Resolve CentOS (Vesta Panel) server downtime issues

I have issue with my VestaCP (CentOS) server. I believe the exim has a problem and probably the panel should be downgraded. Because of this problem my server goes down and offline every once or twice in a day and currently my websites are down. I want to delete unwanted folders and subdomains, optimise the server and correct misconfigurations on it if.

Please bid if you are a server expert.