Email Address - Overview

Overview

The transmission of email over the Internet normally uses the Simple Mail Transfer Protocol (SMTP), defined in Internet standards RFC 5321 and RFC 5322, and extensions like RFC 6531. Mailboxes themselves are most often accessed using the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP).

The general format of an email address is jsmith@example.org. It consists of two parts: the part before the @ sign is the local-part of the address, often the username of the recipient (jsmith), and the part after the @ sign is a domain name to which the email message will be sent (example.org).

It is not clear from the email address domain name what is the actual destination (the mailbox host) of an email. A mail server will use the Domain Name System, which is a distributed database, to find the IP address of the host of the domain. The server queries the DNS for any mail exchanger records (MX records) to find the IP address of a designated mail transfer agent (MTA) for that address. That way, the organization holding the delegation for a given domain – the Mailbox Provider – can define which are 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, it must simply accept mail for the domain. The target hosts are configured with a mechanism to deliver mail to all destination mail boxes. If no MX servers are configured, a mail server queries the A record for the domain. There is a chance that this server will accept email for this domain.

The local-part of an email address has no significance to intermediate mail relay systems other than the final mailbox host. Email programs and intermediate mail relay systems must not assume it to be case-insensitive, since the final mailbox host may or may not treat it as such. The same mailbox can be set up to receive emails from multiple email addresses. Conversely, a single email address may be an alias and have a distribution function 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 such results.

The addresses found in the header fields of an email message are not the ones used by SMTP servers to deliver the message. Servers use the so-called message envelope to route mail. 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.

Further information: Email authentication, Anti-spam techniques

To indicate for whom the message is intended, a user can use the "display name" of the recipient followed by the address specification surrounded by angled brackets, for example: John Smith .

Earlier forms of email addresses included the somewhat verbose notation 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 generally more convenient SMTP form.