I've been doing a little bit more of researching and have found a pair of threads with the same restrictions as my problem:
1) Is the solution in Zimbra 5.5?
Multiple Zimbra Server install to satisfy DMZ requirement
2) Is Zimbra proxy what I need installed in DMZ host, whereas Zimbra server remains in LAN?
https://www.zimbra.com/forums/instal...eb-server.html
What do you think about those?