I have just installed Virtualmin today on a Contabo VPS running Ubuntu 18.04. Took several attempts and had to reinstall the OS about 4 times due to Netplan issues, but I finally got there by rolling back changes made by install.sh to the /etc/01-netcfg.yaml file.
Anyway, I've configured Dovecot and Postfix and I've configured a Virtual Server to use for testing services, on which I used the script to install Roundcube. All good thus far.
However, when i send email from an external unconnected domain to the new user created in the Virtual Server, it is rejected with "Relay access denied".
Looking in the Postfix config, I can see an entry where I think I can include the new domain created and it will thereby work. It's called, "What domains to receive mail for" and it currently contains, "$myhostname, vmi192093.contaboserver.net, s1.snowweb.info, localhost.snowweb.info, localhost".
The reason I haven't done that though is because I suspect VirtualMin has a feature whereby it will automatically add the domains of Virtual Servers to the list and I just haven't enabled it, but I can't find it.
I would really appreciate any insights on this and many thanks for all the time and effort which you've put into building such a useful piece of software.
Warm regards, Peter