Simply run certbot and create your certificate with the desired hostnames. Example for a wildcard certificate:
certbot certonly -d domain.tld -d *.domain.tld
Then prepare the certificate for Grommunio:
cat /etc/letsencrypt/live/<your domain>/fullchain.pem > /etc/grommunio-common/ssl/server-bundle.pem
cat /etc/letsencrypt/live/<your domain>/privkey.pem > /etc/grommunio-common/ssl/server.key
Don't forget to restart the services after.
In order to automatically renew your certificates, create the following cronjob (crontab -e):
0 1 * * * /usr/bin/certbot --reuse-key renew &> /dev/null
If you don't like vim, use nano instead. ;-)
zypper installer nano
echo "export EDITOR=\$(which nano)" >> ~/.bashrc
List of files you have to edit:
/etc/grommunio-admin-common/config.json
/etc/gromox/http.cfg
/etc/gromox/mysql_adaptor.cfg
/etc/gromox/imap.cfg
/etc/gromox/pop3.cfg
/etc/gromox/midb.cfg
/etc/gromox/zcore.cfg
/etc/gromox/delivery.cfg
/etc/postfix/main.cf
/etc/hostname
/etc/hosts
Personally I woudn't do this adventure; better install a second machine and migrate your users.