Following steps must be performed on both nodes.
Before starting we should check that we have updated all installed packages:
Stop and disable the default installed mail service:
In this example we will use only short node names and you could use hostnamectl command to fix it if in your installation is configured in FQDN (Fully Qualified Domain Name) style:
If you prefer to use FQDN hostnames you should change them accordingly everywhere you will find references to short names.
Configure the enp0s3 network interface with a new zone (named ha) and restart it:
If you like to ignore link up / down events on all network interfaces you should configure accordingly (and restart) Network Manager service
At this step it is a good idea to reboot each node, especially if the kernel has been updated.