E-mail address
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General information
An e-mail address identifies a location to which e-mail can be delivered. A modern Internet e-mail address (using SMTP) is a string of the form jsmith@example.com. It should be read as "jsmith at example dot com". 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 a domain name which can be looked up in the Domain Name System to find the mail exchange servers accepting e-mail for that address.
The domain name is often that of the of the e-mail service, such as Microsoft's Hotmail or Google's Gmail it could also be the domain name of the company that the recipient represents or even the domain of the recipient's personal site.
Earlier forms of e-mail address 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 This latter was in wide use for several years, but was superseded by the generally more convenient SMTP form.
Addresses found in the headers of e-mail should not be considered authoritative, because SMTP has no mechanism for authentication. Forged e-mail addresses are often seen in spam and in phishing and similar scams, leading to several initiatives, such as Sender ID, which aim to make such forgeries easier to spot. The most reliable method of authentication, however, is to require that messages be digitally signed.
Limitations
The format of Internet e-mail addresses is defined in RFC 2822, which permits only a subset of ASCII characters in e-mail addresses. RFC 1642, however, defines UTF-7, a way of encoding all Unicode characters using only characters permitted in e-mail; e-mail addresses using this standard are most commonly seen in Asia and not widely used in Europe or North America.
As defined in RFC 2821, the local-part of an e-mail address allows up to 64 characters maximum and the domain name a maximum of 255 characters.