Email address

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. This article uses the term email address to refer to the addr-spec defined in RFC 5322, not to the address that is commonly used; the difference is that an address may contain a display name, a comment, or both. 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. This article uses the term email address to refer to the addr-spec defined in RFC 5322, not to the address that is commonly used; the difference is that an address may contain a display name, a comment, or both. An email address such as John.Smith@example.com is made up of a local-part, an @ symbol, then a case-insensitive domain. Although the standard requiresthe local part to be case-sensitive, it also urges that receiving hosts deliver messages in a case-independent fashion,e.g., that the mail system at example.com treat John.Smith as equivalent to john.smith; some mail systems even treat them as equivalent to johnsmith. Mail systems often limit their users' choice of name to a subset of the technically valid characters, and in some cases 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. 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 local-part@domain, and a specific example is jsmith@example.com. An address consists of two parts. The part before the @ symbol (local-part) identifies the name of a mailbox. This is often the username of the recipient, e.g., jsmith. The part after the @ symbol (domain) is a domain name that represents the administrative realm for the mail box, e.g., a company's domain name, example.com. When delivering email, an SMTP client, e.g., Mail User Agent (MUA), Mail Transfer Agent (MTA), uses the domain name system (DNS) to look up a Resource Record (RR) for the recipient's domain (the part of the email address on the right of @); if there is a mail exchanger Resource Record (MX record) then the returned MX record contains the name of the recipient's mailserver, otherwise the SMTP client uses an address record (A or AAAA). The MTA next connects to this server as an SMTP client.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).