Communication between two email servers

Hi;
I have two domains one is ???A.COM and another is ???B.COM ( ???A.COM and ????B.COM) and for this purpose i have installed two email servers (Kerio mail server)in DMZ zone. I can send emails to any domain and can receive as well one both servers from any domain, the only problem is that i cannot receive emails from MY ???A.COM to ????B.COM.

192.0.0.4 = ???A.COM email server
192.0.0.8 = ????B.COM mail server

I don’t know why 192.0.0.8 cannot receive email from 192.0.0.4. Both have kerio mail servers. Can anyone help me.

Thanks
Bawer