Email address

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An email address identifies an email box to which email messages are delivered. This article covers modern Internet email, but many earlier email systems used different address formats.

An email address such as John.Smith@example.com is made up of a local part, an @ sign, then a domain part. The domain part is not case-sensitive, but local-parts normally are. In practice, the mail system at example.com may choose to treat John.Smith as equivalent john.smith or even johnsmith.[1] Mail systems often limit their users' choice of name to a subset of the technically valid characters, and may in some cases also limit which addresses it is possible to send mail to.

Ongoing internationalization efforts are underway to allow non-ASCII characters to be used in both the local and domain parts of an email address, allowing addresses such as Pelé@example.com and 甲斐@黒川.日本, but these are still constrained by the current generation of email servers and clients.

Overview[edit]

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)[citation needed], though the use of webmail (where the mailbox contents are rendered in a client browser by HTML) is becoming more common.

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 (DNS), 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.

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 <john.smith@example.org>.

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.

Syntax[edit]

The format of email addresses is local-part@domain where the local-part may be up to 64 characters long and the domain name may have a maximum of 255 characters – but the maximum 256 characters length of a forward or reverse path restricts the entire email address to be no more than 254 characters.[2] The formal definitions are in RFC 5322 (sections 3.2.3 and 3.4.1) and RFC 5321 – with a more readable form given in the informational RFC 3696[3] and the associated errata.

Local part[edit]

The local-part of the email address may use any of these ASCII characters RFC 5322 Section 3.2.3, RFC 6531 permits Unicode beyond the ASCII range:

The restrictions for special characters are that they must only be used when contained between quotation marks, and that 2 of them (the backslash \ and quotation mark " (ASCII: 92, 34)) must also be preceded by a backslash \ (e.g. "\\\"").

A quoted string may exist as a dot separated entity within the local-part, or it may exist when the outermost quotes are the outermost characters of the local-part (e.g. abc."defghi".xyz@example.com or "abcdefghixyz"@example.com are allowed. Conversely, abc"defghi"xyz@example.com is not; neither is abc\"def\"ghi@example.com). Quoted strings and characters however, are not commonly used. RFC 5321 also warns that "a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form".

The local-part "postmaster" is treated specially – it is case-insensitive, and should be forwarded to the server's administrator. Technically all other local-parts are case sensitive, therefore jsmith@example.com and JSmith@example.com specify different mailboxes; however, many organizations treat uppercase and lowercase letters as equivalent.

Most organizations do not allow use of many of the technically valid special characters. Organizations are free to restrict the forms of their own email addresses as desired, e.g., Windows Live Hotmail, for example, only allows creation of email addresses using alphanumerics, dot (.), underscore (_) and hyphen (-).[4]

Systems that send mail must be capable of handling outgoing mail for all valid addresses. Contrary to the relevant standards, some defective systems treat certain legitimate addresses as invalid and fail to handle mail to these addresses. Hotmail, for example, refuses to send mail to any address containing any of the following standards-permissible characters: !#$%*/?^`{|}~.[citation needed]

Domain part[edit]

The domain name part of an email address has to conform to strict guidelines: it must match the requirements for a hostname, consisting of letters, digits, hyphens and dots. In addition, the domain part may be an IP address literal, surrounded by square braces, such as jsmith@[192.168.2.1], although this is rarely seen except in email spam. Internationalized domain names (which are encoded to comply with the requirements for a hostname) allow for presentation of non-ASCII domain parts.

Comments are allowed in the domain part as well as in the local part. E.g. "john.smith@(comment)example.com" and "john.smith@example.com(comment)" are equivalent to "john.smith@example.com".

Examples[edit]

Valid email addresses[edit]

Invalid email addresses[edit]

Common local-part semantics[edit]

According to RFC 5321 2.3.11 Mailbox and Address, "...the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address.". This means that no assumptions can be made about the meaning of the local-part of another mail server. It is entirely up to the configuration of the mail server.

Local-part normalization[edit]

Interpretation of the local-part of an email address is dependent on the conventions and policies implemented in the mail server. For example, case sensitivity may distinguish mailboxes differing only in capitalization of characters of the local-part, although this is not very common.[5] Gmail ignores all dots in the local-part for the purposes of determining account identity.[6] This prevents the creation of user accounts your.user.name or yourusername when the account your.username already exists.

Address tags [edit]

Some mail services allow a user to append a tag to their email address (e.g., where joeuser@example.com is the main address, which would also accept mail for joeuser+work@example.com or joeuser-family@example.com). The text of tag may be used to apply filtering and to create single-use addresses.[7] Some IETF standards-track documents, such as RFC 5233 refer to this convention as "sub-addressing". However, many websites' automatic form validation scripts or software will reject + as an invalid character in the email address.

Disposable email addresses of this form, using various separators between the base name and the tag, are supported by several email services, including Runbox (plus and hyphen), Gmail (plus),[8] Yahoo! Mail Plus (hyphen),[9] Apple's MobileMe (this site is now closed and changed to iCloud.com) (plus), Outlook.com (plus),[10] FastMail.FM (plus and Subdomain Addressing),[11] and MMDF (equals).

Most installations[which?] of the qmail and Courier Mail Server products support the use of a hyphen '-' as a separator within the local-part, 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 an application based on the tagging system established.[12][13]

Postfix allows configuring an arbitrary separator from the legal character set. The separator info remains available on the email (address is not rewritten to remove it), and thus is useful in internal mail-routing, filtering, and forwarding via any of the mechanisms existing in Postfix.[14]

Validation & Verification[edit]

Not only are email addresses used in a mail client or on a mail server, but also used in websites where a user-supplied email address is often validated.

