Any program that actually handles a message for delivery to the point where it can be read byan email client application can be considered an MDA. For this reason, some MTAs (such asSendmail and Postfix) can fill the role of an MDA when they append new email messages to alocal user's mail spool file. In general, MDAs do not transport messages between systems nordo they provide a user interface; MDAs distribute and sort messages on the local machine foran email client application to access.2.3. Mail User AgentA Mail User Agent (MUA) is synonymous with an email client application. An MUA is a programthat, at the very least, allows a user to read and compose email messages. Many MUAs arecapable of retrieving messages via the POP or IMAP protocols, setting up mailboxes to storemessages, and sending outbound messages to an MTA.MUAs may be graphical, such as Mozilla Mail, or have a very simple, text-based interface, suchas mutt.3. Mail Transport AgentsRed Hat Enterprise Linux includes two primary MTAs, Sendmail and Postfix. Sendmail isconfigured as the default MTA, although it is easy to switch the default MTA to Postfix.TipFor information about how to switch the default MTA from Sendmail to Postfix,refer to the chapter called Mail Transport Agent (MTA) Configuration in the RedHat Enterprise Linux System Administration Guide.3.1. SendmailSendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually usingthe SMTP protocol. However, Sendmail is highly configurable, allowing control over almostevery aspect of how email is handled, including the protocol used. Many system administratorselect to use Sendmail as their MTA due to its power and scalability.3.1.1. Purpose and LimitationsIt is important to be aware of what Sendmail is and what it can do, as opposed to what it is not.In these days of monolithic applications that fulfill multiple roles, Sendmail may seem like theonly application needed to run an email server within an organization. Technically, this is true,as Sendmail can spool mail to each users' directory and deliver outbound mail for users.However, most users actually require much more than simple email delivery. Users usually wantto interact with their email using an MUA, that uses POP or IMAP, to download their messagesto their local machine. Or, they may prefer a Web interface to gain access to their mailbox.These other applications can work in conjunction with Sendmail, but they actually exist forMail User Agent189
