Email box

Email box

An email box (also email mailbox, e-mailbox) is the equivalent of a letter box for electronic mail. It is the destination where electronic mail messages are delivered.



A mailbox is identified by an email address. 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 may be applied to reach end recipients from such addresses. Electronic mailing lists and email aliases are typical examples.

RFC 5321[1], 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:[2] 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 transmission.


An email client 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.

Popular protocols to retrieve messages are:

  • Post Office Protocol: a client–server method that is most suitable for reading messages from a single client computer because message are removed from the server mailbox after first retrieval.
  • Internet Message Access Protocol: designed to retrieve messages from multiple clients by allowing remote management of the server mailbox by keeping master copies of messages on the server but can save a copy in the local mailbox.

Web-based clients exist that retrieve messages from the server on behalf of the website user and display them to the user in a suitable format in a web browser.

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 programs. There are two kinds of widely used formats:

  • mbox is the original technique of storing all messages in a single file,
  • Maildir is a newer specification that provides for storing all messages in a directory tree, with one file for each message.

Mailbox names

Mailbox names are the first part of an email address, i.e. the local-part before the @ symbol. The format of email addresses is formally defined in RFC 5322 and RFC 5321. It is often the username of the recipient on the mail server or in the destination domain.

The local-part of an email address may be up to 64 characters long and is case-sensitive.

Valid characters

The local-part of the email address may use any of these ASCII characters:

  • Uppercase and lowercase English letters (a–z, A–Z)
  • Digits 0 to 9
  • Characters ! # $ % & ' * + - / = ? ^ _ ` { | } ~
  • Character . (dot, period, full stop) 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.

Reserved names

The following names are sometimes reserved and use internally by various mail handling applications:[citation needed] cur, default, Discard, DRAFTS, INBOX, IMAP, Junk, Junkmail, mbox, mail.txt, new, SENT, SPAM, Spam, tmp, TRASH.


  1. ^ RFC 5321, Simple Mail Transfer Protocol, J. Klensin, The Internet Society (October 2008), Section 2.3.11 (Mailbox and Address)
  2. ^ RFC 5322, Internet Message Format, P. Resnick (Ed.), The Internet Society (October 2008), Section 3.4 (Address Specification)

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