You will have to specify there the IP or fqdn of the mail server.
For example: pop.yourdomain.com, smtp.yourdomain.com, imap.yourdomain.com
But, before this, you will have to create that records in a DNS (a server in charge of manage the relation "domain = ip", or just add that reference to /etc/resolv.conf
I can see that you are relatively new to this world so... if you want to learn more, look in google for topics like: dns basics, dns record basics, network basics, tcp/ip basics, mailserver basics, postfix basics, routing, switch, linux directory structure, etc.
This world appear to be complex, but is more easy than it appears to be :P