Zimbra on Ubuntu OpenVZ CT behind Reverse Proxy, NAT and with Dynamic IP

I'm trying to install and make Zimbra works on Ubuntu OpenVZ Container, behind a Reverse Proxy, NAT and with Dynamic IP available.

I'm started installation script without problem but when I reach menu configuration and I insert domain name I recieve DNS Error (DNS ERROR - none of the MX records for mail.mysite.com
resolve to this host).

I'm using Dyndns service and I put a CNAME entry for mail.mysite.com to myserver.dyndns.org and a MX entry for mail.mysite.com.

I opened NAT on 7071 port to IP of my Zimbra machine.

I've add an entry on my Reverse Proxy machine for mail.mysite.com pointing Zimbra machine private IP address.

What's wrong?