Do you have spams in your inbox? Maybe the answer will definitely YES. Most of our inbox has been filled with spams from all around the world. They can be in plain text or an image (more advanced techniques to bypass the spam filter). How do we solve this? Well, if you use Postfix, then you might try to use this solutions from HowToForge. It will guide you to configure a default Postfix installation and add Spam Assasin with a range of RBL (Realtime Blacklists) and RHBL (Same but different), Greylistings and Helo Checks.
One note before you start implement this tutorial : There's no 100% secured inbox unless you limit your inbox by yourself. If you publish it to the Internet, then someday a spam will come and visit your inbox :D