Exim is a popular, powerful and easily configurable mail transfer agent (MTA) included on most Linux operating systems.
If you want to install Exim on a CentOS VPS, all you have to do is install the EPEL repository:
rpm -Uvh http://fedora-epel.mirror.lstn.net/6/x86_64/epel-release-6-8.noarch.rpm
then, install Exim via yum:
yum install exim