On a default installation of Plexa Panel, Postfix and Dovecot use self-signed SSL. You can now use Let’s Encrypt with your mail server. Navigate to:
https://<IP Address>:8090/manageSSL/sslForMailServer
You will see a list of websites/domain here (which you have created via Plexa Panel 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.
