I have two machines: a server with a static IP address and a client
with a dynamic IP address. Postfix is running on the server. I used
these guides to configure Postfix: [1,2].

I want to read and send emails from the client.

I've been told that I should use POP or IMAP to get emails and SMTP to
push replies back to the server.

Here is what I'm going to use for IMAP. (I used this [3] guide.)
Is it OK? Could you comment on the unspecified options?

(mail-source . (imap :server "mail.example.com"
                                         :port 993
                                         :user "john"
                                         :stream tls
                                         :authentication ;; What should I specify?
                                         :program ;; What should I specify?
                                         :mailbox ;; What should I specify?
                                         :dontexpunge nil))

How to configure ESMTP? There is a howto [4], but I don't understand
it. Could you elaborate?

Is it possible not to store sent emails on the server?

What else should be placed in .gnus.el to use IMAP and ESMTP?

How to check that my connections are encrypted?

Thanks

Instead you should use nnimap.

In Emacs 24, you will be prompted for the SMTP setup the first time you
send email, so no configuration is necessary.