An email[4] address is a unique identifier for an individual’s electronic mailbox, following a specific format of local-part@domain. The local-part, with a maximum length of 64 octets, represents the specific mailbox, while the domain, up to 255 octets, indicates the mail server. Email addresses are essential for communication via PCs, mobile devices, or webmail sites, and are often used for user validation on websites. However, a correct format doesn’t guarantee existence. Techniques like callback verification are used to verify mailbox existence. Emails are transmitted through the Internet[3] using the SMTP protocol defined in RFC5321 and 5322, with the SMTP client using the domain name[1] to locate the mail exchange IP address[2]. Further, the IETF has worked towards internationalizing email addresses, enabling the use of non-ASCII characters. The domain of the email address must meet strict rules, including a limit of 63 characters and the inclusion of letters, digits, and hyphens.
An email address identifies an email box to which messages are delivered. While early messaging systems used a variety of formats for addressing, today, email addresses follow a set of specific rules originally standardized by the Internet Engineering Task Force (IETF) in the 1980s, and updated byRFC 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 from a local-part, the symbol @, and a domain, which may be a domain name or an IP address enclosed in brackets. Although the standard requires the local-part to be 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 them as equivalent to johnsmith. Mail systems often limit the users' choice of name to a subset of the technically permitted characters.
With the introduction of internationalized domain names, efforts are progressing to permit non-ASCII characters in email addresses.