SMTP is a server-to-server protocol that was designed to deliver mail to
systems that are always connected to the Internet. Dial-up systems connect
only at the user’s command; they connect for specific operations and are
frequently disconnected. To accommodate this difference, many mail systems
also include a mail delivery agent (MDA). The MDA transfers mail to systems
without permanent Internet connections. The MDA is similar to an MTA (see
the following note) but does not handle deliveries between systems and does
not provide an interface to the user.
NOTE
Procmail or Spamassassin are examples of MDAs; both provide filtering
services to the MDA while they store messages locally and then make them
available to the MUA or email client for reading by the user.The MDA uses the Post Office Protocol version 3 (POP3) or Internet
Message Access Protocol (IMAP) for this process. In a manner similar to a
post office box at the post office, POP3 and IMAP implement a “store and
forward” process that alleviates the need to maintain a local mail server if all
you want to do is read your mail. For example, dial-up Internet users can
intermittently connect to their ISP’s mail server to retrieve mail using
Fetchmail—the MDA recommended by Ubuntu. (See the section “Using
Fetchmail to Retrieve Mail,” later in this chapter.)
The Mail User Agent
The mail user agent (MUA) is another necessary part of the email system. The
MUA is a mail client, or mail reader, that enables 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 Mail, and Apple Inc.’s Mail.
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