Email client

The MTA, using a suitable mail delivery agent (MDA), adds email messages to a client's storage as they arrive.

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 a 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.

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.

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.

IMAP features an idle extension for real-time updates, providing faster notification than polling, where long-lasting connections are feasible.

Direct access can be more efficient but is less portable as it depends on the mailbox format; it is used by some email clients, including some webmail applications.

To better assist the user with destination fields, many clients maintain one or more address books and/or are able to connect to an LDAP directory server.

The email client is usually set up automatically to connect to the user's mail server, which is typically either an MSA or an MTA, two variations of the SMTP protocol.

Client settings require the name or IP address of the preferred outgoing mail server, the port number (25 for MTA, 587 for MSA), and the user name and password for the authentication, if any.

Examples of concern include the government censorship and surveillance and fellow wireless network users such as at an Internet cafe.

S/MIME employs a model based on a trusted certificate authority (CA) that signs users' public keys.

By contrast, both IMAP and webmail keep messages on the server as their method of operating, albeit users can make local copies as they like.

RFC 5068, Email Submission Operations: Access and Accountability Requirements, provides a survey of the concepts of MTA, MSA, MDA, and MUA.

For MSA, IMAP and POP3, the table reports also the labels that a client can use to query the SRV records and discover both the host name and the port number of the corresponding service.