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I use the free plan from zoho mail. You just need to set up your users and put the mx records on your server. Easy to configure. :)
Take a look at How To Install and Setup Postfix on Ubuntu 14.04. While it is written for Ubuntu, it should still work fine on CentOS 6.5.
Make sure you run
sudo yum install postfix
instead of
sudo apt-get update
sudo apt-get install postfix