How to choose a hostname for the mail server?
By purchasing a mail server you get a server with the CentOS operating system on which PostfixAdmin is installed. To properly configure the mail server when registering/purchasing this service, you need to select a fully qualified domain name (FQDN) as the hostname – the name of the mail server. After the activation of the service, the initial mail settings are automatically executed on the server itself, and can not be changed later. This domain needs to be in the form of mail.domainname. The hostname of the server, that is the main domain, will be linked to all mail system settings: MX record, PTR or reverse DNS record, and will be used as incoming and outgoing mail server in the settings of your mail clients. This is the name of the server that will appear in the mail header sent from this server.