1. Configuring a Linux Email system

Email is probably one of the most popular and useful uses of a Linux based server or desktop system. Given the both the importance and popularity of email it is surprising to some people to find out how complex the email structure is on a Linux system and this complexity can often be a little overwhelming to the Linux newcomer.

The good news is that much of the complexity is there to allow experienced Linux administrators achieve complicated configurations for large scale enterprise installations. The fact is that for most Linux users it is relatively straight forward to set up a basic email systems so that they can send and receive electronic mail.

In this chapter we will explain the basics of Linux based email configuration and step through configuring a basic email environment. We will leave the complexities for the email system for more advanced books on the subject.

1.2 The structure of the Email System

There are a number of components that make up a complete email system. Below is a brief description of each one:

1.2.1 Mail User Agent

This is the part of the system that the user wil be most familiar with. The Mail User Agent (MUA) or mail client is the application that is used to write, send and read email messages. Anyone who has written and sent a message on any computer has used a Mail User Agent. Typical Graphical MUA’s on Linux are Evolution and KMail. For those who prefer a text based mail client there are also the more traditional pine and mail tools.

1.2.3 Mail Transfer Agent

The Mail Transfer Agent (MTA) is the part of the email system that does much of the work of transferring the email messages from one computer to another (either on the same local network or over the internet to a remote system). Once configured correctly most users will not have any direct interaction with their chosen MTA unless they wish to re-configure it for any reason. There are many choices of MTA available for Linux including sendmail, Postfix, Fetchmail, Qmail and Exim.

1.2.4 Mail Delivery Agent

Another part of the infrastructure that is typically hidden from the user. The Mail Delivery Agent (MDA) sits in the background and performs filtering on the email messages between the Mail Transfer Agent and the mail client (MUA). The most popular form of MDA is a spam filter to remove all the unwanted email messages from the system before they reach the inbox of the user’s mail client. Typical MDAs are Spamassassin and Procmail. It is important to note that some Mail User Agent applications (such as Evolution and KMail) include their own MDA filtering. Others, such as pine and Basla, do not. This can be a source of confusion to the Linux beginner.

1.2.5 SMTP

SMTP is an acronym for Simple Mail Transport Protocol. This is the protocol used by the email systems to transfer mail messages from one server to another. This protocol is essentially the communications language that the MTAs use to talk to each other and transfer messages back and forth.