E-mail address

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An "e-mail address" identifies a location to which e-mail messages can be delivered. The word e-mail address is also used as the formal pre-registered authoritative electronic mailing delivery site for an individual (example: an attorney's e-mail address registered for delivery of proof of service digital copies of legal pleadings). 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 the domain-part which may be a host name or 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.

Main article: E-mail

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

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 This latter was widely used for several years, but was superseded by the generally more convenient SMTP form.

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.

Further information: E-mail authentication, Anti-spam techniques (e-mail)

To indicate where the message 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 <ap118@example.com>.

[edit] Limitations

The format of Internet e-mail addresses is defined in RFC 2822, which permits them to consist of only a subset of ASCII characters.

As defined in RFC 2821, the local-part of an e-mail address has a maximum of 64 characters (although servers are encouraged to not limit themselves to accepting only 64 characters) and the domain name a maximum of 255 characters. Unlike everything else in the header, the local-part "MUST BE treated as case sensitive. [...] However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged."

Local part is the portion of a mail address before the @ character. This normally identifies a particular mailbox within a site mail system so is not usually of interest to other mail systems.

According to RFC 2822, the local-part of the address may use any of these ASCII characters:

  • Uppercase and lowercase letters (case sensitive)
  • 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".

Notwithstanding the addresses permitted by these standards, some systems impose more restrictions on email addresses, both in email addresses created on the system and in email addresses to which messages can be sent. Hotmail, for example, only allows creation of email addresses using alphanumerics and . _ - and will not allow sending mail to any email address containing ! # $ % * + / ? | ^ { } ` ~.[1]

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 informational RFC 3696 written by the author of RFC 2821 explains the details in a readable way, with a few minor errors noted in the 3696 errata.

[edit] 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, and the recommendations in RFC 3696, 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.

[edit] References

Wikibooks
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