Did server to server migration (https://www.virtualmin.com/documentation/system/migrate)
Both VPS are on the same Linux distributions. Did “Re-Check Configuration” & “Re-Run Install Wizard” on new machine, no problem everything went smooth but…
Perhaps a silly question, though because I’ve made this mistake a few times myself it’s worth eliminating the possibility of…
Is your MX record pointing to the new VPS?
Sometimes when you migrate a virtual server, the DNS records don’t get updated correctly, so you might still be pointing your MX record to the old VPS.
Failing that, feel free to fire me a message on Skype and I’ll take a look into what may be causing the issue over a screen sharing session (so we can resolve, and you can learn along the way).