Sending email information from root server failed "mail transport unavailable"

I have setup CWPPro from the begining. no modified at all.
At CWP Setting -> Edit Setting, I have checked "Forward server system email"
I can receive email from outside to my email address: info@domain.com
But, all emails from server such as Anacron, RKHunter, CSF informations etc are queue in. It tries to send to info@domain.com, but fail, and bounce back as mailer-daemon with error "mail transport unavailable".
When I click on detail button next to my email address, it said "network unreachable". My email uses google apps and has no problem.

How to fix it?
Thanks.