To install the image on an RPi4, configure port forwarding for your router first (follow instruction for email server). Then, download and save the image to a 64Gb sdcard.
Next, go to your local IP at https://192.168.1.4/postfixadmin/public/raspmailinstall/index.php
Username:ubuntu
Password:berry123
Next, go to Main Domain tab, and put in your company's domain name and submit the form.
Finally, login to https://192.168.1.4/postfixadmin/public/login.php and login with U: admin@admin.com and P:berry123. Create a domain name and mailbox. You may now use your newly created emailbox.