I have a howto on setting up a Centos7 multidomain mail server at:
This is on a CubieTruck armv7, but it is Centos7 which SHOULD mean that you can install the whole suite on a ClearOS7 box.
This is an improvement on my Redsleave6 mailserver. So I have been using this sort of setup for a lot of years.
Just note that amavis and clamav can eat up 2 cores easily.