|
An email address identifies an email box to which email messages are delivered. A wide variety of formats were used in early email systems, but only a single format is used today, following the standards developed for Internet mail systems since the 1980s. An email address such as ''John.Smith@example.com'' is made up of a local part, an @ symbol, then a case-insensitive domain part. Although the standard specifies the local part to be case-sensitive, in practice the mail system at ''example.com'' may treat ''John.Smith'' as equivalent to ''john.smith'' or even as ''johnsmith'',〔(''"...you can add or remove the dots from a Gmail address without changing the actual destination address; and they'll all go to your inbox..."'' ), Google.com〕 and mail systems often limit their users' choice of name to a subset of the technically valid characters. In some cases they also limit which addresses it is possible to send mail to. With the introduction of internationalized domain names, efforts are progressing to permit non-ASCII characters in email addresses. ==Overview== The transmission of electronic mail within the Internet uses the Simple Mail Transfer Protocol (SMTP), defined in Internet standards RFC 5321 and RFC 5322, and extensions like RFC 6531. The mailboxes may be accessed and managed by users with the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) with email client software that runs on a personal computer, mobile device, or with webmail systems that render the messages on a screen or on paper printouts. The general format of an email address is ''localpart@domain'', and a specific example is ''jsmith@example.org''. An address consists of two parts. The part before the @ sign (localpart) identifies the name of a mailbox. This is often the username of the recipient, e.g., ''jsmith''. The part after the @ symbol is a domain name that represents the administrative realm for the mail box, e.g., a company's domain name, ''example.com''. A mail server uses the Domain Name System (DNS) to locate the destination mail server for the domain of the recipient by querying for mail exchanger records (MX records). The organization holding the delegation for a given domain, the mailbox provider, can define the target hosts for all email destined to its domain. The mail exchanger does not need to be located in the domain of the destination mail box, however it must accept mail for the domain. The target hosts are configured with a mechanism to deliver mail to all destination mail boxes. If no mail exchangers are configured, a mail sender directly queries the address record (A record or AAAA record) for the domain name in the email address. The local-part of an email address has no significance for intermediate mail relay systems other than the final mailbox host. Email senders and intermediate relay systems must not assume it to be case-insensitive, since the final mailbox host may or may not treat it as such. A single mailbox may receive mail for multiple email addresses, if configured by the administrator. Conversely, a single email address may be the alias to a distribution list to many mailboxes. Email aliases, electronic mailing lists, sub-addressing, and catch-all addresses, the latter being mailboxes that receive messages regardless of the local part, are common patterns for achieving a variety of delivery goals. The addresses found in the header fields of an email message are not directly used by mail exchangers to deliver the message. An email message also contains a message envelope that contains the information for mail routing. While envelope and header addresses may be equal, forged email addresses are often seen in spam, phishing, and many other Internet-based scams. This has led to several initiatives which aim to make such forgeries easier to spot. To indicate the message recipient, an email address also may have an associated display name for the recipient, which is followed by the address specification surrounded by angled brackets, for example: ''John Smith Earlier forms of email addresses on other networks than the Internet included other notations, such as that required by X.400, and the UUCP ''bang path'' notation, in which the address was given in the form of a sequence of computers through which the message should be relayed. This was widely used for several years, but was superseded by the Internet standards promulgated by the Internet Engineering Task Force (IETF). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Email address」の詳細全文を読む スポンサード リンク
|