Can't setup Virtualmin to use Postfix+LDAP

In addition to Bug #368.
I've installed Virtualmin but can't configure it as it fails while checking Postfix:
The status of your system is being checked to ensure that all enabled features are available, that the mail server is properly configurated, and that quotas are active ..

No virtual domains file was found in your Postfix configuration. Maybe no virtual domain map has been defined.

.. your system is not ready for use by Virtualmin.

Closed (fixed)