Postfix Cent0S Configuration for internal domain

We have installed iRedmail (free edition) on a CentOS 8.2 server. We have also created a domain name (test.local). We need some of these accounts where included to the local domain to connect on a mail server and send external emails. The Server is in a local network, so we cannot provide you remote access for tests. The server also has not a valid domain , as the internal domains are not valid.