Several questions concerning CI4 controller

CI doesn't handle TLS encryption that is a function of the mail transport application (Postfix, in your case).

Read about Postfix and TLS.

CI does not need to " establish the connection to the email server" because the mail server is literally a program residing on the same server as CI and that CI calls directly.

In terms of the connection between a remote mail client and the CI server, the use of TSL depends entirely on the user's email client setup. There isn't anything a remote server can do to influence how the client is set up.

Other information sources you might find useful.

https://kruyt.org/postfix-and-tls-encryption/

https://upcloud.com/community/tutorials/...s-encrypt/