The Change Hostname option enables you to change the hostname of your server. It is important to choose a hostname that will not be used for any account, and the hostname must be socially acceptable, as it will appear in all mail headers. The hostname must be a FQDN (Fully Qualified Domain Name). All valid hostnames must have at least 2 dots in them, for example entropy.yourdomain.com.
WebHost Manager automatically generates 10 random hostnames based on your current hostname as samples which you can use.
NOTE: Your hostname should be a Fully Qualified Domain Name that will not be used elsewhere on the server. You should never set your hostname to an name that is already used (www, ftp, mail, etc) or to the name of an account on the server (www.domain.com). This means that your hostname should never be a domain name and especially not the name of your domain!!