To protect you from spammers, you have to be sure that the company you choose to your VPS server works by using Port 25 and doesn't block it. Purple Hat Insights is usually a proactive management Resource for publicly supported versions of RHEL. Following installing and registering the Insights https://awslinuxsupport89000.gynoblog.com/26473480/helping-the-others-realize-the-advantages-of-whmcs-migration