I wa s installing Virtualmin when the installation stopped on this error which included the following message:

postfix: fatal: file /etc/postfix/main.cf: parameter myhostname: bad parameter value: {MTA_HOSTNAME}

I was able to complete the installation after editing the /etc/postfi/main.cf file and editing the value of myhostname.

In my case, I only had to remove the period after my host name in /etc/postfi/main.cf. Then I saved the file and then started the virtualmin install script again and everything ran just fine.