Certificate status invalid

Hi Guys, 

I am setting up a dev lab of exchange 2013. The env is quite simple - one server as DC, DNS, and Certificate Service (enterprise) and one server as exchange mailbox & CAS server.

I am following these article to issue a cert and import to exchange

exchangeserverpro.com/exchange-2013-ssl-certificate-private-certificate-authority/

michaelvh.wordpress.com/2012/07/22/configuring-certificates-in-exchange-server-2013-preview/

After I import the cert to exchange, its status is Invalid. I no experience on certificate configuration. Which step goes wrong?

I would expect Exchange can pop up a dialog with the detailed reasons or write some logs somewhere if the imported certificate to is invalid.  

Thanks,

Msts