It is important to choose a hostname that you will not be using for any account. You should choose something that will be socially acceptable, as it will appear in all mail headers. Your hostname must be a FQDN (Fully Qualified Domain Name).
The hostname should be a valid subdomain (name and domain name).
Hostnames should never begin with www. Hostnames should also not begin with a digit, nor end with a hyphen (-).
To verify which hostname you have run command "hostname" in SSH. You can permanently change your hostname from your Hypanel account (https://panel.intovps.com) -> Machine Settings tab -> Set Machine Hostname.