When running Virtualmin, and you get the error below, follow the advice later in this article. Error message:
Failed to save enabled features : A problem was found with your Postfix virtual maps : No map sources were found in the Postfix configuration
This is an indication that something is amiss with your Postfix installation. It could mean that something went wrong with the installation of Virtualmin, or later on, perhaps when new packages were upgraded (e.g. MariaDB).
The ideal situation is to run this command:
virtualmin-config-system -i Postfix
See reasons here.
On older installations of Virtualmin you might not have that command available. In those instances you could manually start repairing the Postfix installation, starting by adding the command below:
Either edit the /etc/postfix/main.cf file and add:
virtual_alias_maps = hash:/etc/postfix/virtual