I've changed my Postfix main.cf to be virtual for hostname and now I see this when "Recheck Config":
The status of your system is being checked to ensure that all enabled features are available, that the mail server is properly configured, and that quotas are active .. No sender canonical map file for outgoing addresses was found in your Postfix configuration. You should either add the appropriate map to Postfix, or disable this feature in Virtualmin's module config page.
.. your system is not ready for use by Virtualmin. Should I worry and fix something?
Postfix seems fine.