An email address identifies a mailbox to which email messages are delivered. While early messaging systems used a variety of address formats, email addresses today follow a specific set of rules originally standardized by the Internet Engineering Task Force (IETF) in the 1980s, and updated by . The term "email address" in this article refers to just the addr-spec in Section 3.4 of . The RFC defines address more broadly as either a mailbox or group. A mailbox value can be either a name-addr, which contains a display-name and addr-spec, or the more common addr-spec alone.
An email address, such as john.smith@example.com, is made up of a local-part, the symbol @, and a domain, which may be either a domain name or an IP address enclosed in brackets. Although the standard stipulates that the local-part is case-sensitive, it also urges that receiving hosts deliver messages in a case-independent manner, e.g., that the mail system in the domain example.com treat John.Smith as equivalent to john.smith. (Some mail systems even treat both as equivalent to johnsmith.) Mail systems often limit the user's choice of name to a subset of the characters permitted by the RFCs. With the introduction of internationalized domain names, efforts are progressing to permit non-ASCII characters in email addresses.
Due to the ubiquity of email today, email addresses are used as regular usernames by many websites and services that provide a user profile or account. For example, if a user wants to log in to their Xbox Live video gaming profile, they would use their email address as the username ID of their Microsoft account, even though the service in this case is not email.
An email address consists of two parts, a local-part (sometimes a user name, but not always) and a domain. If the domain is a domain name rather than an IP address, then the SMTP client looks up the mail-exchange (MX) IP address of that domain name. The general format of an email address is local-part@domain, e.g. jsmith@<nowiki>[</nowiki>192.168.1.2<nowiki>]</nowiki>, jsmith@example.com. The SMTP client transmits the message to the mail exchange, which may forward it to another mail exchange, until it ultimately arrives at the host of the recipient's mail system.
The transmission of electronic mail from the sender's computer and between mail hosts in the Internet uses the Simple Mail Transfer Protocol (SMTP), defined in , and extensions such as . The mailboxes may be accessed and managed by applications on personal computers, mobile devices or webmail sites using the SMTP protocol and either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP).
When transmitting email messages, mail user agents (MUAs) and mail transfer agents (MTAs) use the domain name system (DNS) to look up a resource record (RR) for the recipient's domain. A mail exchange resource record (MX record) contains the name of the recipient's mail server. In the absence of an MX record, an address record (whether A or AAAA) directly specifies the mail host.
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 (mailboxes that receive messages regardless of the local-part) are common patterns for achieving a variety of delivery goals.
The addresses contained in the header fields of an email message are not directly used by mail exchanges to deliver the message. In SMTP mail transfer, the sending system communicates the source and destination addresses; this information is sometimes called the "message envelope". While the envelope and header addresses may be the same, this is not strictly required, and forged or "spoofed" email addresses are often seen in spam, phishing, and many other Internet-based scams. This has led to several initiatives aimed at making such forgeries of fraudulent emails easier to spot.
The format of an email address is local-part@domain, where the local-part may be up to 64 octets long and the domain may have a maximum of 255 octets. The formal definitions are in RFC 5322 (sections 3.2.3 and 3.4.1) and RFC 5321âÂÂwith a more readable form given in the informational RFC 3696 (written by J. Klensin, the author of RFC 5321) and the associated errata.
An email address also may have an associated "display-name" (Display Name) for the recipient, which precedes the address specification, surrounded by angled brackets in that case, for example: John Smith <john.smith@example.org>. Email spammers and phishers will often use "Display Name spoofing" to trick their victims, by using a false Display Name, or by using a different email address as the Display Name.
Earlier forms of email addresses for 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).
The local-part of the email address may be unquoted or may be enclosed in quotation marks.
If unquoted, it may use any of these ASCII characters: