To those who are using Linux on their mail server will find this tutorial useful. Whilst Linux is not really prone to getting virus unlike that other operating system from Redmond, more often than not, clients connecting to the Linux-based mail server run that OS from Redmond. It is important to add virus-scanning tool in the server to filter all viruses before they reach the client.
In addition, spam is a universal problem, even if you are running Linux or Mac. Filtering spam is also a must-have feature on mail servers.
Whilst the tutorial was written for Ubuntu and, it should not be that complex to port it to other Linux distributions. Good luck!
(image from MrDLogan.)