Zentyal with LetsEncrypt

Dear all, I have setup my Zentyal Server and I can access the webmail fine and able to send/receive emails fine, also the initial setup with outlook is fine and it reads my first and last name .. yet I am getting this error "there is a problem with the proxy server's security certificate. The name on the security certificate is invalid or does not match the name on the target site [login to view URL] , outlook is unable to connect to the proxy server. (Error code 10) what is required? 1- Fixing this issue on all outlook from 2007 till 2016 without the need of installing the certificate on the clients' PC (including written documentation on how that was done for upcoming similar incidents and new installs) 2- the integration and configuration of CPanel where clients have CPanel interested to have Exchange services from Zentyal instead of the ordinary CPanel POP and IMAP emails NOTE: I have searched around and found that the best practice is to integrate third party SSL , for such instance I would recommend to use LetsEncrypt [login to view URL] and here is a form with the documentation on how to do it [login to view URL] after all, this is just a suggestion, or you can go with your way if you think you have a better idea or vendor but has to be opensource vendor , Thanks