The installation of Mail-in-a-Box worked like a charm.
However, there is a small security issue:
Reconfigure server to use forward secrecy and authenticated encryption
Even though this server supports TLS 1.2, the cipher suite configuration is suboptimal. We recommend that you reconfigure the server so that the cipher suites providing forward secrecy (ECDHE or DHE in the name, in this order of preference) and authenticated encryption (GCM or CHACHA20 in the name) are at the top. The server must also be configured to select the best-available suite.
Can that be fixed?