Linux Babe published a tutorial about setting up Postfix send-only SMTP Server on CentOS 8.
Set Up Postfix Send-Only SMTP Server on CentOS 8
In this tutorial, I will show you how to set up Postfix as a send-only SMTP server on CentOS 8. You can apply the steps for multiple domains if you need to.
Set Up Postfix Send-Only SMTP Server on CentOS 8 - LinuxBabe