I use a custom port for SSH.
After installing Mail-in-a-Box I was no longer able to SSH into my server as the firewall was blocking the port.
Thankfully I’m using DigitalOcean and was able to use their virtual console to SSH into the server and
sudo ufw allow 1330; reload ssh to reopen the port.
So if you are using a custom port to connect with SSH, make sure you open up the port after installing Mail-in-a-Box before you exit the session and close the terminal.