primary domain email using hostname

Hi, i'v installed cpanel. in a vps test.
After the initial configuration and installation, i go trough webmail and send an email test.

the email is coming as account@host.domain.com

as should it be

account@domain.com

This hostname from initial configuration of whm is a subdomain?
I have to create it on cpanel or add a host. cname?

I think i am miss something about hostname.

Thanks