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A mailboxISO/IEC 2382:2015 (also electronic mailbox, email box, email mailbox, e-mailbox) is the destination to which
electronic mail Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic (digital) version of, or counterpart to, mail, at a time when "mail" meant ...
messages are delivered. It is the equivalent of a
letter box A letter box, letterbox, letter plate, letter hole, mail slot or mailbox is a receptacle for receiving incoming mail at a private residence or business. For outgoing mail, Post boxes are often used for depositing the mail for collection, althou ...
in the postal system.


Definitions

A mailbox is identified by an
email address An email address identifies an email box to which messages are delivered. While early messaging systems used a variety of formats for addressing, today, email addresses follow a set of specific rules originally standardized by the Internet Engineer ...
. However, not all email addresses correspond to a storage facility. The term ''pseudo-mailbox'' is sometimes used to refer to an address that does not correspond to a definitive mail store.
Email forwarding Email forwarding generically refers to the operation of re-sending an email message delivered to one email address to one or more different email addresses. The term ''forwarding'', used for mail since long before electronic communications, has no ...
may be applied to reach end recipients from such addresses. Electronic mailing lists and
email alias An email alias is simply a forwarding email address. The term ''alias expansion'' is sometimes used to indicate a specific mode of email forwarding, thereby implying a more generic meaning of the term ''email alias'' as an address that is forwarde ...
es are typical examples. RFC 5321, defines an ''email address'' as a character string that identifies a user to whom mail will be sent or a location into which mail will be deposited. The term ''mailbox'' refers to that depository. In that sense, the terms ''mailbox'' and ''address'' can be used interchangeably. RFC 5322 defines a mailbox as follows: ''A mailbox receives mail. It is a 'conceptual entity' that does not necessarily pertain to file storage.'' It further exemplifies that some sites may choose to print mail on a printer and deliver the output to the addressee's desk, much like a traditional
fax Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer o ...
transmission.


Access

Access to a mailbox is controlled by a
mailbox provider A mailbox provider, mail service provider or, somewhat improperly, email service provider is a provider of email hosting. It implements email servers to send, receive, accept, and store email for other organizations or end users, on their behalf ...
. Usually, anyone can send messages to a mailbox while only authenticated users can read or delete from their own mailboxes. An
email client An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email. A web application which provides message management, composition, and reception functio ...
retrieves messages from one or more mailboxes. The database (file, directory, storage system) in which the client stores the messages is called the ''local mailbox''.


Read access

Popular client–server protocols to retrieve messages are: *
Post Office Protocol In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. POP version 3 (POP3) is the version in common use, and along with IMAP the most common ...
(POP): a method that is most suitable for reading messages from a single client computer. Usually messages are removed from the server mailbox after retrieval. Anyway, the master copy of a message is the one in the local mailbox. *
Internet Message Access Protocol In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection. IMAP is defined by . IMAP was designed with the goal of per ...
(IMAP): designed to retrieve messages from multiple clients by allowing remote management of the server mailbox. Master copies stay on the server, but a copy can be saved locally. * Webmail over
HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
: messages are served to a user's browser in a server-defined format. Master copies stay on the server, possibly in the original format, which may be downloadable. IMAP and webmail can go along with each other more or less seamlessly. POP, if configured to leave messages on server, can be compatible with them. Internet message format, currently defined by RFC 5322, dates back to 1982 (RFC 822). That is what POP and IMAP clients expect to retrieve.


Write access

Messages sent to a mailbox are written by a
mail delivery agent A message delivery agent (MDA), or mail delivery agent, is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's mailbox., ''Internet Mail Architecture'', D. Crocker (July 2009) It is also call ...
into the server's local mailbox, which, for remote users, is a remote mailbox that they own on that server. IMAP clients can copy, move, and delete messages in remote mailboxes.


Size quota

Mailboxes have a size limit, either determined implicitly by available memory, or after quota definitions for that mailbox or folders thereof. Besides administrative trivia, quota limits help mitigate
email bomb On Internet usage, an email bomb is a form of net abuse that sends large volumes of email to an address to overflow the mailbox, overwhelm the server where the email address is hosted in a denial-of-service attack (DoS attack) or as a smoke scre ...
attacks. An IMAP extension for quota was standardized in 1997.


Storage format

Any kind of database can be used to store email messages. However, some standardization has resulted in several well-known file formats to allow access to a given mailbox by different
computer program A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components. A computer program ...
s. There are two kinds of widely used formats: *
mbox Mbox is a generic term for a family of related file formats used for holding collections of email messages. It was first implemented in Fifth Edition Unix. All messages in an mbox mailbox are concatenated and stored as plain text in a single f ...
is the original technique of storing all messages in a single file, *
Maildir The Maildir e-mail format is a common way of storing email messages in which each message is stored in a separate file with a unique name, and each mail folder is a file system directory. The local file system handles file locking as messages are ...
is a newer specification that provides for storing all messages in a directory tree, with one file for each message.


Mailbox names

A mailbox name is the first part of an email address, also known as ''local-part''; that is, the part before the @ symbol. Its format is formally specified by RFC 5322 and RFC 5321. It is often the
username A user is a person who utilizes a computer or network service. A user often has a user account and is identified to the system by a username (or user name). Other terms for username include login name, screenname (or screen name), accoun ...
of the recipient on the mail server or in the destination domain. The local-part may be up to 64 characters long and, in theory, is case-sensitive. It can consist of either a sequence of ''valid characters'' (described below) or a quoted string, which can also contain spaces and special characters. Using
SMTPUTF8 The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typica ...
extension of SMTP it is also possible to use non-ASCII characters. Some common sense is needed when creating new mailbox names, in order to avoid common pitfalls. In the words of RFC 5321, very wary of imposing restrictions:


Valid characters

The following characters may appear in a local-part without quoting: * Uppercase and lowercase English letters (a–z, A–Z), and
UTF-8 UTF-8 is a variable-length character encoding used for electronic communication. Defined by the Unicode Standard, the name is derived from ''Unicode'' (or ''Universal Coded Character Set'') ''Transformation Format 8-bit''. UTF-8 is capable of ...
sequences if using SMTPUTF8 * Digits 0 to 9 * Characters ! # $ % & ' * + - / = ? ^ _ ` ~ * Character . (dot) provided that it is not the first or last character, and provided also that it does not appear two or more times consecutively (e.g. John..Doe@example.com).


Reserved names

The names "postmaster", "abuse", and others correspond to well-known roles and functions, and are required to be valid. Some names are known to cause troubles, possibly because they conflict with names used internally by (some parts of) the mail software, including mail filters, or because the underlying
storage system Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a computer ...
chokes on them. A number of lists exist, for example on
GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, continu ...
.


References

{{Portal, Internet Email