Installed the free version of Virtualmin GPL on a clean install of Ubuntu 20.04. Didn't change my Contabo VPS servers default hostname before the install. So changed the hostname after the install with Webmin. While configuring Postfix I noticed /etc/postfix/main.cf in the mydestination parameter didn't update all the way with my new domain.. It had a localhost.contabo.net value that was the default from my VPS Contabo. It didn't' change to localhost.mydomain.com with Webmin hostname update. Don't know if it broke anything but just changed it manually.
Issue in Postfix /etc/postfix/main.cf file. mydestination didn't update the localhost.domain.com with my updated domain