Many providers set a default hostname, that is subdomain of the providers domain. It is better change this to your own subdomain.
hostname is used by mail servers. If you use providers sub domain as your servers host name and there is some black list blocking the domain, it can affect your email delivery ability. Using your own hostname make it look more professional.
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If you have non cpanel server, you can use the command hostnamectl to set hostname for the server.
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