Thank you very much for your feedback, however I have a hard time understanding your issue. grommunio delivers an out-of-the-box experience with its configuration parameters, especially in your described scenario.
The responsible configuration directives are:
virtual_mailbox_domains = mysql:/etc/postfix/grommunio-virtual-mailbox-domains.cf
virtual_transport = smtp:[::1]:24
This way, any domain created within grommunio is automatically available for mail exchange on SMTP level.
If any further configuration options are wanted, we recommend to use the dbconf feature, which directly configures services such as postfix, documented here: https://docs.grommunio.com/admin/administration.html#db-configuration