An email address is generally recognized as having two parts joined with an at-sign (@); this in itself is a basic form of validation. However, the technical specification detailed in RFC 822 and subsequent RFCs go far beyond this, offering very complex and strict restrictions.[15]

It is impossible to match these restrictions with a single technique and using for instance regular expressions will result in long and still not a perfect pattern.[16]

Unfortunately syntactically correct, "verified" email addresses will not guarantee Email box existence. Thus many mail servers use other techniques and check the mailbox existence against relevant systems such as DNS for the domain part or using callback verification to check if the mailbox exists. (This is however often disabled to avoid Directory Harvest Attack).

Assuring an email address is of a good quality, requires a combination of various validation techniques. Large websites, bulk mailers or spammers, require fast algorithms that will predict validity of email address. Such methods depend heavily on Heuristic algorithms and Statistical_models.[17]

Conversely, many websites check and validate email addresses much more strictly than the standard specifies, rejecting addresses containing valid characters like + or / signs, or setting arbitrary length limitations (e.g., 30 characters). RFC 3696 was written to give specific advice for validating internet identifiers, including email addresses.

With many browsers now having implemented support for HTML5 forms, using the new 'email' state of the input element allows email address validation to be handled by the browser.

Identity validation[edit]

Despite the growth of the World Wide Web as a primary interface for communication, email addresses continue to remain the primary means (besides cell phone number validation, postal mail validation, fax validation, etc.) of identity validation for website account activation. This is usually accomplished by the website sending a temporary hyperlink to the inbox of the user-provided email address in order to open, immediately activating the account. Email addresses are also useful as means of forwarding messages from the website (i.e., user messages, user actions, etc.) to the email inbox.

Internationalization[edit]

The IETF conducts a technical and standards working group devoted to internationalization issues of email addresses, entitled Email Address Internationalization (EAI, also known as IMA – Internationalized Mail Address).[18] This group produced RFCs 6530, 6531, 6532, and 6533, and continues to work on additional EAI related RFCs.

The IETF's EAI Working group published RFC 6530 "Overview and Framework for Internationalized Email", which enabled non-ASCII characters to be used in both the local and domain parts of an email address. RFC 6530 provides for email based on the UTF-8 encoding, which permits the full repertoire of Unicode. RFC 6531 provides a mechanism for SMTP servers to negotiate transmission of the SMTPUTF8 content.

The basic EAI concepts involve exchanging mail in UTF-8. Though the original proposal included a downgrading mechanisms for legacy systems this has now been dropped.[19] The local servers are responsible for the "local" part of the address, whereas the domain portion would be restricted by the rules of internationalized domain names, though still transmitted in UTF-8. The mail server is also responsible for any mapping mechanism between the IMA form and any ASCII alias.

EAI enables users to have a localized address in a native language script or character set, as well as an ASCII form for communicating with legacy systems or for script-independent use. Applications that recognize internationalized domain names and mail addresses must have facilities to convert these representations.

Significant demand for such addresses is expected in China, Japan, Russia, and other markets that have large user bases in a non-Latin based writing system.

Internationalization examples[edit]

These addresses are not compliant with RFC 5322 and will therefore not work with many of the current generation of email servers and clients. RFC 6530 compliant servers should be able to handle these.

Internationalization support[edit]

Modified versions of sendmail and postfix exist that support the proposed EAI rules.[citation needed] Google has limited support for the earlier experimental RFC.

See also[edit]

References[edit]

  1. ^ "...you can add or remove the dots from a Gmail address without changing the actual destination address; they'll all go to your inbox...", Google.com
  2. ^ RFC 5321, section 4.5.3.1. Size Limits and Minimums explicitly details protocol limits.
  3. ^ Written by J. Klensin, the author of RFC 5321
  4. ^ The character limitation is written in plain English in the subscription page "Sign up for Windows Live". Retrieved 2008-07-26. . However, the phrase is hidden, thus one has to either check the availability of an invalid ID, e.g. me#1, or resort to alternative displaying, e.g. no-style or source view, in order to read it.
  5. ^ Are Email Addresses Case Sensitive? by Heinz Tschabitscher
  6. ^ Google Mail – help center article
  7. ^ "Instant disposable Gmail addresses" by Gina Trapani 2005
  8. ^ Using an address alias
  9. ^ help.yahoo.com
  10. ^ Outlook.com supports simpler "+" email aliases too
  11. ^ FastMail's subdomain addressing
  12. ^ "dot-qmail – control the delivery of mail messages". Retrieved 27 January 2012. 
  13. ^ Sill, Dave. "4.1.5. extension addresses". Life with qmail. Retrieved 27 January 2012. 
  14. ^ Postfix configuration parameters: recipient_delimiter
  15. ^ I Knew How To Validate An Email Address Until I Read The RFC]
  16. ^ Mail::RFC822::Address
  17. ^ Verification & Validation Techniques for Email Address Quality Assurance by Jan Hornych 2011, University of Oxford
  18. ^ "Eai Status Pages". Email Address Internationalization (Active WG). IETF. March 17, 2006-March 18, 2013. Retrieved July 26, 2008. 
  19. ^ "Email Address Internationalization (eai)". IETF. Retrieved November 30, 2010. 

External links[edit]