cannot received email

I was installed the IREDMAIL with the following documentation link.

http://www.iredmail.org/install_iredmail_on_rhel.html

I was using a local IP address 10.0.0.0

and I setup the domain and build a  user..

After that I can send to gmail with my postmaster account.

but when we send from gmail to that postmaster account, we got mail delivery error.

I am sure that I am missing somethings.

Please someone guide me to correct the configuration

thanks,
joe