Mail app sends my IP in email headers I just sent my first email using the Mail app through a fully functional email Postfix+Dovecot server. 

I am worried by the fact that it also sends my IP address and hostname along with it.

The header that is causing this is Received:

Received: from mycomp-mbp.domain_not_set.invalid (cpe-IP-HERE.dynamic.ISPHERE.net [IPHERE]) by mydomain.com (Postfix) with ESMTPSA id 70F1D1B459 for &lt;recipient.of.email@gmail.com&gt;; Fri, 1 Feb 2019 11:17:45 +0100 (CET)


How can I instruct Mail app to not send this information along the way?
That line is added by the machine that received the mail from your Mac, so you don't get to control that from Mail.app or macOS.

This is very intentional, so unless your mail company wants to be taking responsibility for all the mails you send them, they might not be willing to "own" the mails you ask them to forward.

The machine that received the mail already knows the IP address and what it thinks is your host name so you would open a ticket with the team that manages that server if you needed to change the source of your mails or alter that header content in any way.