[enh] Enable Postfix on submission port and update TLS related conf

Update main configuration for TLS support and enable submission service (port 587) to allow only TLS connection on it - based on [this tutorial](http://docs.homelinux.org/postfix:configure_postfix_with_tls).

I'm not sure about the *smtpd_tls_auth_only* since - as better explained [here](http://www.postfix.org/TLS_README.html#server_tls_auth) - it can be either a security risk or a non-compatibility issue with non-TLS clients... What's the most important? Since at the moment it's still possible to connect with SSL on the - yet depreciated - port 465.