E-mail address
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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.
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[edit] Overview
Most e-mail on the internet uses the Simple Mail Transfer Protocol (SMTP), which is defined in the internet standards RFC 2821 and RFC 2822.
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. 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.
When a host receives an e-mail, it will be delivered to an e-mail mailbox. Some hosts allow 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 email would be delivered to a configured and existing email 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 organisation 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.
- Further information: E-mail authentication, Anti-spam techniques (e-mail)
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 <john.smith@example.com>.
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.
[edit] Restrictions
E-mail addresses are formally defined in RFC 2822 (mostly section 3.4.1) and to a lesser degree RFC 2821. An email address is a string of a subset of ASCII characters separated into 2 parts by an "@" (at sign), a "local-part" and a domain, that is, local-part@domain.
The "local-part" of an e-mail address is can be up to 64 characters (however servers are encouraged to not limit themselves to accepting only 64 characters) and the domain name a maximum of 255 characters.
The local-part of the e-mail address may use any of these ASCII characters:
- Uppercase and lowercase letters
- Digits
0through9 - Characters
! # $ % * / ? | ^ { } ` ~ & ' + - = _ - Character
.provided that it is not the first nor last character, nor may it appear two or more times consecutively.
Additionally, quoted-strings (ie: "John Doe"@example.com) are permitted, thus allowing characters that would otherwise be prohibited, however they do not appear in common practice. RFC 2821 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 is case sensitive, so "jsmith@example.com" and "JSmith@example.com" may be delivered to different people. This practice is, however, discouraged by RFC 2821.
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, dot (.), underscore (_) and hyphen (-), and will not allow sending mail to any email address containing ! # $ % * / ? | ^ { } ` ~.[citation needed]
The domain name is much more restricted, as they must match the requirements for a hostname, basically letters, digits, hyphens and dots. In addition, the domain may be an IP address literal, surrounded by square braces, such as jsmith@[192.0.2.1], although this is rarely used in practice, except by spammers.
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] Examples
Valid e-mail addresses
- Abc@example.com
- Abc.123@example.com
- 1234567890@domain.com
- abcd@example-one.com
- _______@domain.com
- user+mailbox/department=shipping@example.com
- !#$%&'*+-/=?^_`.{|}~@example.com
- "Abc@def"@example.com
- "Fred Bloggs"@example.com
- "Joe.\\Blow"@example.com
Invalid e-mail addresses
- Abc.example.com (char @ is missing)
- Abc.@example.com (last char of local part is a dot(.))
- Abc..123@example.com (char dot(.) is double)
[edit] Plus (or Minus) addressing
According to RFC 2821 2.3.10 Mailbox and Address, "...the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address.".
Plus addressing is one of the benefits of this limitation. Some mail services 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 a dash '-' 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 application based on the tagging system established.
Disposable e-mail addresses of this form, using various separators between the base name and tag are supported by several email services, including Runbox (plus and minus), Google Mail (plus), Yahoo! Mail Plus (minus)[1], and FastMail (plus)[2].
[edit] Validation & Verification
Determining the validity of an e-mail address is a common and, unfortunately, difficult task.[3] The problem arises for example in online forms, where the user is asked to enter an e-mail address to allow them to be contacted later. As this is often the only contact information available, there are good reasons for trying to ensure that the address given is indeed correct.
In general, two types of validity checking may be desired:
- determining whether an address is syntactically valid according to the rules above, or
- determining whether e-mail can actually be delivered to the address.
The former may be accomplished by parsing the address according to the syntax rules described above, and possibly subjecting the domain name part to further validity checks. Unfortunately, many widespread approaches, often based on regular expressions, tend to match only a subset of all valid addresses, potentially causing difficulties for users whose address doesn't happen to match the programmer's expectations. Often, the best approach may be to simply check for the few features that can be relied on to be present in any valid address, such as the presence of an @ sign. Such a check will accept many invalid addresses, but will hopefully at least ensure that the user has entered something that might be an e-mail address, rather than, say, their street address.
In general, the only way to reliably determine whether an address can actually receive e-mail is in the form of verification, which usually means sending a test message to it and have the recipient confirm that they've seen it, for example by entering a randomly generated code included in the message or accessing a URL containing such a code. It's worth noting that merely the apparently successful delivery of a message without errors or bounce messages is not sufficient to guarantee validity, since many e-mail servers may accept and silently discard messages sent to nonexistent addresses.
Before attempting delivery to an address, some intermediate checks, such as querying the domain name system to ensure that the hostname in the address has a valid MX record, may be performed. Such checks are of limited use in cases where delivery would be attempted anyway if they pass, since the mail transport agent responsible for delivering the message will be doing the same checks in any case. They may, however, be useful additions to pure syntax validity checks, either in situations where the sending of test messages is considered too onerous, or as an initial check before the user has yet committed to having a test message sent to them.
In online applications based on HTML forms, validity checks may be done either on the server receiving the content after submission, or directly at the client end using client side scripting languages such as JavaScript. The latter have the advantage of being able to provide immediate feedback to the user, but are not always supported or enabled in all browsers, necessitating the implementation of redundant server-side checks. It can also be difficult to reliably implement anything more than simple syntax validity checking on the client side, due to variations in the client environment. A solution that can combine some of the advantages of both approaches is to use techniques like AJAX to have the client automatically contact the server to do the checking, allowing the same server-side code used for the final validation to be applied interactively.
[edit] References
- RFC 2821: Simple Mail Transfer Protocol
- RFC 2822: Internet Message Format
- RFC 3696: Application Techniques for Checking and Transformation of Names
- RFC 2142: Mailbox names for common services, roles and functions
