An Email mailbox is the email equivalent of a Letter box, it is where email messages are delivered.

Usage

An Email client retrieves messages from one or more mailboxes. The file or directory where the client stores the messages is called the local mailbox.

Popular protocols to retrieve messages are

the Post Office Protocol 

usually eliminates messages from the server's mailbox, and

the Internet Message Access Protocol

designed to retrieve messages from different hosts or clients, usually keeps messages on the server but can save a copy on the local mailbox.

Messages can also be retrieved using a web browser if the server hosts a suitable service.

Formats

Any kind of database can be used to store mail messages. However, some standardization is desirable in order 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.

Role in the Email Transport System

A mailbox can be identified by an email address. In that sense, it can receive email messages. However, not all email addresses correspond to a physical storage location. The term pseudo-mailbox is sometimes used to refer to an address that does not correspond to a final user. Email forwarding may be applied to reach real recipients from such addresses. Mailing lists and email aliases are typical examples.

RFC 5321, section 2.3.11 Mailbox and Address, defines that an email address is 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 is slightly different. In section 3.4. Address Specification, it mentions just that 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 fax transmission.