An e-mail address identifies a location to which e-mail messages can be delivered. An e-mail address on the modern Internet looks like, for example, jsmith@example.com and is usually read as "jsmith at example dot com". Many earlier e-mail systems had different formats for e-mail addresses and because modern e-mail systems are partially based on, and compatible with these older systems, the exact format of an e-mail address is complicated and frequently misunderstood.

Overview

Most e-mail on the internet uses the Simple Mail Transfer Protocol (SMTP), which is defined in the internet standards RFC 5321 and RFC 5322.

E-mail addresses, such as jsmith@example.com, have two parts. The part before the @ sign is the local-part of the address, often the username of the recipient, and the part after the @ sign is the domain which is a hostname where the e-mail will be sent. Roughly speaking, the hostname is looked up in the Domain Name System to find the mail transfer agent or Mail eXchangers (MXs) accepting e-mail for that address.

The domain may have semantic meaning for any mail-system handling the address; the meaning is well-defined and changes to meaning involve changing every mail-server in existence. The local-part, by contrast, is supposed to be opaque to every mail-system except the system which is authoritative for the domain. This is what makes e-mail a federated system.

When a host receives an e-mail, it will be delivered to an e-mail mailbox. Some hosts allow more than one e-mail address to be sent to the same mailbox via an e-mail alias or even allow a catch-all address where the local-part can be undefined and e-mail would be delivered to a configured and existing e-mail address.

Often, the domain of an e-mail address is that of an e-mail service, such as Google's Gmail, Microsoft's Hotmail, etc. The domain can also be the domain name of the organization that the recipient represents, or of the recipient's personal site.

Addresses found in the header fields of e-mail should not be considered authoritative, because SMTP has no generally-required mechanisms for authentication. Forged e-mail 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 who the message is intended for, 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 e-mail 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.