centos7 - Setting FQDN with external domain name

I have a Centos 7 server which has tens of domain names and IPs.

The IP addresses are pointing to my server.

Each domain name is pointing to its own IP via A record.

I want to configure my FQDN in order to install Postfix.

/etc/hostname contains myproject.localdomain

/etc/hosts contains:

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4

::1         localhost localhost.localdomain localhost6 localhost6.localdomain6

Command hostname returns:

myproject.localdomain

hostname -f returns:

localhost

domainname returns (none)

Do I have to choose a real domain purchased and prepend to it the hostname like this:

ip    hotname.domainame    hostname