E-mail address

An e-mail address, also known as an eddress (from electronic address) or sometimes (in a technical misuse of the word), simply as one's email, identifies a location to which e-mail can be delivered. A modern Internet e-mail address (using SMTP or Usenet) 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 transfer agent or Mail eXchangers (MXs) accepting e-mail for that address.

Overview

The domain name of an e-mail address is often that of the e-mail service, such as Microsoft's Hotmail or Google's Gmail. The domain name can be also the domain name of the company that the recipient represents or 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 This latter was in wide use for several years, but was superseded by the generally more convenient SMTP form.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 generally required mechanisms for authentication. Forged e-mail addresses are often seen in spam, phishing, and similar scams, leading to several initiatives, which aim to make such forgeries easier to spot.

To indicate where the e-mail should go, a user normally types the "display name" of the recipient followed by the address specification surrounded by angled brackets, for example "John Smith ".

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.

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. The local-part "MUST BE treated as case sensitive. [...] However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged."

According to RFC 2822, the local-part of the e-mail may use any of these ASCII characters:
* Uppercase and lowercase letters
* The digits 0 through 9
* The characters, ! # $ % & ' * + - / = ? ^ _ ` { | } ~
* The character "." provided that it is not the first or last character in the local-part.

Additionally, RFC 2821 and RFC 2822 allow the local-part to be a quoted-string, as in "John Doe"@example.com, thus allowing characters in the local-part that would otherwise be prohibited. However, RFC 2821 warns: "a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form".

The domain name is much more restricted. The dot separated domain labels are limited to "letters, digits, and hyphens drawn from the ASCII character set ... Mailbox domains are not case sensitive."

The informationalRFC 3696written by the author of RFC 2821 explains the details in a readable way, with a few minor errorsnoted in the 3696 errata.

Plus (or Minus) addressing

According to RFC 2821, "the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address. In particular, for some hosts the user "smith" is different from the user "Smith".

Plus addressing is one of the benefits of this limitation. Some mail servers allow a user to append +tag to their email address (joeuser+tag@example.com). The text of tag can be used to apply filtering.

Some systems violate RFC 2822 by refusing to send mail addressed to a user on another system merely because the local-part of the address contains the plus sign (+). Users of these systems cannot use plus addressing.

On the other hand, most qmail installations support the use of '-' as a separator between local-address and domain parts. Such as joeuser-tag@example.com or joeuser-tag-sub-anything-else@example.com. This allows qmail through .qmail-default or .qmail-tag-sub-anything-else files to sort, filter, forward, or run application based on the tagging system established. Procmail and SpamAssassin are common applications to use with qmail to help sort out spam or further filter incoming email.

References

*RFC 2821: Simple Mail Transfer Protocol
*RFC 2822: Internet Message Format
*RFC 3696: Application Techniques for Checking and Transformation of Names
* E-mail addresses with a "+" are VALID
* Discussion of plus addressing
* Verify email address: Verify the syntax & format, the domain name, a valid user and actual mailbox. (RFC 2822 compliant online free tool).
* Another verify email address tool: Verify the syntax & format, the domain name, a valid user and actual mailbox. (RFC 2822 compliant online free tool).