Mailgun Exposed our IP?

Hello,

When using Mailgun it’s exposed our IP, and after contact them they advice to use API with html instead…

If there is a way to do that please advice; or advice how can we apply with PostFIX like discourse.org

from localhost.localdomain (unknown [IPv6:2602:fd3f:2:ff02::e4]) by mx-out-01b.yyz2.discourse.cloud (Postfix)

How to use IPv6 instead of the Postfix IPv4 as well ?

Thanks!