Different Types Of Email Programs - Red Hat LINUX 7.2 Reference Manual

262 Chapter 16:Email
answers with a 250 line containing the various SMTP extensions it supports. Then, the connecting
server can use the supported extensions as it wishes to accomplish the goals of the communication.
One noticeable extension concerns the addition of SMTP Authentication through the AUTH command
as outlined in RFC-2554. Another widely used SMTP extension is detailed in RFC-2034, which
discusses the use of dot-separated, standardized error codes to be used between SMTP applications.
Reading the various RFCs that involve SMTP provides a background to the way email moves around
the Internet. In addition, you can connect to an SMTP server via telnet by specifying port 25, such as
telnet localhost 25. Executing a few commands and sending a mail manually is a good way
to get a handle on how SMTP communications occur.
Red Hat Linux uses Sendmail as its SMTP program by default, although various other applications
are available that have many of the same features but are easier to use, such as Postfix. Many email
client applications connect to an SMTP server directly to send messages, although you can configure
a locally running SMTP service to deliver your email for you, either directly to the final destination
or to a central email server to then be forwarded on.

16.2 Different Types of Email Programs

There are three types of email programs, all of which perform a specific role in the process of moving
and managing email messages. While most users are only aware of the specific email program they
use to receive and send messages, each of these types is important for making sure email arrives at the
correct destination.
16.2.1 Mail User Agent
A Mail User Agent (MUA) is a program that, at the very least, allows a user to read and compose
email messages. An MUA is often referred to as an email client. Of course, many MUAs help users do
more than that, including retrieving messages via the POP or IMAP protocols, setting up mailboxes to
store messages, or helping present new messages to a Mail Transfer Agent program that will deliver
them to their final destination.
MUA programs may be graphical, such as Mozilla Mail, or have a very simple, text-based interface,
such as Mutt, or Pine.
16.2.2 Mail Transfer Agent
A Mail Transfer Agent (MTA) transfers email messages between machines using SMTP. A message
may involve several MTAs as it moves to its intended destination. Most users are totally unaware of
the presence of MTAs, even though every email message is sent through at least one MTA.
While the delivery of messages between machines may seem rather straightforward, the entire process
of deciding if a particular MTA can or should accept a message for delivery to a remote host is quite