There’s a python script which grants permissions to your Dovecot and Postfix database users.

user@host:~$ python /path/to/vmm-0.6.2/pgsql/set-permissions.py -h
Usage: set-permissions.py OPTIONS

Set permissions for Dovecot and Postfix in the vmm database.

Options:
  -h, --help            show this help message and exit
  -a, --askpass         Prompt for the database password.
  -H HOST, --host=HOST  Hostname or IP address of the database server. Leave
                        blank in order to use the default Unix-domain socket.
  -n NAME, --name=NAME  Specifies the name of the database to connect to.
                        Default: mailsys
  -p PASS, --pass=PASS  Password for the database connection.
  -P PORT, --port=PORT  Specifies the TCP port or the local Unix-domain socket
                        file extension on which the server is listening for
                        connections. Default: 5432
  -U USER, --user=USER  Connect to the database as the user USER instead of
                        the default: root
  -D USER, --dovecot=USER
                        Database user name of the Dovecot database user.
                        Default: dovecot
  -M USER, --postfix=USER
                        Database user name of the Postfix (MTA)  database
                        user. Default: postfix
user@host:~$ python /path/to/vmm-0.6.2/pgsql/set-permissions.py -a -H 127.0.0.1 -U vmm
Password:
user@host:~$