Based on this post: https://www.virtualmin.com/node/42012, i would suggest the following:
Update Webmin, that it installs letsencrypt /certbot by checking out the git given. check whether the FQDN is connected to an external ip use let's encrypt to create a cert for the given FQDN one has to have in order to get rid of those self signed cert problems. add it to crontab so it will be recreated at least once a week. Add the option to add the FQDN certificate to proftpd / postfix / dovecot and set the settings for all of those correctly
Update Virtualmin with a new feature option under SSL website (autocreate letsencrypt certificate?) While creating the new virtual server, add the domain to the cli ini file and run letsencrypt script to add that domain to the certificate. Use the according lines in the virtual server conf as outlined in above mentioned post.
Would be brilliant :-)
Best