Ubuntu: How to setup TLS on postfix and dovecot

[adrianknight]

The mails from my server are send from the Hostname of the server (hostname.domain.tld) and the MX record point to mail.domain.tld.

I pointed the PTR to mail.domain.tld but now changed it to hostname.domain.tld because first I thought the mails are send from mail.domain.tld.

Now where should I setup the TLS to? mail.domain.tld (Which is pointed by the MX record) or hostname.domain.tld (Which is the sender of the emails)?

Thank you