Linux is a version of the UNIX operating system that has gained in popularity over the past few years. As an operating system, it is reliable, stable and can be downloaded free of cost. Due to these factors, several Internet Service Providers and other users use Linux as an operating system of choice.

The Linux Kernel (core) was created by a Finnish student, Linus Torvalds, who named it Linux. Torvalds then made available this code available to the world for free and invited others to add to it provided they kept their contributions free. Thousands of coders from around the world worked on enhancing Linux and it grew in features and the number of users. Today, Linux has evolved to become a full featured operating system and is poised to challenge the domination of Microsoft Windows.

Due to the low cost and reliability of Linux, it has become a popular operating system to host web servers and e-mail servers. Although Linux is harder to manage compared to Microsoft Windows, it offers more flexibility and configuration options.

Several open source e-mail servers are available for the Linux platform. Open source is a term used for software application whose source code is freely available. Coders participating in the open source initiative freely share their source code for applications believing that by allowing anyone who's interested to modify the source code, the application will be more useful and error-free over the long term. Open source applications are available without any charge. Linux is a part of open source software.

Popular open source Linux based e-mail servers include Exim, qmail and Qpoppter. Exim is a message transfer agent and was developed at the University of Cambridge. qmail is an Internet Mail Transfer Agent (MTA) and is a replacement for the Sendmail system bundled with the Unix operating systems. qmail uses the Simple Mail Transfer Protocol (SMTP) to exchange messages with message transfer agents on other computers.

Qpoppter is one of the most widely used server applications for the POP3 protocol. It supports the latest standards and includes a large number of optional features. Unlike Exim and qmail, Qpoppter is not available for free.