Also if your old box is still up and running, you can rsync the /home/user-data/ from the old to the new. Then delete the contents of the /home/user-data/ssl/ directory. THEN rerun sudo mailinabox. You should then have all of your users and email in place on the new box.
For some reason, the SSL Certificate of box.domainname.com didn’t go over and would not let me do the auto Let’s Encrypt, do I manually inserted a Zero SSL certificate.