I have been working on this problem for the past few days and still cant get the email notifications to work. I have installed and setup according to the quickstart guide on the nagios website. I first tried ubuntu then Centos because I thought maybe it was a bug in ubuntu but I have the same problem in both. I have tried configuring postfix and failed, I then tried sendmail and again failed. My question is does anyone know of a complete step by step guide how to setup nagios and also the email part, the OS really doesnt matter because at this point I will go with what works. From what I have found it should work but its not. Any help would be great.