An e-mail client, aka Mail User Agent (MUA), aka e-mail reader is a frontend computer program used to manage e-mail.

Sometimes, the term e-mail client is also used to refer to any agent acting as a client toward an e-mail server, independently of it being a real MUA, a relaying server, or a human typing directly on a telnet terminal. In addition, a web application providing the relevant functionality is sometimes considered an e-mail client.

Functionality and configuration of a MUA

Although mail user agents aim at enabling users to deal with their mail with minimal technical knowledge, some understanding of the operations involved is useful for making savvy configuration decisions.

Retrieving messages from a mailbox

Like most client programs, a MUA is only active when a user runs it. Messages arrive on the Mail Transfer Agent (MTA) server. Unless the MUA has access to the server's disk, messages are stored on a remote server and the MUA has to request them on behalf of the user.

In the first case, shared disk, a user logs on a server and runs a MUA on that machine. The MUA reads messages from a conventionally formatted storage, typically mbox, within the user's HOME directory. The MTA uses a suitable Mail Delivery Agent (MDA) to add messages to that storage, possibly in concurrence with the MUA. This is the default setting on many Unix systems. Webmail applications running on the relevant server can also benefit from direct disk access to the mail storage.

For personal computing, and whenever messages are stored on a remote system, a mail user agent connects to a remote mailbox to retrieve messages. Access to remote mailboxes comes in two flavors. On the one hand, the Post Office Protocol (POP) allows the client to download messages one at a time and only delete them from the server after they have been successfully saved on local storage. It is possible to leave messages on the server in order to let another client download them. However, there is no provision for flagging a specific message as seen, answered, or forwarded, thus POP is not convenient for users who access the same mail from different machines or clients. On the other hand, the Internet Message Access Protocol (IMAP) allows users to keep messages on the server, flagging them as appropriate. IMAP provides sub-folders. Typically, the Sent, Drafts, and Trash folders are created by default.

Both POP and IMAP clients can be configured to access more mailboxes at the same time, as well as to check each mailbox every given number of minutes. IMAP features an idle extension for real time updates, that may work better than polling when long lasting connections are feasible.