Securing ISPConfig 3.1 With a Free Let's Encrypt SSL Certificate
This post is from 2018. Versions, commands and links in it may be outdated. The current release is ISPConfig 3.3.2p1.
This tutorial shows how to create and configure a free Let's encrypt SSL certificate for the ISPconfig interface (port 8080), the email system (Postfix and Dovecot/Courier), the FTP server (pure-ftpd) and Monit.
https://www.howtoforge.com/tutorial/securing-ispconfig-3-with-a-free-lets-encrypt-ssl-certificate/