Recently I had to fix problems on an existing mail server, that had a really messy configuration. So I resolved to start the whole thing again and used a fantastic script found at http://www.qmailtoaster.com
Installed CentOS 7, downloaded the script and launched, answered a few questions about the MariaDB database configuration, finally entered the email domain.
In 10 minutes I had a fully fledged mail server with spam+virus check running!
Linux Love <3