Email management is a tricky business as email deliverability and troubleshooting such issues can have factors outside of provided software. I provide and develop Centmin Mod for free to users so I don't have the time to delve into email deliverability or troubleshooting issues which may have nothing to do with software I provide i.e. spam blacklists, viruses, dkim, spf, dmarc policy misconfigurations or lack of such preventing emails from reaching your intended recipients. A 3rd party service is paid or solely focused on dealing with those issues. Though, Centmin Mod latest beta has an addon script to auto generate the server's main hostname's dkim and spf settings so you can manually add to your DNS settings to ensure better email deliverability for server outbound emails at server level.
Also for proper anti-virus/malware scanning of locally hosted email software, you will increase your server resources dramatically for memory, disk and mysql usage if you have considerable traffic. You can expect memory usage requirements easily increase by 2-8GB of memory if you want to properly host your own email @domain.com with proper anti-virus/malware, anti-spam etc. It's why you may have read cpanel/WHM like control controls have higher memory requirements as well.
Try running cpanel/WHM on 256MB VPS server for instance - you won't be able to ! Centmin Mod has minimum memory requirements and disk on official site's install page but on right CentOS 6 32bit OS you can run on 128-256MB VPS, for 64bit OS 256-512MB VPS server. CentOS 7 based generally needs min 1GB memory regardless of Centmin Mod or cpanel/WHM. So for CentOS 7 + cpanel/whm may need at least 2GB memory for the same site on Centmin Mod LEMP can get by with 1GB provided your site resource usage needs are met. Part of why this is possible is Centmin Mod doesn't handle @domain.com email management and the other part is because of LEMP - Nginx usage compared to cpanel/WHM LAMP - Apache usage.
Also uptime and reliability of 3rd party @domain.com providers like Google App/Gsuite, Zoho etc important. It makes moving between web hosts easier as your @domain email operation is unaffected as the mail server is off server. Even if you server is down, your 3rd party @domain.com email will still be operational and working uninterrupted