On a default installation of CyberPanel, Postfix and Dovecot use self-signed SSL. You can now use Let's Encrypt with your mail server. Navigate to:
You will see a list of websites/domain here (which you have created via CyberPanel already). Make sure to choose the domain which has RDNS record against your IP Address. Once SSL is successfully issued, Postfix and Dovecot will start using Let's Encrypt SSL.