ERROR: You don't have a valid hostname set!

Hi..
This is my problem.. How to solve?

Your Hostname is: DonaGest and it resolves to IP: (ERROR: You don't have a valid hostname set!)

I've create a Free DNS account and I've inserted the ns1.centos-webpanel.com and ns2.centos-webpanel.com to the nameservers in the account of my domain..
In the CWP I'm going to Edit Nameservers IPs and I've setted the same servers with the IP of my website..
Now I'm go to Edit Hostname and I've set the name DonaGest on hostname and I'm go to httpd.conf and I've inserted DonaGest in ServerName..
It's correct?
I receive the same error!

Solved inserting the mail hostname.. But not work for receiving mails, just for send..!

Now work..!
Scuse me!