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
In English, a hostname should consist of a name and a domain name.
All valid hostnames should have 2 dots in them.
Hostnames should never begin with “www”.
Hostnames should also not begin with a digit, nor end with a hyphen (“-”).