I'm still new in Zimbra

I have 3 Zimbra servers
version 5 ~ local server, local domain (send and receive email from internal network only)
version 7 ~ local server, local domain (send and receive email from internal network only)
version 8 ~ new, have public domain (can send and receive email from external)
currently I set the DNS of the new server to the Internet so it can send and receive from external
but when I sent email to other local server it couln't delivered
How to make my new zimbra server can send and receive email from external and to my other local servers?

Thanks