A software application used to compose, send, receive and view e-mails. Some common examples include Outlook, Thunderbird, Apple Mail and Google Gmail. Like most client programs, an email client is only active when a user runs it. The most common arrangement is for an email user (the client) to make an arrangement with a remote Mail Transfer Agent (MTA) server for the receipt and storage of the client’s emails. The MTA, using a suitable mail delivery agent (MDA), adds email messages to a client’s storage as they arrive. The remote mail storage is referred to as the user’s mailbox. The default setting on many Unix systems is for the mail server to store formatted messages in mbox, within the user’s HOME directory. Of course, users of the system can log-in and run their mail client on the same computer that hosts their mailboxes; in which case, the server is not actually remote, other than in a generic sense.
Emails are stored in the user’s mailbox on the remote server until the user’s email client requests them to be downloaded to the user’s computer, or can otherwise access the user’s mailbox on the possibly remote server. The email client can be set up to connect to multiple mailboxes at the same time and to request the download of emails either automatically, such as at pre-set intervals, or the request can be manually initiated by the user. A user’s mailbox can be accessed in two dedicated ways. The Post Office Protocol (POP) allows the user to download messages one at a time and only deletes them from the server after they have been successfully saved on local storage. Alternatively, the Internet Message Access Protocol (IMAP) allows users to keep messages on the server, flagging them as appropriate. IMAP provides folders and sub-folders, which can be shared among different users with possibly different access rights.