Make TSL/SSL/STARTSSL Encryption for the Email Client (for example Outlook and co) required (as a setting)
Thank you for your input! We will consider functionality to implement ability to set TLS as required for email connections in upcoming releases if it will be popular.
Everyone, please continue voting for this feature if you consider it important.
Please note that STARTSSL is a service of StartCom and there are issues with them to stay in CA role http://www.pcworld.com/article/3129725/certificate-policy-violations-force-reform-at-startcom-and-wosign.html so STARTSSL support will not be implemented in Plesk.
I would disable unencrypted email server connection on default (except localhost).
Till now I always edited the dovecot and postfix config files myself, but this should be possible by a click of a button/ticking of a checkbox in the mail server settings.
A Warning should show up if there is an invalid or self signed certificate, that might cause for trouble with the mail clients though.