The mail user agent, or MUA, is another necessary part of the email system. The MUA is a mail client, or mail reader, that allows the user to read and compose email and provides the user interface. (It is the email application itself that most users are familiar with as "email.") Some popular UNIX command-line MUAs are elm, pine, and mutt. Ubuntu also provides modern GUI MUAs: Evolution, Thunderbird, Mozilla Mail, Balsa, Sylpheed, and KMail. For comparison, common non-UNIX MUAs are Microsoft Outlook, Outlook Express, Pegasus, Eudora, and Netscape Messenger.
The Microsoft Windows and Macintosh MUAs often include some MTA functionality; UNIX does not. For example, Microsoft Outlook can connect to your Internet provider's mail server to send messages. On the other hand, UNIX MUAs generally rely on an external MTA such as Sendmail. This might seem like a needlessly complicated way to do things, and it is if used to connect a single user to her ISP. For any other situation, however, using an external MTA allows you much greater flexibility because you can use any number of external programs to handle and process your email functions and customize the service. Having the process handled by different applications gives you great control over how you provide email service to users on your network, as well as to individual and small office/home office (SOHO) users.
For example, you could do the following:
• Use Evolution to read and compose mail
• Use Sendmail to send your mail
• Use xbiff to notify you when you have new mail
• Use Fetchmail to retrieve your mail from a remote mail server
• Use Procmail to automatically sort your incoming mail based on sender, subject, or many other variables
• Use Spamassassin to eliminate the unwanted messages before you read them
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