Email address

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 RFC 5322 and 6854. The term "email address" in this article refers to just the addr-spec in Section 3.4 of RFC 5322. 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.

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