (the only exception is a coincidence if the sender and receiver use the same encoding scheme).
[55] Most modern graphic email clients allow the use of either plain text or HTML for the message body at the option of the user.
Some web-based mailing lists recommend all posts be made in plain text, with 72 or 80 characters per line for all the above reasons,[57][58] and because they have a significant number of readers using text-based email clients such as Mutt.
Various informal conventions evolved for marking up plain text in email and usenet posts, which later led to the development of formal languages like setext (c. 1992) and many others, the most popular of them being markdown.
When opening an email, it is marked as "read", which typically visibly distinguishes it from "unread" messages on clients' user interfaces.
[62] Such webmail interfaces allow users to access their mail with any standard web browser, from any computer, rather than relying on a local email client.
Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage.
[63][64] Many clients also support query string parameters for the other email fields, such as its subject line or carbon copy recipients.
[67][68] The Internet Message Access Protocol (IMAP) provides features to manage a mailbox from multiple devices.
Messaging Application Programming Interface (MAPI) is used by Microsoft Outlook to communicate to Microsoft Exchange Server—and to a range of other email server products such as Axigen Mail Server, Kerio Connect, Scalix, Zimbra, HP OpenMail, IBM Lotus Notes, Zarafa, and Bynari where vendors have added MAPI support to allow their products to be accessed directly via Outlook.
A sponsored 2010 study on workplace communication found 83% of U.S. knowledge workers felt email was critical to their success and productivity at work.
[69] It has some key benefits to business and other organizations, including: Email marketing via "opt-in" is often successfully used to send special sales offerings and new product information.
[73] As of 2010[update], the number of Americans visiting email web sites had fallen 6 percent after peaking in November 2009.
Technology writer Matt Richtel said in The New York Times that email was like the VCR, vinyl records and film cameras—no longer cool and something older people do.
[77][78][79] Furthermore, due to technical reasons, attachment sizes as seen by these transport systems can differ from what the user sees,[80] which can be confusing to senders when trying to assess whether they can safely send a file by email.
Some observers even argue it could have a significant negative economic effect,[85] as efforts to read the many emails could reduce productivity.
Email spam and phishing methods typically use spoofing to mislead the recipient about the true message origin.
Additionally, many mail user agents do not protect logins and passwords, making them easy to intercept by an attacker.
Harvard University law professor Lawrence Lessig is credited with coining this term, but he may only have popularized it.
[1][105] The original SMTP mail service provides limited mechanisms for tracking a transmitted message, and none for verifying that it has been delivered or read.
[nb 3] Many ISPs now deliberately disable non-delivery reports (NDRs) and delivery receipts due to the activities of spammers: In the absence of standard methods, a range of system based around the use of web bugs have been developed.
