27/09/2021
1. PTR/RDNS Setup:
###.###.###.### PTR smtp1.yourdomain.com
2. A/CNAME record Setup:
A record smtp1.yourdomain.com ###.###.###.###
3. Disable Cloud Network Configuration
/etc/cloud/cloud.cfg.d/99-disable-network-config.cfg
4. Add IP to Network configuration file
sudo nano /etc/netplan 50-cloud-init.yaml